Microstructures and Mechanical Properties of Dissimilar Al / Steel Butt Joints Produced by Autogenous Laser Keyhole Welding

Dissimilar Al/steel butt joints of 6.0 mm thick plates have been achieved using fiber laser keyhole welding autogenously. The cross sections, interface microstructures, hardness and tensile properties of Al/steel butt joints obtained under different travel speeds and laser beam offsets were investigated. The phase morphology and thickness of the intermetallic compound (IMC) layers at the interface were analyzed by scanning electronic microscopes (SEM) using the energy-dispersive spectrometry (EDS) and electron back-scattered diffraction (EBSD) techniques. The results show that travel speeds and laser beam offsets are of considerable importance for the weld shape, morphology and thickness of IMC layers, and ultimate tensile strength (UTS) of Al/steel butt joints. This proves that the IMC layers consist of Fe2Al5 phases and Fe4Al13 phases by EBSD phase mapping. Increasing laser beam offsets from 0.3 mm to 0.7 mm significantly decreases the quantity of Fe4Al13 phases and the thickness of Fe2Al5 layers at the interface. During tensile processing, the Fe2Al5 layer with the weakest bonding strength is the most brittle region at the interface. However, an intergranular fracture that occurred at Fe2Al5 layers leads to a relatively high UTS of Al/steel butt joints.


Introduction
Dissimilar joining of Al/steel joints has become increasingly significant in industrial applications with a particular weight-saving interest [1,2].Although research on joining dissimilar steel/Al alloy joints with fusion-based welding processes is extensive, most previous studies have been focused on lapped joints or overlap joints of less than 2 mm thick sheets for automotive applications [2,3].In recent years, the development of fast-speed vessels requires improved solutions to the dissimilar joining of Al alloy superstructures to the steel hull for achieving weight reduction in the shipbuilding industry [4,5].Thus, the dissimilar joining of thick Al/steel joints has received considerable attention in shipbuilding applications [6].In general, an explosive Al/steel transition joint is used in the shipyard for the dissimilar joining of Al/steel joints.However, the cost of the transition joints bonded by explosive welding is high due to the complex manufacturing process [4,5,7].Therefore, it is highly important to develop new welding process for dissimilar joining Al/steel joints directly.
It is well known that dissimilar joining Al/steel joints is extremely challenging due to the huge disparity in thermal-physical properties between steels and Al alloys [4,8,9].One of the main issues associated with welding Al/steel joints is the formation of thick intermetallic compound (IMC) layers at the interface [10,11].While the IMC layer is necessary for achieving a reliable Al/steel joint, the brittleness of Al/steel joints is increased and the mechanical property is reduced if the IMC layer thickness is beyond a certain range [12,13].The key to improving the mechanical properties of the joints is to control the formation of IMCs and limit IMCs thickness within 10 µm [14].Therefore, considerable effort has been put into inhibiting the growth of IMC layers at the interface of Al/steel joints.The results show that welding processes allowing precise control to potentially minimize heat input (by an optimization of process parameters) are required to obtain thin IMC layers [15,16].In view of this, a laser is regarded as a desirable heating source for joining dissimilar steel to Al alloys because of its characteristics of high power density, low welding heat input, and precise control of the location of laser focus, etc., which allow better control of the IMC layer thickness at the interface [7,17,18].
Some work has been reported on the laser welding-brazing process for dissimilar steel/Al alloy joining.During the welding, metals with a low melting point (e.g., Al alloy) and the filler wire are melted by laser or a laser-arc hybrid heating source, while metals with a high melting point (e.g., steel) is brazed by the molten metal in the solid state [1,16,17].Thus, the resulted joint has dual characteristics: it is a welded joint on the low melting point metal side and a brazed joint on the high melting point metal side [19].Compared to the fusion-welding processes, this process is effective for limiting the mixture of the dissimilar molten metal and inhibiting the formation of brittle IMCs at the interface.However, it is inefficient when the laser is irradiated at the low melting point Al alloy side, because Al has a high reflectivity for lasers.In addition, using laser welding-brazing can achieve an acceptable steel/Al alloy joint, but the wettability is not good when liquid Al alloy meets the solid steel.So the addition of flux, coating or other methods is usually applied to improve wettability [14,18,20,21].
Laser keyhole welding is one of the most widely used of the various laser-welding processes in dissimilar joints.With the laser keyhole-welding technique, rather good mechanical properties were obtained by irradiating steels placed on Al sheets, and limiting Al to steels mixed with a reduced steel penetration in Al alloys [8].This indicates that the laser keyhole welding process was mainly used to lapped or overlapped joints [12,13,15,[22][23][24].It can be, but rarely is, used for welding Al/steel butt joints requiring accurate groove preparations, flux and filler wires [24][25][26][27].Although the use of filler wires or flux leads to some improvement in the joint properties, laser welding with filler materials has usually been considered too difficult for industrial applications because there are too many parameters and the requirements for wire positioning are too stringent [28], thereby reducing efficiency and productivity.Thus, the possibility to work in autogenous laser keyhole welding is of important technological interest, as it allows a reduction of the time taken in joint preparation and has fewer parameters to control, making it easier to obtain reproducible and stable results [29].
In the present study, an autogenous laser keyhole welding process was introduced to join 6 mm thick steel to Al alloy plates in a butt configuration.The primary objective of this work was to understand the influences of travel speeds and laser beam offsets on the weld shapes, the IMC layers and ultimate tensile strength (UTS) of Al/steel butt joints.Using electron back-scattered diffraction (EBSD) phase mapping, the phase identification and the fracture behavior of IMC layers at the interface were investigated.Based on microstructual and tensile testing, correlations between the processing parameters, IMC layers, and the UTS of Al/steel butt joints were established.

Materials and Methods
The materials studied in the present work were 6 mm thick 5083 alloy and Q235 low carbon steel plates arranged in a butt configuration, and their nominal compositions are shown in Table 1.Before welding, the oxides on the surface of the aluminum alloy sheet were removed with abrasive paper, and then the surface of the aluminum alloy and steel plates were cleaned with acetone to remove grease and residue.With particular attention to the butt joint, the Al and steel plate edges of the butt joint were also ground, brushed using a stainless steel wire brush, and cleaned thoroughly with ethanol in order to remove the oxide layer in the welding gap.The autogenous laser keyhole welding was carried out by a 6 kW YLR-6000 fiber laser (IPG Photonics, Oxford, MA, USA) with an emission wavelength of 1.07 µm delivering in continuous wave mode through an output fiber core diameter of 100 µm.The laser beam was transmitted through a processing fiber with a diameter of 300 µm, collimated by a lens of 150 mm focal length, and then focused on the materials by a focusing lens with 200 mm focal length.Accordingly, the spot size of focused laser beam was approximately 0.3 mm when the beam-defocused distance was 0 mm.Throughout the experiments, the primary and back-shielding gas, using ultrahigh purity argon gas, was supplied at flow rates of 15 L/min.
The schematic illustration of the welding setup and the principle of laser keyhole-welded Al/steel butt joints is presented in Figure 1.During welding, the focused laser beam was directed onto the steel plate to generate a keyhole, and the solidification of the welding pool formed a weld penetration.Important process parameters for laser keyhole welding affected the weld shapes, including laser power (P), travel speed (v), defocused distance (f ), and laser beam offset (∆d).P and v are generally important process variables affecting the heat input, as in regular arc welding.∆d refers to the distance from the laser focus to the interface illustrated in Figure 1b, and is an important parameter only in laser welding, which also controls the welding thermal cycles and cooling rates at the interface [24,30,31].As a result, the process parameters of ∆d and v were considered for studying the process parameter influence upon the laser keyhole welded Al/steel butt joints.Thus, ∆d = 0.3-0.7 mm and v = 0.6-1.2m/min were varied discretely under the constant conditions of P = 3.25 kW and f = 0 mm.The autogenous laser keyhole welding was carried out by a 6 kW YLR-6000 fiber laser (IPG Photonics, Oxford, MA, USA) with an emission wavelength of 1.07 μm delivering in continuous wave mode through an output fiber core diameter of 100 μm.The laser beam was transmitted through a processing fiber with a diameter of 300 μm, collimated by a lens of 150 mm focal length, and then focused on the materials by a focusing lens with 200 mm focal length.Accordingly, the spot size of focused laser beam was approximately 0.3 mm when the beam-defocused distance was 0 mm.Throughout the experiments, the primary and back-shielding gas, using ultrahigh purity argon gas, was supplied at flow rates of 15 L/min.
The schematic illustration of the welding setup and the principle of laser keyhole-welded Al/steel butt joints is presented in Figure 1.During welding, the focused laser beam was directed onto the steel plate to generate a keyhole, and the solidification of the welding pool formed a weld penetration.Important process parameters for laser keyhole welding affected the weld shapes, including laser power (P), travel speed (v), defocused distance (f), and laser beam offset (∆d).P and v are generally important process variables affecting the heat input, as in regular arc welding.∆d refers to the distance from the laser focus to the interface illustrated in Figure1b, and is an important parameter only in laser welding, which also controls the welding thermal cycles and cooling rates at the interface [24,30,31].As a result, the process parameters of ∆d and v were considered for studying the process parameter influence upon the laser keyhole welded Al/steel butt joints.Thus, ∆d = 0.3-0.7 mm and v = 0.6-1.2m/min were varied discretely under the constant conditions of P = 3.25 kW and f = 0 mm.The quality of welded joints is significantly affected by the defects generated in the process of welding dissimilar materials.Thus, after welding, the welding quality was evaluated according to the Chinese welding procedure qualification code DL/T868-2004 and the international standard EN ISO 6520-1 for dissimilar welded joints.An acceptable joint is one in which the weld ripples, the weld width is uniform, and the weld surface should have no visible porosity or hot cracking.Also, there should be no excessive asymmetry or top and root concavity.After the weld surface was checked, Xray inspection was performed, and the determined quality should be less than grade II.Then, the acceptable Al/steel butt joints were cut by electro-discharge machining, and this was followed by grinding, polishing and etching.Keller's reagent and Nital acid were used for etching the Al alloys and steels, respectively.The weld appearance and cross-sections were examined using a BX51M The quality of welded joints is significantly affected by the defects generated in the process of welding dissimilar materials.Thus, after welding, the welding quality was evaluated according to the Chinese welding procedure qualification code DL/T868-2004 and the international standard EN ISO 6520-1 for dissimilar welded joints.An acceptable joint is one in which the weld ripples, the weld width is uniform, and the weld surface should have no visible porosity or hot cracking.Also, there should be no excessive asymmetry or top and root concavity.After the weld surface was checked, X-ray inspection was performed, and the determined quality should be less than grade II.Then, the acceptable Al/steel butt joints were cut by electro-discharge machining, and this was followed by grinding, polishing and etching.Keller's reagent and Nital acid were used for etching the Al alloys and steels, respectively.The weld appearance and cross-sections were examined using a BX51M optical microscope (Olympus, Tokyo, Japan).The top width (W 1 ) and bottom width (W 2 ) of the weld formed in the steel side, and the area (S) of the heat-affected zone (HAZ) formed in the Al alloy side, were used to evaluate the weld shapes of the Al/steel butt joints, as illustrated in Figure 2.
Metals 2017, 7, 492 4 of 18 optical microscope (Olympus, Tokyo, Japan).The top width (W1) and bottom width (W2) of the weld formed in the steel side, and the area (S) of the heat-affected zone (HAZ) formed in the Al alloy side, were used to evaluate the weld shapes of the Al/steel butt joints, as illustrated in Figure 2. Vickers microhardness was measured using a HXD-10007 digital and intelligent micro-hardness tester (Caikon, Shanghai, China) with a load of 100 N and 10 s holding time.For each value, the average of five measurements was taken.Uniaxial transverse tensile tests of the Al/steel butt joints were carried out in accordance with the GB/T 228-2002 and GB/T 2651-2001 standards at room temperature using a MTS810 testing machine (MTS, Eden Prairie, MN, USA) operating with a load rate of 1 mm/min.The average UTS value was determined by calculating three tensile specimens.
The IMC layers at the interface of the Al/steel butt joints were preliminarily examined using scanning electron microscopy (SEM, FEI, Hillsboro, OR, USA) with a QUANTA FEG 650 equipped with an energy-dispersive X-ray spectrometer (EDS).Then, EBSD phase mapping was examined in a rectangular zone using a step size of 0.3 μm between two measurements in order to further identify the IMC phases.The EBSD analysis was conducted by a Quanta FEI 650 SEM field emission gun operated at 30 kV and 1.0 nm.The phases and thickness of the IMC layers were analyzed with the HKL-Channel software package (Oxford Instruments, Witney, Oxon, UK).After tensile testing, the failed samples were observed using the SEM and EBSD techniques to study the facture behavior of the IMC layers.

Weld Appearance of Al/Steel Butt Joints
Figure 3 shows a representative weld appearance of an Al/steel butt joint under the conditions of ∆d = 0.6 mm, v = 1.0 m/min, P = 3250 W and f = 0 mm.It is obvious that the top and bottom appearance of the weld are uniform, with regular ripples, except for a little irregularity in proximity to the Al alloys.This means that a relatively acceptable weld appearance of Al/steel butt joints has been achieved by the autogenous laser keyhole-welding process under the optimum welding conditions.Vickers microhardness was measured using a HXD-10007 digital and intelligent micro-hardness tester (Caikon, Shanghai, China) with a load of 100 N and 10 s holding time.For each value, the average of five measurements was taken.Uniaxial transverse tensile tests of the Al/steel butt joints were carried out in accordance with the GB/T 228-2002 and GB/T 2651-2001 standards at room temperature using a MTS810 testing machine (MTS, Eden Prairie, MN, USA) operating with a load rate of 1 mm/min.The average UTS value was determined by calculating three tensile specimens.
The IMC layers at the interface of the Al/steel butt joints were preliminarily examined using scanning electron microscopy (SEM, FEI, Hillsboro, OR, USA) with a QUANTA FEG 650 equipped with an energy-dispersive X-ray spectrometer (EDS).Then, EBSD phase mapping was examined in a rectangular zone using a step size of 0.3 µm between two measurements in order to further identify the IMC phases.The EBSD analysis was conducted by a Quanta FEI 650 SEM field emission gun operated at 30 kV and 1.0 nm.The phases and thickness of the IMC layers were analyzed with the HKL-Channel software package (Oxford Instruments, Witney, Oxon, UK).After tensile testing, the failed samples were observed using the SEM and EBSD techniques to study the facture behavior of the IMC layers.

Weld Appearance of Al/Steel Butt Joints
Figure 3 shows a representative weld appearance of an Al/steel butt joint under the conditions of ∆d = 0.6 mm, v = 1.0 m/min, P = 3250 W and f = 0 mm.It is obvious that the top and bottom appearance of the weld are uniform, with regular ripples, except for a little irregularity in proximity to the Al alloys.This means that a relatively acceptable weld appearance of Al/steel butt joints has been achieved by the autogenous laser keyhole-welding process under the optimum welding conditions.

Cross Sections of Al/Steel Butt Joints
Figure 4 shows the cross sections of the Al/steel butt joints obtained at different travel speeds under the condition of ∆d = 0.6 mm, P = 3.25 kW and f = 0 mm.For all the travel speeds, each weld is fully penetrated in keyhole mode.The cross sections of the welds at a travel speed of 0.6 m/min and 0.8 m/min have a "champagne glass" shape, while the weld at 1.0 m/min has a "nail" tape.The welds at 1.1 m/min and 1.2 m/min fall between the two types, with a hybrid of both "nail" and "champagne glass" shapes.Thus, the travel speed plays a major role in forming the weld shape of Al/steel butt joints.In addition, it is obvious that the weld width and weld area decrease rapidly as travel speed increases from 0.6 m/min to 1.0 m/min.At slower travel speeds of 0.6 and 0.8 m/min, a relatively large keyhole weld is generated due to the relatively high welding heat input compared with that of 1.0 m/min.As travel speeds increase from 1.0 to 1.2 m/min, although the amount of time that the laser beam is over a particular zone along the joint is less [32], the resulting level of the top part of the welds increases to some extent, whereas the middle part and bottom part of the welds have a relatively regular interface, resulting in a welding-brazing Al/steel butt joint.Thus, the optimum travel speeds for the best cross section obtained by laser keyhole welding is 1.0 and 1.1 m/min.

Cross Sections of Al/Steel Butt Joints
Figure 4 shows the cross sections of the Al/steel butt joints obtained at different travel speeds under the condition of ∆d = 0.6 mm, P = 3.25 kW and f = 0 mm.For all the travel speeds, each weld is fully penetrated in keyhole mode.The cross sections of the welds at a travel speed of 0.6 m/min and 0.8 m/min have a "champagne glass" shape, while the weld at 1.0 m/min has a "nail" tape.The welds at 1.1 m/min and 1.2 m/min fall between the two types, with a hybrid of both "nail" and "champagne glass" shapes.Thus, the travel speed plays a major role in forming the weld shape of Al/steel butt joints.In addition, it is obvious that the weld width and weld area decrease rapidly as travel speed increases from 0.6 m/min to 1.0 m/min.At slower travel speeds of 0.6 and 0.8 m/min, a relatively large keyhole weld is generated due to the relatively high welding heat input compared with that of 1.0 m/min.As travel speeds increase from 1.0 to 1.2 m/min, although the amount of time that the laser beam is over a particular zone along the joint is less [32], the resulting level of the top part of the welds increases to some extent, whereas the middle part and bottom part of the welds have a relatively regular interface, resulting in a welding-brazing Al/steel butt joint.Thus, the optimum travel speeds for the best cross section obtained by laser keyhole welding is 1.0 and 1.1 m/min.

Cross Sections of Al/Steel Butt Joints
Figure 4 shows the cross sections of the Al/steel butt joints obtained at different travel speeds under the condition of ∆d = 0.6 mm, P = 3.25 kW and f = 0 mm.For all the travel speeds, each weld is fully penetrated in keyhole mode.The cross sections of the welds at a travel speed of 0.6 m/min and 0.8 m/min have a "champagne glass" shape, while the weld at 1.0 m/min has a "nail" tape.The welds at 1.1 m/min and 1.2 m/min fall between the two types, with a hybrid of both "nail" and "champagne glass" shapes.Thus, the travel speed plays a major role in forming the weld shape of Al/steel butt joints.In addition, it is obvious that the weld width and weld area decrease rapidly as travel speed increases from 0.6 m/min to 1.0 m/min.At slower travel speeds of 0.6 and 0.8 m/min, a relatively large keyhole weld is generated due to the relatively high welding heat input compared with that of 1.0 m/min.As travel speeds increase from 1.0 to 1.2 m/min, although the amount of time that the laser beam is over a particular zone along the joint is less [32], the resulting level of the top part of the welds increases to some extent, whereas the middle part and bottom part of the welds have a relatively regular interface, resulting in a welding-brazing Al/steel butt joint.Thus, the optimum travel speeds for the best cross section obtained by laser keyhole welding is 1.0 and 1.1 m/min.Figure 5 shows the cross sections of Al/steel butt joints obtained at different laser beam offsets under the condition of v = 1.0 m/min, P = 3.25 kW and f = 0 mm.All the welds possess a similar "nail" type penetration, meaning that the laser beam offsets have no obvious influence on the weld type of Al/steel butt joints.Regarding the weld quality, at relatively low laser beam offsets of 0.3-0.5 mm, the top surfaces of the welds are slightly concave, which is usually caused by excessive metallurgical reactions resulting from the high welding heat input [31].As the laser beam offset increases to 0.6 and 0.7 mm, the concavity of the welds is considerably reduced due to the decrease of the heat input.In addition, at relatively low laser beam offsets of 0.3-0.4mm, the interface boundary is seriously uneven along the joint thickness because of the excessive metallurgical reactions happening at the interface.When the laser beam offsets continuously increase to 0.6 and 0.7 mm, the excessive metallurgical reaction of the interface is lessened.As a result, the acceptable laser beam offsets for laser keyhole welding of Al/steel butt joints is 0.6 and 0.7 mm.
Al/steel butt joints.Regarding the weld quality, at relatively low laser beam offsets of 0.3-0.5 mm, the top surfaces of the welds are slightly concave, which is usually caused by excessive metallurgical reactions resulting from the high welding heat input [31].As the laser beam offset increases to 0.6 and 0.7 mm, the concavity of the welds is considerably reduced due to the decrease of the heat input.
In addition, at relatively low laser beam offsets of 0.3-0.4mm, the interface boundary is seriously uneven along the joint thickness because of the excessive metallurgical reactions happening at the interface.When the laser beam offsets continuously increase to 0.6 and 0.7 mm, the excessive metallurgical reaction of the interface is lessened.As a result, the acceptable laser beam offsets for laser keyhole welding of Al/steel butt joints is 0.6 and 0.7 mm.
However, the thermal cycle suffered at the interface from the top to the bottom of the joints is different, because laser welding has a high temperature gradient of through-thickness, which induces uneven distribution of reaction layer morphology, leading to different reaction modes.At the top part of the steel/Al alloy joints, the interface generated an excessive metallurgical reaction showing fusion joint characteristics as a result of the relatively high heat input.However, in some regions of the interface, such as in the middle part, or the one close to the bottom, a relatively regular interface line due to the reduced heat input is revealed.It is found that a narrow zone of solid steel HAZ is formed between the fusion weld and the Al alloy, and the interfacial reaction mode is transformed from fusion-melting to brazing, which proves that a welding-brazing butt joint has been obtained by the keyhole laser welding process.The effects of travel speeds and laser offsets on W1, W2, S are illustrated in Figure 6.As shown in Figure 6a, S decreases sharply from 6.84 to 3.42 mm 2 with the increase of travel speeds from 0.6 to 1.0 m/min, and decreases slowly from 2.76 to 2.10 mm 2 with increasing travel speeds from 1.1 to 1.2 m/min; indicating that S is decreased continuously with increasing travel speeds from 0.6 to 1.2 m/min.The decrease of S is due to the fact that the laser beam interacts with the steels for a shorter period of time as travel speed increases [33], and thus less molten weld metal is produced, causing S to decrease continuously.As shown in Figure 6b, the S decreases sharply from 4.56 to 2.64 mm 2 , and then decreases slowly from 2.22 to 1.92 mm 2 with increasing laser offsets from 0.3 to 0.7 mm.The decrease of S with increasing laser beam offsets is due to the fact that the heat input at the interface is decreased continuously with increasing laser beam offsets [9].This indicates that the travel speeds and laser beam offsets directly impact the molten zone of the Al alloy side, which contributes to the final weld quality of the interface.However, the thermal cycle suffered at the interface from the top to the bottom of the joints is different, because laser welding has a high temperature gradient of through-thickness, which induces uneven distribution of reaction layer morphology, leading to different reaction modes.At the top part of the steel/Al alloy joints, the interface generated an excessive metallurgical reaction showing fusion joint characteristics as a result of the relatively high heat input.However, in some regions of the interface, such as in the middle part, or the one close to the bottom, a relatively regular interface line due to the reduced heat input is revealed.It is found that a narrow zone of solid steel HAZ is formed between the fusion weld and the Al alloy, and the interfacial reaction mode is transformed from fusion-melting to brazing, which proves that a welding-brazing butt joint has been obtained by the keyhole laser welding process.
The effects of travel speeds and laser offsets on W 1 , W 2 , S are illustrated in Figure 6.As shown in Figure 6a, S decreases sharply from 6.84 to 3.42 mm 2 with the increase of travel speeds from 0.6 to 1.0 m/min, and decreases slowly from 2.76 to 2.10 mm 2 with increasing travel speeds from 1.1 to 1.2 m/min; indicating that S is decreased continuously with increasing travel speeds from 0.6 to 1.2 m/min.The decrease of S is due to the fact that the laser beam interacts with the steels for a shorter period of time as travel speed increases [33], and thus less molten weld metal is produced, causing S to decrease continuously.As shown in Figure 6b, the S decreases sharply from 4.56 to 2.64 mm 2 , and then decreases slowly from 2.22 to 1.92 mm 2 with increasing laser offsets from 0.3 to 0.7 mm.The decrease of S with increasing laser beam offsets is due to the fact that the heat input at the interface is decreased continuously with increasing laser beam offsets [9].This indicates that the travel speeds and laser beam offsets directly impact the molten zone of the Al alloy side, which contributes to the final weld quality of the interface.Concerning the weld width, the W1 decreases sharply from 4.92 to 1.56 mm, and then increases slowly from 1.81 to 2.44 mm, as travel speed increases from 0.6 to 1.2 m/min.The W2 decreases gradually from 2.34 to 0.97 mm with the increase of travel speeds from 0.6 to 1.0 m/min, and then increases a little from 1.02 to 1.31 mm with the increase of travel speeds from 1.0 to 1.2 m/min.This indicates that travel speed has a significant effect on W1, but has less influence on W2.In the case of laser beam offsets, W1 decreases gradually from 2.47 to 1.67 mm and W2 decreases a little from 1.5 to 1.0 mm, as the laser beam offsets increase from 0.3 to 0.7 mm, indicating that a varying laser beam offset influences W1 and W2 somewhat.Consequently, varying travel speeds and laser beam offsets have a more pronounced effect on S than on weld width.

Microstructures of Al/Steel Joint
The typical cross section of an Al/steel joint with a laser beam offset 0.3 mm was observed and analyzed.Several different zones shown in Figure 7a and the characteristics of each are discussed in detail.Figure 7b,d,f presents an enlarged view of zones A, B and C as shown in Figure 7a. Figure 7b shows the microstructure of the steel-fusion zone, steel HAZ and steel base metal, showing the variation of grain morphology along with the decline of heat input.Figure 7c,e shows highmagnification microstructures of steel fusion and steel HAZ, corresponding to zones marked by white dashes and red dashes shown in Figure 7b, respectively.It can be noticed that the steel-fusion zone has a coarser lath structure, which contains martensite microstructures after rapid cooling, while the steel HAZ consists of granular bainite.Figure 7d shows an enlarged view of zone B shown in Figure 7a, and displays the typical microstructure of a steel seam which is mainly composed of columnar crystal.Grain size is from 100 to 200 μm.Moreover, columnar crystal was nearly perpendicular to the fusion boundary, which is induced by the higher cooling rate in this direction and the characteristic of the preferred growth of crystallized grains [24].
Figure 7f displays an enlarged view of zone C shown in Figure 7a, revealing the microstructure of the Al side of the Al/steel joint.It can be seen that the welding joint is formed between the molten aluminum weld metal and local molten steel.IMC layers were produced along the Al/steel interface, with broken IMC phases in the neighboring Al alloy weld metal.In this area, the changes of grain size can be observed, which may confirm the fact that heat input affects the process of crystallization and growth.Figure 7g shows the Al fusion zone marked by white dashes in Figure 7f.The microstructure of the base metal is very different to that of the fusion zone.The external Al fusion zones close to the base metal (shown in Figure 7g), are characterized by dendritic growth, which corresponds to the zones with higher solidification rates [34].Concerning the weld width, the W 1 decreases sharply from 4.92 to 1.56 mm, and then increases slowly from 1.81 to 2.44 mm, as travel speed increases from 0.6 to 1.2 m/min.The W 2 decreases gradually from 2.34 to 0.97 mm with the increase of travel speeds from 0.6 to 1.0 m/min, and then increases a little from 1.02 to 1.31 mm with the increase of travel speeds from 1.0 to 1.2 m/min.This indicates that travel speed has a significant effect on W 1 , but has less influence on W 2 .In the case of laser beam offsets, W 1 decreases gradually from 2.47 to 1.67 mm and W 2 decreases a little from 1.5 to 1.0 mm, as the laser beam offsets increase from 0.3 to 0.7 mm, indicating that a varying laser beam offset influences W 1 and W 2 somewhat.Consequently, varying travel speeds and laser beam offsets have a more pronounced effect on S than on weld width.

Microstructures of Al/Steel Joint
The typical cross section of an Al/steel joint with a laser beam offset 0.3 mm was observed and analyzed.Several different zones shown in Figure 7a and the characteristics of each are discussed in detail.Figure 7b,d,f presents an enlarged view of zones A, B and C as shown in Figure 7a. Figure 7b shows the microstructure of the steel-fusion zone, steel HAZ and steel base metal, showing the variation of grain morphology along with the decline of heat input.Figure 7c,e shows high-magnification microstructures of steel fusion and steel HAZ, corresponding to zones marked by white dashes and red dashes shown in Figure 7b, respectively.It can be noticed that the steel-fusion zone has a coarser lath structure, which contains martensite microstructures after rapid cooling, while the steel HAZ consists of granular bainite.Figure 7d shows an enlarged view of zone B shown in Figure 7a, and displays the typical microstructure of a steel seam which is mainly composed of columnar crystal.Grain size is from 100 to 200 µm.Moreover, columnar crystal was nearly perpendicular to the fusion boundary, which is induced by the higher cooling rate in this direction and the characteristic of the preferred growth of crystallized grains [24].
Figure 7f displays an enlarged view of zone C shown in Figure 7a, revealing the microstructure of the Al side of the Al/steel joint.It can be seen that the welding joint is formed between the molten aluminum weld metal and local molten steel.IMC layers were produced along the Al/steel interface, with broken IMC phases in the neighboring Al alloy weld metal.In this area, the changes of grain size can be observed, which may confirm the fact that heat input affects the process of crystallization and growth.Figure 7g shows the Al fusion zone marked by white dashes in Figure 7f.The microstructure of the base metal is very different to that of the fusion zone.The external Al fusion zones close to the base metal (shown in Figure 7g), are characterized by dendritic growth, which corresponds to the zones with higher solidification rates [34].

Phase Identification of Intermetallic Compound (IMC) Layers
Figure 8 shows a back-scattered scanning electron (BSE) micrograph of a thick IMC layer at the interface of Al/steel butt joints produced under the same conditions of ∆d = 0.4 mm, P = 3.25 kW, f = 0 mm and v = 1.0 m/min.It was found that the IMC layers were composed of a large needle-like phase ("1") dispersed in the Al alloy side, a compact lath structure ("2") having a relatively smooth boundary adjacent to the steel side, and a fine serrated phase ("3") grown from the "2" layer.An elemental composition analysis from EDS was conducted on the location "1", "2" and "3", and the results are shown in Figure 8.The composition of location "1" is 77.09 at % Al and 22.91 at % Fe, which is similar to that of location "3", and the one for location "2" is 72.81 at % Al and 27.19 at % Fe.Compared to the measured composition with the Fe-Al phase diagram, the compositions of location

Phase Identification of Intermetallic Compound (IMC) Layers
Figure 8 shows a back-scattered scanning electron (BSE) micrograph of a thick IMC layer at the interface of Al/steel butt joints produced under the same conditions of ∆d = 0.4 mm, P = 3.25 kW, f = 0 mm and v = 1.0 m/min.It was found that the IMC layers were composed of a large needle-like phase ("1") dispersed in the Al alloy side, a compact lath structure ("2") having a relatively smooth Metals 2017, 7, 492 9 of 19 boundary adjacent to the steel side, and a fine serrated phase ("3") grown from the "2" layer.An elemental composition analysis from EDS was conducted on the location "1", "2" and "3", and the results are shown in Figure 8.The composition of location "1" is 77.09 at % Al and 22.91 at % Fe, which is similar to that of location "3", and the one for location "2" is 72.81 at % Al and 27.19 at % Fe.Compared to the measured composition with the Fe-Al phase diagram, the compositions of location "1" and "3" are in accordance with FeAl 3 , and the location "2" can be the Fe 2 Al 5 phase.Therefore, EDS analysis shows that the IMC layers at the interface consists of FeAl 3 and Fe 2 Al 5 phases.Concerning the formation sequence and phase growth, some references have shown that the Fe 2 Al 5 would be formed first.An indication that the Fe 2 Al 5 is the phase that is formed first is its wavy interface towards the FeAl 3 , which indicates in a binary system that it was a former interface between a liquid and a solid, while the straight interface of the Fe 2 Al 5 on the steel side indicates solid-state diffusion [35].A further indication of the prior formation of the Fe 2 Al 5 is the fact that each Fe 2 Al 5 grain is a direct neighbor of several FeAl 3 grains, thus it appears likely that the FeAl 3 nucleated at the interface of the already formed Fe 2 Al 5 [36].
Metals 2017, 7, 492 9 of 18 EDS analysis shows that the IMC layers at the interface consists of FeAl3 and Fe2Al5 phases.
Concerning the formation sequence and phase growth, some references have shown that the Fe2Al5 would be formed first.An indication that the Fe2Al5 is the phase that is formed first is its wavy interface towards the FeAl3, which indicates in a binary system that it was a former interface between a liquid and a solid, while the straight interface of the Fe2Al5 on the steel side indicates solid-state diffusion [35].A further indication of the prior formation of the Fe2Al5 is the fact that each Fe2Al5 grain is a direct neighbor of several FeAl3 grains, thus it appears likely that the FeAl3 nucleated at the interface of the already formed Fe2Al5 [36].An interface region including the Al alloy, IMC layers and steel base metal was measured using EBSD phase mapping to obtain statistically significant phase information formed at the steel/Al alloy butt joints, as shown in Figure 9.The phase type of the IMC layers was accurately determined by Kikuchi patterns and the lattice constants using EBSD phase mapping performed at a selected rectangular zone near the middle part (plotted in Figure 5) of the Al/steel butt joints.The colors in phase mapping is based on a color-coded legend, in which the red, yellow, blue and green correspond to the α-Fe, α-Al, Fe2Al5, and Fe4Al13 phases, respectively.As shown in Table 2, the Fe4Al13 phase had a monoclinic unit cell with a = 15.49Å, b = 8.08 Å, c = 12.48 Å, and β = 107.72°,and the Fe2Al5 phase had an orthorhombic crystal with lattice constants of a = 7.66 Å, b = 6.42 Å, c = 4.22 Å, and β = 90°, which is consistent with that of the previous studies [25,37].Note that the Fe4Al13 phase here was the same as the equilibrium FeAl3 phase identified by preliminary EDS analysis.Figure 8 displays EBSD phase mapping of the IMC layers at the interface for the Al/steel butt joints obtained at a laser beam offset of 0.3 mm.According to Table 2 and Figure 8, the needle-like, serrated Fe4Al13 phase and lath Fe2Al5 phases were formed at the interface, which agrees with the early results obtained by Dybkov [38] and Shi et al. [37].They have proposed that the reaction products are a solid solution based upon the Fe4Al13 and Fe2Al5 [15].Therefore, by EDS and EBSD analysis, it can be confirmed that the IMC layers at the interface were composed of Fe4Al13 and Fe2Al5 phases.An interface region including the Al alloy, IMC layers and steel base metal was measured using EBSD phase mapping to obtain statistically significant phase information formed at the steel/Al alloy butt joints, as shown in Figure 9.The phase type of the IMC layers was accurately determined by Kikuchi patterns and the lattice constants using EBSD phase mapping performed at a selected rectangular zone near the middle part (plotted in Figure 5) of the Al/steel butt joints.The colors in phase mapping is based on a color-coded legend, in which the red, yellow, blue and green correspond to the α-Fe, α-Al, Fe 2 Al 5 , and Fe 4 Al 13 phases, respectively.As shown in Table 2, the Fe 4 Al 13 phase had a monoclinic unit cell with a = 15.49Å, b = 8.08 Å, c = 12.48 Å, and β = 107.72 • , and the Fe 2 Al 5 phase had an orthorhombic crystal with lattice constants of a = 7.66 Å, b = 6.42 Å, c = 4.22 Å, and β = 90 • , which is consistent with that of the previous studies [25,37].Note that the Fe 4 Al 13 phase here was the same as the equilibrium FeAl 3 phase identified by preliminary EDS analysis.Figure 8 displays EBSD phase mapping of the IMC layers at the interface for the Al/steel butt joints obtained at a laser beam offset of 0.3 mm.According to Table 2 and Figure 8, the needle-like, serrated Fe 4 Al 13 phase and lath Fe 2 Al 5 phases were formed at the interface, which agrees with the early results obtained by Dybkov [38] and Shi et al. [37].They have proposed that the reaction products are a solid solution based upon the Fe 4 Al 13 and Fe 2 Al 5 [15].Therefore, by EDS and EBSD analysis, it can be confirmed that the IMC layers at the interface were composed of Fe 4 Al 13 and Fe 2 Al 5 phases.

Morphology and Thickness of IMC Layers
Figure 10 shows the BSE micrographs of the IMC layers at the interface of the Al/steel butt joints with different travel speeds.At a travel speed of 0.6 m/min, the IMC morphology formed at the interface exhibits a number of large needle-like FeAl3 phases and thick Fe2Al5 layers owing to the high heat input induced by the slow travel speeds.With the increase of travel speeds from 0.8 to 1.2 m/min, the quantity of large needle-like FeAl3 phases was significantly decreased and the thickness of the Fe2Al5 layers was also reduced.In particular, the large needle-like FeAl3 phases dispersed in the Al alloy completely disappeared when the travel speed was increased from 1.0 m/min to 1.2 m/min.The effect of the travel speeds on the quantity of the needle-like FeAl3 may be attributed to the decrease of the cooling rate of the interface [11].Figure 10f displays the thickness of the IMC layers as a function of travel speeds.Here, the thickness of Fe2Al5 layers is used to evaluate the IMC layer because of its regularity.The thickness of Fe2Al5 layers decreased sharply from 47.7 to 15.1 μm from 0.6 to 0.8 m/min, and then decreased slowly from 7.9 to 4.7 μm with the increase of travel speeds from 1.0 to 1.2 m/min.The decrease of Fe2Al5 layer thickness at the interface can be explained by the reduction of linear energy input with the increase of travel speeds [33].

Morphology and Thickness of IMC Layers
Figure 10 shows the BSE micrographs of the IMC layers at the interface of the Al/steel butt joints with different travel speeds.At a travel speed of 0.6 m/min, the IMC morphology formed at the interface exhibits a number of large needle-like FeAl 3 phases and thick Fe 2 Al 5 layers owing to the high heat input induced by the slow travel speeds.With the increase of travel speeds from 0.8 to 1.2 m/min, the quantity of large needle-like FeAl 3 phases was significantly decreased and the thickness of the Fe 2 Al 5 layers was also reduced.In particular, the large needle-like FeAl 3 phases dispersed in the Al alloy completely disappeared when the travel speed was increased from 1.0 m/min to 1.2 m/min.The effect of the travel speeds on the quantity of the needle-like FeAl 3 may be attributed to the decrease of the cooling rate of the interface [11].Figure 10f displays the thickness of the IMC layers as a function of travel speeds.Here, the thickness of Fe 2 Al 5 layers is used to evaluate the IMC layer because of its regularity.The thickness of Fe 2 Al 5 layers decreased sharply from 47.7 to 15.1 µm from 0.6 to 0.8 m/min, and then decreased slowly from 7.9 to 4.7 µm with the increase of travel speeds from 1.0 to 1.2 m/min.The decrease of Fe 2 Al 5 layer thickness at the interface can be explained by the reduction of linear energy input with the increase of travel speeds [33].
The IMC layers at the interface of the Al/steel butt joints with different laser beam offsets are shown in Figure 11.At relatively low laser beam offsets of 0.3-0.5 mm, it was obvious that a great number of large needle-like FeAl 3 phases and thick Fe 2 Al 5 layers were formed at the interface.With increasing laser beam offsets from 0.6 to 0.7 mm, the quantity of large needle-like FeAl 3 phases dispersed in the Al alloy significantly decreased, and only compact lath Fe 2 Al 5 layer and fine serrated FeAl 3 phases attached to Fe 2 Al 5 layer were observed at the interface.Therefore, that laser beam offset decreases greatly the quantity of large needle-like FeAl 3 phases at the interface.Concerning the effect of laser beam offsets on the IMC layer thickness, it was found that the thickness of Fe 2 Al 5 layers decreased quickly from 21.0 to 9.6 µm with the increase of laser beam offsets from 0.3 to 0.4 mm, and then decreased gradually from 7.8 to 4.1 µm as laser beam offsets increase from 0.5 to 0.7 mm, as shown in Figure 11f.The decrease of the Fe 2 Al 5 layer thickness with the increase of laser beam offsets was mainly caused by the decrease in the maximum temperature and the reaction time at the interface [30,39].With the increase of the laser offsets, the maximum temperature at the interface tends to be lower and the time for diffusion and dissolution of Fe atoms decreases, giving a thinner IMC layer.
Fe2Al5 layers was also reduced.In particular, the large needle-like FeAl3 phases dispersed in the Al alloy completely disappeared when the travel speed was increased from 1.0 m/min to 1.2 m/min.The effect of the travel speeds on the quantity of the needle-like FeAl3 may be attributed to the decrease of the cooling rate of the interface [11].Figure 10f displays the thickness of the IMC layers as a function of travel speeds.Here, the thickness of Fe2Al5 layers is used to evaluate the IMC layer because of its regularity.The thickness of Fe2Al5 layers decreased sharply from 47.7 to 15.1 μm from 0.6 to 0.8 m/min, and then decreased slowly from 7.9 to 4.7 μm with the increase of travel speeds from 1.0 to 1.2 m/min.The decrease of Fe2Al5 layer thickness at the interface can be explained by the reduction of linear energy input with the increase of travel speeds [33].The IMC layers at the interface of the Al/steel butt joints with different laser beam offsets are shown in Figure 11.At relatively low laser beam offsets of 0.3-0.5 mm, it was obvious that a great number of large needle-like FeAl3 phases and thick Fe2Al5 layers were formed at the interface.With increasing laser beam offsets from 0.6 to 0.7 mm, the quantity of large needle-like FeAl3 phases dispersed in the Al alloy significantly decreased, and only compact lath Fe2Al5 layer and fine serrated FeAl3 phases attached to Fe2Al5 layer were observed at the interface.Therefore, that laser beam offset decreases greatly the quantity of large needle-like FeAl3 phases at the interface.Concerning the effect of laser beam offsets on the IMC layer thickness, it was found that the thickness of Fe2Al5 layers decreased quickly from 21.0 to 9.6 μm with the increase of laser beam offsets from 0.3 to 0.4 mm, and then decreased gradually from 7.8 to 4.1 μm as laser beam offsets increase from 0.5 to 0.7 mm, as shown in Figure 11f.The decrease of the Fe2Al5 layer thickness with the increase of laser beam offsets was mainly caused by the decrease in the maximum temperature and the reaction time at the interface [30,39].With the increase of the laser offsets, the maximum temperature at the interface tends to be lower and the time for diffusion and dissolution of Fe atoms decreases, giving a thinner IMC layer.

Microhardness Profile of Al/Steel Butt Joints
Figure 12 shows the microhardness distribution of the Al/steel butt joints with different laser beam offsets.It was found that the average microhardness of 5083 Al alloys and Q235 steels is 74.1 and 142.3 HV, while the weld and HAZ microhardness at the steel side reach 258.4 and 185.5 HV, respectively.The higher hardness in the weld is mainly caused by the steel hardening because of the quenching effect, and the high hardness in the HAZ is related to the high heating and cooling rate experienced in the HAZ resulting in a fine microstructure [40].
The hardness at the interface of the Al/steel butt joints increased abruptly in the range from 600 to 800 HV from the laser beam offset of 0.3 to 0.7 mm, as seen in Figure 11a.The higher hardness at the interface was due to the presence of the brittle Fe2Al5 and Fe4Al13 phases [13].In addition, the

Microhardness Profile of Al/Steel Butt Joints
Figure 12 shows the microhardness distribution of the Al/steel butt joints with different laser beam offsets.It was found that the average microhardness of 5083 Al alloys and Q235 steels is 74.1 and 142.3 HV, while the weld and HAZ microhardness at the steel side reach 258.4 and 185.5 HV, respectively.The higher hardness in the weld is mainly caused by the steel hardening because of the quenching effect, and the high hardness in the HAZ is related to the high heating and cooling rate experienced in the HAZ resulting in a fine microstructure [40].
Metals 2017, 7, 492 12 of 18 interface consisted of only thin lath Fe2Al5 layer and fine serrated Fe4Al13 phases as a result of the lesser heat input.Thus, at the laser beam offset of 0.7 mm, the hardness at the interface mainly depended on Fe2Al5 layers, which have a higher hardness than Fe4Al13 layers [41].This explained why the joint with a laser beam offset of 0.7 mm had the higher hardness at the interface.

Tensile Properties of the Al/Steel Butt Joints
The UTS of the Al/steel butt joints was influenced significantly by travel speeds and laser beam offsets, as shown in Figure 13. Figure 13a shows the UTS of the Al/steel butt joints increased rapidly from 64.0 to 110.4 MPa with the increase of travel speeds from 0.6 to 1.0 m/min, and then decreased to 77.3 MPa at the travel speed of 1.2 m/min, which indicates that the UTS of the Al/steel butt joints reached its maximum at the travel speed of 1.0 m/min.It can be seen from Figure 13b that the UTS of the Al/steel butt joints was increased from 42.0 to 107.0 MPa by increasing laser beam offsets from 0.3 to 0.6 mm, while the UTS of the Al/steel butt joints decreased to 96.0 MPa as laser beam offset continuously increased to 0.7 mm.Thus, the maximum UTS of 107.0 MPa was obtained at a laser beam offset of 0.6 mm.Consequently, the UTS of the Al/steel butt joints reached its maximum in the conditions of a laser beam offset of 0.6 mm or a travel speed of 1.0 m/min.The relatively low UTS of Al/steel butt joints is primarily due to the formation of brittle IMC phases at the interface, which is the main cause of embrittlement [42].All the Al/steel butt joints failed at the IMC layers at the interface, as shown in Figure 14.During the tensile test, the crack firstly The hardness at the interface of the Al/steel butt joints increased abruptly in the range from 600 to 800 HV from the laser beam offset of 0.3 to 0.7 mm, as seen in Figure 11a.The higher hardness at the interface was due to the presence of the brittle Fe 2 Al 5 and Fe 4 Al 13 phases [13].In addition, the lowest hardness of 0.3 mm was 671.3 HV, and the highest hardness was 765.4 HV for the laser beam offset of 0.7 mm.The difference in hardness at the interface between the laser beam offsets of 0.3 mm and 0.7 mm was closely associated with the different morphology of the IMC layers generated at the interface.As mentioned above, at a laser beam offset of 0.3 mm, the IMC layers at the interface were composed of a thick lath Fe 2 Al 5 layer, large needle-like and fine serrated Fe 4 Al 13 phases, due to the excessive reactions at the interface as a result of the high heat input caused by a small laser beam offset of 0.3 mm.However, at a relatively higher laser beam offset of 0.7 mm, the IMC layers at the interface consisted of only thin lath Fe 2 Al 5 layer and fine serrated Fe 4 Al 13 phases as a result of the lesser heat input.Thus, at the laser beam offset of 0.7 mm, the hardness at the interface mainly depended on Fe 2 Al 5 layers, which have a higher hardness than Fe 4 Al 13 layers [41].This explained why the joint with a laser beam offset of 0.7 mm had the higher hardness at the interface.

Tensile Properties of the Al/Steel Butt Joints
The UTS of the Al/steel butt joints was influenced significantly by travel speeds and laser beam offsets, as shown in Figure 13. Figure 13a shows the UTS of the Al/steel butt joints increased rapidly from 64.0 to 110.4 MPa with the increase of travel speeds from 0.6 to 1.0 m/min, and then decreased to 77.3 MPa at the travel speed of 1.2 m/min, which indicates that the UTS of the Al/steel butt joints reached its maximum at the travel speed of 1.0 m/min.It can be seen from Figure 13b that the UTS of the Al/steel butt joints was increased from 42.0 to 107.0 MPa by increasing laser beam offsets from 0.3 to 0.6 mm, while the UTS of the Al/steel butt joints decreased to 96.0 MPa as laser beam offset continuously increased to 0.7 mm.Thus, the maximum UTS of 107.0 MPa was obtained at a laser beam offset of 0.6 mm.Consequently, the UTS of the Al/steel butt joints reached its maximum in the conditions of a laser beam offset of 0.6 mm or a travel speed of 1.0 m/min.
reached its maximum at the travel speed of 1.0 m/min.It can be seen from Figure 13b that the UTS of the Al/steel butt joints was increased from 42.0 to 107.0 MPa by increasing laser beam offsets from 0.3 to 0.6 mm, while the UTS of the Al/steel butt joints decreased to 96.0 MPa as laser beam offset continuously increased to 0.7 mm.Thus, the maximum UTS of 107.0 MPa was obtained at a laser beam offset of 0.6 mm.Consequently, the UTS of the Al/steel butt joints reached its maximum in the conditions of a laser beam offset of 0.6 mm or a travel speed of 1.0 m/min.The relatively low UTS of Al/steel butt joints is primarily due to the formation of brittle IMC phases at the interface, which is the main cause of embrittlement [42].All the Al/steel butt joints failed at the IMC layers at the interface, as shown in Figure 14.During the tensile test, the crack firstly The relatively low UTS of Al/steel butt joints is primarily due to the formation of brittle IMC phases at the interface, which is the main cause of embrittlement [42].All the Al/steel butt joints failed at the IMC layers at the interface, as shown in Figure 14.During the tensile test, the crack firstly initiated at the top part ("I" zone in Figure 14a) or the bottom part ("III" zone in Figure 14c) of the IMC layers.Finally, it propagated into the middle part ("II" zone in Figure 14b) of the IMC layer, which led to the failure of the Al/steel butt joint.The fracture occurring at the IMC layer was related to the brittleness and morphology of the IMC layers at the interface.At the top part of the IMC layer, a thick Fe 2 Al 5 layer and a number of the large needle-like Fe 4 Al 13 phases were observed owing to the high heat input.For the bottom part of the IMC layer, some welding undercuts were observed at the root of the joint in the Al alloy side close to the interface, which was more inclined to initiate microcracks at high loads during the tensile test.Therefore, it can be supposed that the crack starts from the bottom or top part, and propagates into the middle part of the IMC layer at the interface.
Metals 2017, 7, 492 13 of 18 initiated at the top part ("I" zone in Figure 14a) or the bottom part ("III" zone in Figure 14c) of the IMC layers.Finally, it propagated into the middle part ("II" zone in Figure 14b) of the IMC layer, which led to the failure of the Al/steel butt joint.The fracture occurring at the IMC layer was related to the brittleness and morphology of the IMC layers at the interface.At the top part of the IMC layer, a thick Fe2Al5 layer and a number of the large needle-like Fe4Al13 phases were observed owing to the high heat input.For the bottom part of the IMC layer, some welding undercuts were observed at the root of the joint in the Al alloy side close to the interface, which was more inclined to initiate microcracks at high loads during the tensile test.Therefore, it can be supposed that the crack starts from the bottom or top part, and propagates into the middle part of the IMC layer at the interface.Figure 15 shows typical results from SEM investigations of fracture surfaces at the steel side and Al alloy side of the position "II" in the Al/steel butt joints, as this region is the final failure location of the joint during tensile tests.The fracture surface at the Al alloy side can be seen to be mostly flat, and the microcrack grows through the grains.Also, there was evidence of some broken Fe4Al13 phases and the needle-like Fe2Al5, as shown in Figure 15a, which depicts brittle transgranular fracture.The factographic surface in the steel side shown in Figure 15b presented the typical cleavage fracture mode, with river pattern strips of particular orientation on the fracture surface, which occupies the largest fraction area of the sample surface exhibiting interfacial failure.Even though the failure of all the samples was always located at the brazing interface during the tensile test, the specimens endured some plastic deformation, as shown in Figure 15c; some regions of the factographic surface in the steel side appear to show some dimples accompanied by some local plasticity, and the crack grew along the grain boundary, showing some intergranular fracture characteristics.Although these fractures were brittle, the latter had a certain resistance to the crack propagation of the IMC layers at the steel side, which enhanced its mechanical resistance at the steel side.As a result, the IMC layer region close to the Al base material was the most brittle region having the weakest bonding strength at the interface.Figure 15 shows typical results from SEM investigations of fracture surfaces at the steel side and Al alloy side of the position "II" in the Al/steel butt joints, as this region is the final failure location of the joint during tensile tests.The fracture surface at the Al alloy side can be seen to be mostly flat, and the microcrack grows through the grains.Also, there was evidence of some broken Fe 4 Al 13 phases and the needle-like Fe 2 Al 5 , as shown in Figure 15a, which depicts brittle transgranular fracture.The factographic surface in the steel side shown in Figure 15b presented the typical cleavage fracture mode, with river pattern strips of particular orientation on the fracture surface, which occupies the largest fraction area of the sample surface exhibiting interfacial failure.Even though the failure of all the samples was always located at the brazing interface during the tensile test, the specimens endured some plastic deformation, as shown in Figure 15c; some regions of the factographic surface in the steel side appear to show some dimples accompanied by some local plasticity, and the crack grew along the grain boundary, showing some intergranular fracture characteristics.Although these fractures were brittle, the latter had a certain resistance to the crack propagation of the IMC layers at the steel side, which enhanced its mechanical resistance at the steel side.As a result, the IMC layer region close to the Al base material was the most brittle region having the weakest bonding strength at the interface.
and the microcrack grows through the grains.Also, there was evidence of some broken Fe4Al13 phases and the needle-like Fe2Al5, as shown in Figure 15a, which depicts brittle transgranular fracture.The factographic surface in the steel side shown in Figure 15b presented the typical cleavage fracture mode, with river pattern strips of particular orientation on the fracture surface, which occupies the largest fraction area of the sample surface exhibiting interfacial failure.Even though the failure of all the samples was always located at the brazing interface during the tensile test, the specimens endured some plastic deformation, as shown in Figure 15c; some regions of the factographic surface in the steel side appear to show some dimples accompanied by some local plasticity, and the crack grew along the grain boundary, showing some intergranular fracture characteristics.Although these fractures were brittle, the latter had a certain resistance to the crack propagation of the IMC layers at the steel side, which enhanced its mechanical resistance at the steel side.As a result, the IMC layer region close to the Al base material was the most brittle region having the weakest bonding strength at the interface.

Effect of IMC Layer Thickness on Ultimate Tensile Strength (UTS)
The relationship between the IMC layer thickness and the UTS of Al/steel joints has been proposed by Borrisutthekul [12], showing that the UTS was increased with decreasing thickness of the reaction layer.In the present study, increasing travel speeds from 0.6 to 1.0 m/min improved the UTS of Al/steel joints, as shown in Figure 13a, due to the decreased thickness of the IMC layer continuously from 47.7 to 7.9 μm (Figure 10f).In Figure 13b, with the increase of laser beam offsets from 0.3 to 0.6 mm, the UTS of Al/steel joints was increased as the IMC layer thickness decreased from 21.0 to 5.6 μm (Figure 11f).In these cases, the increased tendency of the UTS with decreasing IMC layer thickness was clearly observed, which is in good agreement with the results reported by Borrisutthekul [12].However, with continuously increasing travel speeds to 1.2 m/min or increasing laser beam offset to 0.7 mm, the UTS of the Al/steel joints was not increased continuously with further reductions in IMC layer thickness.This indicates that the UTS of Al/steel joints does not always increase with a decrease in IMC layer thickness [3].Gao et al. [17] has proposed that there is an optimal range of IMC layer thickness for improving the UTS of Al/steel joints, because an IMC layer that is too thin or too thick results in low UTS.The reasons are due to the fact that an IMC layer that

Effect of IMC Layer Thickness on Ultimate Tensile Strength (UTS)
The relationship between the IMC layer thickness and the UTS of Al/steel joints has been proposed by Borrisutthekul [12], showing that the UTS was increased with decreasing thickness of the reaction layer.In the present study, increasing travel speeds from 0.6 to 1.0 m/min improved the UTS of Al/steel joints, as shown in Figure 13a, due to the decreased thickness of the IMC layer continuously from 47.7 to 7.9 µm (Figure 10f).In Figure 13b, with the increase of laser beam offsets from 0.3 to 0.6 mm, the UTS of Al/steel joints was increased as the IMC layer thickness decreased from 21.0 to 5.6 µm (Figure 11f).In these cases, the increased tendency of the UTS with decreasing IMC layer thickness was clearly observed, which is in good agreement with the results reported by Borrisutthekul [12].
However, with continuously increasing travel speeds to 1.2 m/min or increasing laser beam offset to 0.7 mm, the UTS of the Al/steel joints was not increased continuously with further reductions in IMC layer thickness.This indicates that the UTS of Al/steel joints does not always increase with a decrease in IMC layer thickness [3].Gao et al. [17] has proposed that there is an optimal range of IMC layer thickness for improving the UTS of Al/steel joints, because an IMC layer that is too thin or too thick results in low UTS.The reasons are due to the fact that an IMC layer that is too thin means an insufficient interface reaction leads to the lack of fusion, and the IMC layer that is too thick resulting from an excessive growth of the IMC phases increased the interface brittleness [17].Therefore, it is concluded that there is a relationship between optimum IMC layer thickness and the UTS of Al/steel butt joints.However, this is not the sole condition.When comparing the thickness of the IMC layer of the joints welded with a welding speed of 0.6 m/min and those welded with a laser offset of 0.5 mm, it is that the thickness for a welding speed of 0.6 m/min is about ~48 µm, and the joint has a low UTS of 64.0 MPa.For a laser beam offset of 0.5 mm, the IMC layers have thin thickness of 7.8 µm, whereas the joints also exhibit low UTS of 66.5 MPa.Therefore, for thickness of more than 10 µm, the increased tendency of UTS with decreasing IMC layer thickness is clearly observed, which is in good agreement with the results reported by Borrisutthekul [12].However, with continuously decreasing thickness to less than 10 µm, the UTS of Al/steel joints not only largely depends on the IMC layer thickness, but also on the IMC phase type and morphology at the interface.

Fracture Behavior of IMC Layers
It is well accepted that the UTS of Al/steel joints largely depends on the IMC layers (thickness and phase type) at the interface [43].However, regarding the fracture behavior of IMC layers during tensile processing, there are some contradictory results.Springer et al. pointed out that the crack propagated in the brittle IMC phases and cut through the remainder of the steel at the irregular, wavy steel/Fe 2 Al 5 interface [44].However, Chen et al. claimed that the Fe 2 Al 5 layer was the primary and most brittle phase at the interface [43].These observations are evidence that the fracture of IMC layers associated with tensile testing are not well understood.Thus, in the present study, an EBSD phase mapping of the fractured surfaces of Al/steel butt joints under three laser beam offsets was conducted in order to understand the fracture behavior of the IMC layers at the interface.At a laser beam offset of 0.3 mm, it is evident that the crack travels through Fe 2 Al 5 layers and the fracture path is smooth, as shown in Figure 16a, while the Fe 4 Al 13 /Fe 2 Al 5 and Fe 2 Al 5 /steel phase interface bond well.This indicates that the Fe 2 Al 5 layer is the weakest in terms of bonding strength of the IMC layers at the interface, and a transgranular fracture that occurred at the Fe 2 Al 5 layers is evident due to the higher hardness of the Fe 2 Al 5 layers [41] (Figure 12). Figure 16b shows a brittle transgranular fracture still occurring at Fe 2 Al 5 layer at a laser beam offset of 0.5 mm, but some second cracks can be observed in the Al base material, showing that some deformation has happened at the interface.This explains why the Al/steel butt joint at a laser beam offset of 0.5 mm has relatively high UTS compared to that of a laser beam offset of 0.3 mm.When the laser beam offset is 0.7 mm (Figure 16c), it clearly reveals that the crack travels along the boundary of the Fe 2 Al 5 grains and the fracture path shows the obvious irregularity, which demonstrates evidence that an intergranular fracture occurred at the Fe 2 Al 5 layers.Thus, the intergranular fracture occurring at the Fe 2 Al 5 layer is thought to be attributable to the relatively high UTS of the Al/steel butt joints with a laser beam offset of 0.7 mm.Consequently, the present study of EBSD phase mapping shows that the Fe 2 Al 5 layer is the most brittle region having the weakest bonding strength, which is entirely consistent with a previous study reported by Chen et al. [43].Moreover, an intergranular fracture that occurred at the Fe 2 Al 5 layers led to a relatively high UTS of the Al/steel butt joints.
Thus, the intergranular fracture occurring at the Fe2Al5 layer is thought to be attributable to the relatively high UTS of the Al/steel butt joints with a laser beam offset of 0.7 mm.Consequently, the present study of EBSD phase mapping shows that the Fe2Al5 layer is the most brittle region having the weakest bonding strength, which is entirely consistent with a previous study reported by Chen et al. [43].Moreover, an intergranular fracture that occurred at the Fe2Al5 layers led to a relatively high UTS of the Al/steel butt joints.

Conclusions
Based on the above results and discussion, butt joints of 6.0 mm thick 5083 Al alloys to Q235 steel plates have been achieved by using autogenous laser keyhole welding.The following conclusions can be drawn from this work: (1) Varying travel speeds and laser beam offsets have a more pronounced effect on the area of HAZ S than on weld width.By increasing the laser beam offsets and welding speed, the S in the Al alloy side is decreased continuously.However, travel speed has a significant effect on W 1 , but has less influence on W 2 , whereas varying laser beam offset influences W 1 and W 2 a little.(2) The thickness of Fe 2 Al 5 layers decreases from 47.7 to 4.7 µm with increasing travel speeds from 0.6 to 1.2 m/min.As laser beam offset increases from 0.3 to 0.7 mm, the thickness of Fe 2 Al 5 layers decreases from 21.0 to 4.1µm.Thus, increasing travel speeds or laser beam offsets obviously decreases the quantity of long needle-like FeAl 3 phases and the thickness of the Fe 2 Al 5 layer.(3) The UTS of Al/steel butt joints is influenced significantly by the tested travel speeds and laser beam offsets.The UTS reaches its maximum under the conditions of a travel speed of 1.0 m/min or a laser beam offset of 0.6 mm.There should be a matching relationship between the IMC layer thickness and UTS of Al/steel butt joints.(4) EBSD phase mapping proves that the IMC layers consist of Fe 2 Al 5 phases and Fe 4 Al 13 phases.
Increasing laser beam offsets from 0.3 to 0.7 mm significantly decreases the quantity of Fe 4 Al 13 phases and the thickness of Fe 2 Al 5 layers at the interface of Al/steel butt joints.

Figure 1 .
Figure 1.Schematic illustration of laser keyhole welding of Al/steel butt joints: (a) welding setup; (b) principle of laser keyhole welding.

Figure 1 .
Figure 1.Schematic illustration of laser keyhole welding of Al/steel butt joints: (a) welding setup; (b) principle of laser keyhole welding.

Figure 2 .
Figure 2. Diagram of dimensions of the weld cross section in the Al/steel butt joints.

Figure 2 .
Figure 2. Diagram of dimensions of the weld cross section in the Al/steel butt joints.

Figure 3 .
Figure 3. Weld appearance of Al/steel butt joints produced by autogenous laser keyhole welding: (a) top surface; (b) bottom surface.

Figure 5
Figure5shows the cross sections of Al/steel butt joints obtained at different laser beam offsets under the condition of v = 1.0 m/min, P = 3.25 kW and f = 0 mm.All the welds possess a similar "nail" type penetration, meaning that the laser beam offsets have no obvious influence on the weld type of

Figure 3 .
Figure 3. Weld appearance of Al/steel butt joints produced by autogenous laser keyhole welding: (a) top surface; (b) bottom surface.

Figure 3 .
Figure 3. Weld appearance of Al/steel butt joints produced by autogenous laser keyhole welding: (a) top surface; (b) bottom surface.

Figure 5
Figure5shows the cross sections of Al/steel butt joints obtained at different laser beam offsets under the condition of v = 1.0 m/min, P = 3.25 kW and f = 0 mm.All the welds possess a similar "nail" type penetration, meaning that the laser beam offsets have no obvious influence on the weld type of

Figure 6 .
Figure 6.Relationship between process parameters and weld width or the heat-affected zone (HAZ) of the Al side: (a) travel speeds; (b) laser beam offsets.Blue color corresponds to HAZ; Black and red color correspond to top width (W1) and bottom width (W2) of the welds.

Figure 6 .
Figure 6.Relationship between process parameters and weld width or the heat-affected zone (HAZ) of the Al side: (a) travel speeds; (b) laser beam offsets.Blue color corresponds to HAZ; Black and red color correspond to top width (W 1 ) and bottom width (W 2 ) of the welds.

Metals 2017, 7 , 492 8 of 18 Figure 7 .
Figure 7. Cross-section microstructure of laser welding-brazing joint of aluminum and steel: (a) cross section of joint; (b) enlarged zone A in (a); (c) enlarged zone marked by white dashes in (b); (d) enlarged zone B in (a); (e) enlarged zone marked by red dashes in (b); (f) enlarged zone C in (a); (g) enlarged zone marked by white dashes in (f).

Figure 7 .
Figure 7. Cross-section microstructure of laser welding-brazing joint of aluminum and steel: (a) cross section of joint; (b) enlarged zone A in (a); (c) enlarged zone marked by white dashes in (b); (d) enlarged zone B in (a); (e) enlarged zone marked by red dashes in (b); (f) enlarged zone C in (a); (g) enlarged zone marked by white dashes in (f).

Figure 8 .
Figure 8. Back-scattered scanning electron (BSE) micrograph and energy dispersive spectrometry (EDS) analysis result of intermetallic compound (IMC) layers at the interface.

Figure 8 .
Figure 8. Back-scattered scanning electron (BSE) micrograph and energy dispersive spectrometry (EDS) analysis result of intermetallic compound (IMC) layers at the interface.

Figure 9 .
Figure 9. EBSD phase mappings of the interface of Al/steel joints at a laser beam offset of 0.3 mm.

Figure 10 .
Figure 10.IMC layer microstructures at the interface of the Al/steel butt joints with different travel speeds: (a) 0.6 m/min; (b) 0.8 m/min; (c) 1.0 m/min; (d) 1.1 m/min; (e) 1.2 m/min; (f) thickness of the IMC layers (contained in the second panel).

Figure
Figure EBSD phase mappings of the interface of Al/steel joints at a laser beam offset of 0.3 mm.

Figure 10 .
Figure 10.IMC layer microstructures at the interface of the Al/steel butt joints with different travel speeds: (a) 0.6 m/min; (b) 0.8 m/min; (c) 1.0 m/min; (d) 1.1 m/min; (e) 1.2 m/min; (f) thickness of the IMC layers (contained in the second panel).

Figure 10 .
Figure 10.IMC layer microstructures at the interface of the Al/steel butt joints with different travel speeds: (a) 0.6 m/min; (b) 0.8 m/min; (c) 1.0 m/min; (d) 1.1 m/min; (e) 1.2 m/min; (f) thickness of the IMC layers (contained in the second panel).

Figure 12 .
Figure 12.Microhardness distribution in the Al/steel butt joints.

Figure 13 .
Figure 13.Effect of laser-welding parameters on the ultimate tensile strength (UTS) of the Al/steel butt joints: (a) travel speeds; (b) laser beam offsets.

Figure 12 .
Figure 12.Microhardness distribution in the Al/steel butt joints.

Figure 13 .
Figure 13.Effect of laser-welding parameters on the ultimate tensile strength (UTS) of the Al/steel butt joints: (a) travel speeds; (b) laser beam offsets.

Figure 13 .
Figure 13.Effect of laser-welding parameters on the ultimate tensile strength (UTS) of the Al/steel butt joints: (a) travel speeds; (b) laser beam offsets.

Figure 14 .
Figure 14.Fracture path of the interface at the steel/Al alloy butt joint: (a) top position (b) middle position (c) bottom position.

Figure 14 .
Figure 14.Fracture path of the interface at the steel/Al alloy butt joint: (a) top position (b) middle position (c) bottom position.

Figure 15 .
Figure 15.Scanning electronic microscope (SEM) micrographs of the middle position at the interface of the steel/Al alloy butt joint: (a) Al alloy side; (b) steel side; (c) steel side showing some dimples.

Figure 15 .
Figure 15.Scanning electronic microscope (SEM) micrographs of the middle position at the interface of the steel/Al alloy butt joint: (a) Al alloy side; (b) steel side; (c) steel side showing some dimples.

Figure 16 .
Figure 16.EBSD phase mapping of the failed tensile samples for the Al/steel joint with different laser beam offsets: (a) 0.3 mm; (b) 0.5 mm; (c) 0.7 mm.Figure 16.EBSD phase mapping of the failed tensile samples for the Al/steel joint with different laser beam offsets: (a) 0.3 mm; (b) 0.5 mm; (c) 0.7 mm.

Figure 16 .
Figure 16.EBSD phase mapping of the failed tensile samples for the Al/steel joint with different laser beam offsets: (a) 0.3 mm; (b) 0.5 mm; (c) 0.7 mm.Figure 16.EBSD phase mapping of the failed tensile samples for the Al/steel joint with different laser beam offsets: (a) 0.3 mm; (b) 0.5 mm; (c) 0.7 mm.