Special Issue "Characterization of Welded Joints"

A special issue of Metals (ISSN 2075-4701).

Deadline for manuscript submissions: 19 July 2019

Special Issue Editor

Guest Editor
Prof. Dr. Francisco J. G. Silva

ISEP–School of Engineering, Polytechnic Institute of Porto, 4249-015 Porto, Portugal
Website | E-Mail
Interests: manufacturing processes; material properties; microhardness; microstructures; hard coatings; wear; friction

Special Issue Information

Dear Colleagues,

Welding remains as one of the most important manufacturing processes in the metalworking industry. Its critical importance is revealed in many applications, from small devices such as pacemakers to huge metallic structures. The development of new metallic materials is an important challenge for welding, because it presents new difficulties that need to be overcome. Furthermore, welding processes such as friction stir welding, laser, and electron beam have significant potential for investigation. Thus, this Special Issue intends to disseminate high-quality research carried out in the area of the welding processes, namely, in the characterization of welded joints considering different processes and challenging alloys, taking advantage of welding parameters’ regulation and the use of different filler metals, thus improving by the properties of welded joints.

Prof. Francisco J. G. Silva
Guest Editor

Manuscript Submission Information

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Keywords

  • advanced welding processes
  • welding-parameters optimization
  • heat-affected zone
  • weld nugget
  • filler metals
  • laser welding
  • electron beam welding
  • friction stir welding
  • microstructure
  • hardness

Published Papers (8 papers)

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Research

Open AccessArticle
Interface Behavior and Impact Properties of Dissimilar Al/Steel Keyhole-Free FSSW Joints
Metals 2019, 9(6), 691; https://doi.org/10.3390/met9060691
Received: 28 April 2019 / Revised: 11 June 2019 / Accepted: 13 June 2019 / Published: 18 June 2019
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Abstract
This work systematically investigates the interface behavior and impact properties of the keyhole-free friction stir spot welding (FSSW) of a dissimilar metal AA6082-T4 Al alloy and DP600 galvanized steel. The keyhole is eliminated by pin retraction technology. The welding process is in accordance [...] Read more.
This work systematically investigates the interface behavior and impact properties of the keyhole-free friction stir spot welding (FSSW) of a dissimilar metal AA6082-T4 Al alloy and DP600 galvanized steel. The keyhole is eliminated by pin retraction technology. The welding process is in accordance with the welding temperature curve and the maximum temperature of the periphery of the shoulder, measured at about 500 °C. The transition layers were formed at the interface, in which the Al, Fe, and Zn elements form an inhomogeneous diffusion. A cloud cluster-like mechanical mixing of the Al and steel components is formed in the stirring zone. The impact toughness of the specimen with a welding parameter of 1000 rpm is the best. To a certain extent, the factors affecting the impact energy are not the maximum impact load but the maximum impact deformation. The maximum impact deformation directly reflects the post-crack propagation energy, which significantly affects its impact toughness. In addition, the impact fracture showed a mixed ductile and brittle fracture mode with a brittle–ductile transition zone. Most of the impact energy was absorbed by the ductile fracture. Full article
(This article belongs to the Special Issue Characterization of Welded Joints)
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Graphical abstract

Open AccessArticle
Microstructure Evolution and Mechanical Properties of Titanium/Alumina Brazed Joints for Medical Implants
Metals 2019, 9(6), 644; https://doi.org/10.3390/met9060644
Received: 7 May 2019 / Revised: 31 May 2019 / Accepted: 31 May 2019 / Published: 3 June 2019
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Abstract
Medical titanium and alumina (Al2O3) bioceramic are widely utilized as biomaterials. A reliable brazed joint of titanium and alumina was successfully obtained using biocompatible Au foil for implantable devices in the present study. The interfacial microstructure and reaction products [...] Read more.
Medical titanium and alumina (Al2O3) bioceramic are widely utilized as biomaterials. A reliable brazed joint of titanium and alumina was successfully obtained using biocompatible Au foil for implantable devices in the present study. The interfacial microstructure and reaction products of titanium/Au/Al2O3 joints brazed under different conditions were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). In this study, the typical interfacial microstructure of the titanium/Au/Al2O3 joint was titanium/Ti3Au layer/TiAu layer/TiAu2 layer/TiAu4 layer/Au + granular TiAu4 layer/TiOx phase/Al2O3 ceramic. With increasing brazing temperature or holding time, the thicknesses of Ti3Au + TiAu + TiAu2 layers adjacent to the titanium substrate increased gradually. Shear tests indicated that the joint brazed at 1115 °C for 3 min exhibited the highest shear strength of 39.2 MPa. Typical fracture analysis displayed that the crack started at the Al2O3 ceramic and propagated along the interface of TiAu2 and TiAu4 reaction layers. Full article
(This article belongs to the Special Issue Characterization of Welded Joints)
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Open AccessArticle
Numerical Simulation of Material Flow and Analysis of Welding Characteristics in Friction Stir Welding Process
Metals 2019, 9(6), 621; https://doi.org/10.3390/met9060621
Received: 5 May 2019 / Revised: 24 May 2019 / Accepted: 25 May 2019 / Published: 28 May 2019
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Abstract
Friction stir welding (FSW) material flow has an important influence on weld formation. The finite element model of the FSW process was established. The axial force and the spindle torque of the welding process were collected through experiments. The feasibility of the finite [...] Read more.
Friction stir welding (FSW) material flow has an important influence on weld formation. The finite element model of the FSW process was established. The axial force and the spindle torque of the welding process were collected through experiments. The feasibility of the finite element model was verified by a data comparison. The temperature field of the welding process was analyzed hierarchically. It was found that the temperature on the advancing side is about 20 °C higher than that on the retreating side near the welding seam, but that the temperature difference between the two sides of the middle and lower layers was decreased. The particle tracking technique was used to study the material flow law in different areas of the weld seam. The results showed that part of the material inside the tool pin was squeezed to the bottom of the workpiece. The material on the upper surface tends to move downward under the influence of the shoulder extrusion, while the material on the lower part moves spirally upward under the influence of the tool pin. The material flow amount of the advancing side is higher than that of the retreating side. The law of material flow reveals the possible causes of the welding defects. It was found that the abnormal flow of materials at a low rotation speed and high welding speed is prone to holes and crack defects. The forming reasons and material flow differences in different regions are studied through the microstructure of the joint cross section. The feasibility of a finite element modeling and simulation analysis is further verified. Full article
(This article belongs to the Special Issue Characterization of Welded Joints)
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Open AccessArticle
Interface Characterization of Ultrasonic Spot-Welded Mg Alloy Interlayered with Cu Coating
Metals 2019, 9(5), 532; https://doi.org/10.3390/met9050532
Received: 8 April 2019 / Revised: 30 April 2019 / Accepted: 5 May 2019 / Published: 8 May 2019
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Abstract
The effect of Cu coating metallic interlayer on the weldability, joint strength, and interfacial microstructure during high-power ultrasonic spot welding (HP-USW) of AZ31B Mg alloy has been studied. Interlayered samples exhibited good weldability and they resulted in strong sound joints with nearly the [...] Read more.
The effect of Cu coating metallic interlayer on the weldability, joint strength, and interfacial microstructure during high-power ultrasonic spot welding (HP-USW) of AZ31B Mg alloy has been studied. Interlayered samples exhibited good weldability and they resulted in strong sound joints with nearly the same strength as joints without interlayer, with the distinction of lower energy being required. The Cu interlayer affected the thermal and vibrational properties of the interface, as the maximum interface temperature decreased and approached better uniformity across the weld nugget. The base metal grain structure changed to equiaxed larger grains after ultrasonic welding and a chain of parent metal small grains were observed around the interface. A binary intermetallic compound product of Mg-Cu, which was rich in Mg, has been found around the interface that was diffused toward base metal. According to the electron probe micro-analyzer (EPMA) results, alongside temperature measurements and hardness data, the formation of Mg2Cu is suggested in this region. At the interface centerline, a narrow region was identified that was composed of Mg, Cu, and Al. Complementary transmission electron microscopy analysis estimated that Al-containing reaction product is a ternary alloy of the MgCuxAly type. The dispersion of fine grain intermetallic compounds as discrete particles inside Mg substrate in both interfacial regions formed a composite like structure that could participate in joint strengthening. Full article
(This article belongs to the Special Issue Characterization of Welded Joints)
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Open AccessArticle
Effects of a Post-Weld Heat Treatment on the Mechanical Properties and Microstructure of a Friction-Stir-Welded Beryllium-Copper Alloy
Metals 2019, 9(4), 461; https://doi.org/10.3390/met9040461
Received: 11 March 2019 / Revised: 16 April 2019 / Accepted: 17 April 2019 / Published: 19 April 2019
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Abstract
This paper investigated the microstructure and mechanical properties of a friction-stir-welded beryllium-copper alloy, which is difficult to weld with conventional fusion welding processes. Friction stir welding (FSW) was successfully conducted with a tungsten-carbide (WC) tool. Sound joints without defects were obtained with a [...] Read more.
This paper investigated the microstructure and mechanical properties of a friction-stir-welded beryllium-copper alloy, which is difficult to weld with conventional fusion welding processes. Friction stir welding (FSW) was successfully conducted with a tungsten-carbide (WC) tool. Sound joints without defects were obtained with a tool rotational speed of 700 RPM and tool travel speed of 60 mm/min. A post-weld heat treatment (PWHT) of the FSW joints was performed to analyze the evolution of the microstructure at 315 °C for a half, one, two, three, four, five and eight hours, respectively. The microstructures of the joints were observed using an optical microscope (OM), a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Observed softening of microstructure is suggested to be due to the dissolution of the strengthening precipitates during the FSW process, whereas the strength of the joints was recovered via the formation of the CuBe (γ′) phase during the post-weld heat treatment. However, the strength was decreased upon an excessive post-weld heat treatment exceeding three hours. It is considered that the formation of the γ phase and the coarse γ′ phase contributed to the reduction in the strength. Full article
(This article belongs to the Special Issue Characterization of Welded Joints)
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Open AccessArticle
Effects of Cerium on Weld Solidification Crack Sensitivity of 441 Ferritic Stainless Steel
Metals 2019, 9(3), 372; https://doi.org/10.3390/met9030372
Received: 26 February 2019 / Revised: 9 March 2019 / Accepted: 13 March 2019 / Published: 22 March 2019
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Abstract
The addition of rare earth element Ce in ferritic stainless steel can improve the high temperature performance to meet the service requirements of automobile exhaust systems at high temperatures. Automobile exhaust systems are generally applied as welded pipes, so it is necessary to [...] Read more.
The addition of rare earth element Ce in ferritic stainless steel can improve the high temperature performance to meet the service requirements of automobile exhaust systems at high temperatures. Automobile exhaust systems are generally applied as welded pipes, so it is necessary to study the effect of Ce on the weldability of ferritic stainless steel. In this study, the Trans-varestraint test method was used to test the solidification crack sensitivities of 441 and 441Ce ferritic stainless steel. The 441Ce steel, which has added Ce, showed poor resistance to weld solidification cracking. Using Thermo-Calc software, Ce was observed to expand the solidification temperature range of 441 ferritic stainless steel, increase the time for solid–liquid coexistence during solidification, and increase the sensitivity of solidification cracking. Further, from scanning electron microscopy and energy dispersive spectrometer analysis, the addition of Ce was found to reduce high temperature precipitation (Ti,Nb)(C,N), reduce or even eliminate the “pinning” effect during solidification, and increase solidification crack sensitivity of 441 ferritic stainless steel. Full article
(This article belongs to the Special Issue Characterization of Welded Joints)
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Open AccessArticle
Effect of Forced Air Cooling on the Microstructures, Tensile Strength, and Hardness Distribution of Dissimilar Friction Stir Welded AA5A06-AA6061 Joints
Metals 2019, 9(3), 304; https://doi.org/10.3390/met9030304
Received: 29 January 2019 / Revised: 22 February 2019 / Accepted: 4 March 2019 / Published: 7 March 2019
Cited by 3 | PDF Full-text (9397 KB) | HTML Full-text | XML Full-text
Abstract
Friction stir welding (FSW) is a promising welding method for welding dissimilar materials without using welding flux. In the present work, 5A06-H112 and 6061-T651 aluminium alloys were successfully welded by friction stir welding with forced air cooling (FAC) and natural cooling (NC). Nanoindentation [...] Read more.
Friction stir welding (FSW) is a promising welding method for welding dissimilar materials without using welding flux. In the present work, 5A06-H112 and 6061-T651 aluminium alloys were successfully welded by friction stir welding with forced air cooling (FAC) and natural cooling (NC). Nanoindentation tests and microstructure characterisations revealed that forced air cooling, which can accelerate the cooling process and suppress the coarsening of grains and the dissolution of precipitate phases, contributes to strengthening and narrowing the weakest area of the joint. The tensile strength of joints with FAC were commonly improved by 10% compared to those with NC. Scanning electron microscopy (SEM) images of the fracture surface elucidated that FSW with FAC tended to increase the number and reduce the size of the dimples. These results demonstrated the advantages of FSW with FAC in welding heat-sensitive materials and provide fresh insight into welding industries. Full article
(This article belongs to the Special Issue Characterization of Welded Joints)
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Open AccessArticle
Microstructure and Properties of Spot Welded Joints of Hot-Stamped Ultra-High Strength Steel Used for Automotive Body Structures
Metals 2019, 9(3), 285; https://doi.org/10.3390/met9030285
Received: 1 February 2019 / Revised: 24 February 2019 / Accepted: 26 February 2019 / Published: 2 March 2019
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Abstract
Hot-stamped ultra-high strength steels have been widely used in automobile structural parts. Considering the high splash tendency in resistance spot welding due to their extremely high hardness, in this work, microstructural characteristics and mechanical performance of the resistance spot welded ultra-high strength steels [...] Read more.
Hot-stamped ultra-high strength steels have been widely used in automobile structural parts. Considering the high splash tendency in resistance spot welding due to their extremely high hardness, in this work, microstructural characteristics and mechanical performance of the resistance spot welded ultra-high strength steels are investigated. The results indicate that the interface between the nugget and heat-affected zone (HAZ) is the weakest zone where fractures initiate. In tensile shearing tests, a qualified spot welding joint failed with a button-shaped fracture. Welding defects would significantly decrease the load-carrying capacity and lead to interfacial fracture, except for a button-shaped fracture. In spot welding, it was found that a specific mid-frequency alternating current (AC) input mode, in which a 6 ms cooling cycle was inserted between every two neighboring current pulses, can avoid the splash in the spot welding of hot-stamped hardened steels. Full article
(This article belongs to the Special Issue Characterization of Welded Joints)
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