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Article

A Test for Susceptibility to Solidification Cracking and Liquation Cracking in Additive Manufacturing

by
Soumyadeep Dasgupta
1,
Dan Thoma
2 and
Sindo Kou
2,*
1
Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA
2
Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706, USA
*
Author to whom correspondence should be addressed.
Metals 2025, 15(10), 1147; https://doi.org/10.3390/met15101147
Submission received: 3 September 2025 / Revised: 2 October 2025 / Accepted: 7 October 2025 / Published: 16 October 2025
(This article belongs to the Special Issue Editorial Board Members’ Collection Series: “Laser Welding and Additive Manufacturing”)
Browse Figures
Versions Notes

Abstract

Solidification cracking and liquation cracking have been reported frequently in additive manufacturing (AM) as well as welding. In the vast majority of weldability tests, a single-pass, single-layer weld is tested, though multiple-pass, multiple-layer welding is common in welding practice. In AM, evaluating the cracking susceptibility based on the total number or length of cracks per unit volume requires repeated cutting and polishing of a built object, and the cracks are often too small to open easily for fracture-surface examination. The present study identified an existing weldability test and modified it to serve as a cracking susceptibility test for AM. A single-pass, single-layer deposit of metal powder was made along a slender specimen that was pulled like in tensile testing but with acceleration. Cracks were visible on the deposit surface and opened easily for examination. The critical pulling speed, i.e., the minimum pulling speed required to cause cracking, was determined as an index for the cracking susceptibility. The lower the critical pulling speed is, the higher the cracking susceptibility. As a result, 6061 Al showed solidification cracking, and 7075 Al showed liquation cracking, consistent with their high susceptibility to such cracking.

1. Introduction

Liquid metals tend to shrink upon solidification, called solidification shrinkage (e.g., 4% for Al alloys) [1], and solid metals tend to shrink upon cooling (i.e., thermal contraction). Free contraction of an alloy can be obstructed during solidification and cooling, e.g., by the walls of the casting mold and the solid materials connected to the weld being made. Consequently, tension can be induced, and cracking can occur along a thin grain-boundary liquid that exists during the terminal stage of solidification of an alloy. The resultant intergranular cracking is called hot tearing in casting [1] and solidification cracking in both welding and additive manufacturing (AM) [2]. In any case, the fracture surface is dendritic because cracking occurs during the terminal stage of dendritic solidification. Grain refining can reduce solidification cracking [3,4,5,6], undercooling due to fast cooling can increase solidification cracking [7], and back diffusion can reduce solidification cracking [8]. The first two of these three factors are relevant to AM. In arc welding, the most effective way to reduce solidification cracking is to change the composition of the weld metal by using a filler metal different in composition from the base metal [2]. This option, unfortunately, is not available in AM. The effect of the composition, back diffusion, grain refining and undercooling can all be shown by a simple index for the susceptibility to solidification cracking proposed by Kou [3,7,8,9].
In the welding of an alloy, the fusion zone refers to the region in which the alloy melts completely and solidifies, and the partially melted zone (PMZ) refers to the region immediately outside the fusion zone, heated above the liquid-forming temperature, e.g., the eutectic temperature in most Al alloys [2]. Melting occurs only along grain boundaries and at isolated spots within grains. So, liquid formation, i.e., liquation, occurs along grain boundaries in the PMZ to form a thin grain-boundary liquid, and intergranular cracking can occur along the grain-boundary liquid in the PMZ, called liquation cracking. The fracture surface is not dendritic because non-dendritic grains already exist in the PMZ before cracking. Like solidification cracking in arc welding, the most effective way to reduce liquation cracking is to change the composition of the weld metal by using a filler metal different in composition [2], but this is not feasible in AM. Decreasing the energy per unit length of the weld can decrease the extent of liquation and hence liquation cracking [2]. This is true both in welding and AM.
Kannengiesser and Boellinghaus [10] reviewed the weldability tests developed to evaluate the susceptibility of alloys to hot cracking during welding, i.e., solidification cracking, liquation cracking and ductility-dip cracking. In the vast majority of weldability tests, a single-pass, single-layer weld is tested, though multiple-pass, multiple-layer welding is common in practice. This is because a weldability test is typically designed to show cracks clearly and evaluate the cracking susceptibility easily, not to duplicate the welding procedure, even when it is a multiple-pass, multiple-layer one. For example, the effect of the alloy composition on the cracking susceptibility can be shown quickly to decide how to adjust the composition to reduce the cracking susceptibility. For another example, the effect of inoculation can be shown quickly to determine which inoculant is effective and how much is needed.
As in welding, solidification cracking and liquation cracking have also been reported frequently in additive manufacturing (AM) [11,12,13,14,15] and shown to degrade mechanical properties [12]. In AM, the PMZ of the current pass can be located inside either the substrate or the fusion zone of a previous pass or layer next to the current pass.
Similarly to welding, grain refining has been used in AM to reduce solidification cracking, e.g., by inoculation [16,17,18]. A master alloy containing a grain-refining agent was added to the melt and atomized [16]. Particles of a grain-refining agent were attached to the powder for AM by a special coating procedure [17]. Ultrasonic grain refining was applied during deposition [19]. Substrate preheating has been reported to reduce solidification cracking in AM [20,21]. Substrate cooling, on the other hand, was reported to reduce liquation cracking in AM [22].
In AM, most cracks are located inside a built object, not visible from the outside. Cracks can be found by cutting, polishing, and etching at various depths under the surface of the object. Cracks are often small and difficult to open up to examine the fracture surfaces to identify the type of cracking or further analyze cracking. The cracking susceptibility is often assessed based on the total number or length of cracks per unit volume. Naturally, this can be time-consuming, and some cracks may be missed.
The purpose of the present study was to identify a weldability test that could be modified and used as a test for evaluating the susceptibility to solidification cracking and liquation cracking in AM. To prove the concept of the test, the present study was conducted to determine the following: (1) If the test could be conducted inside an Ar-filled chamber for AM. (2) If specimens could be tested one by one without opening the chamber. (3) If cracks could be readily visible. (4) If fracture surfaces could be accessed easily to identify the type of cracking. (5) If the cracking susceptibility under the deposition and testing conditions used could be determined.

2. Materials and Methods

2.1. Identifying a Weldability Test for AM

A series of specimens needs to be tested to see the differences in their cracking susceptibility in order to determine the effect of a certain factor on the cracking susceptibility, e.g., the alloy composition or a process parameter. This is easy in arc welding because it can be performed in open air. However, this can be challenging in AM because it needs to be performed in an Ar-filled chamber, e.g., a LENS MR7 chamber for laser directed-energy deposition. If the chamber has to be opened in order to remove one tested specimen and mount the next specimen, overnight Ar purging will be required to remove oxygen from the chamber. So, specimens need to be tested one by one without opening the chamber. Typically, two long rubber gloves at the front panel of the chamber are available for specimen manipulation inside the chamber.
Since many weldability tests have been developed and their reliability has been verified extensively [9], it is desirable to identify a weldability test that can be modified into a test for evaluating the cracking susceptibility of alloys in AM. However, the authors are unaware of any existing weldability tests being used in AM.
In the present study, the first step was to identify an existing weldability test that potentially could be modified into a cracking susceptibility test for AM. Which weldability test is a good candidate may become obvious after it has been identified and tried but certainly not before. During the course of the present study, it became increasingly clear to the authors that a weldability test with any of the following requirements might be difficult to use for AM:
(1)
Lengthy specimen mounting/removal: A weldability test such as the Sigmajig test [23] uses many (ten) screws to clamp down a specimen and pre-stretch it. This lengthy procedure can be difficult to perform with gloves from outside an Ar-filled chamber.
(2)
Welding along a joint: Since a single-pass, single-layer deposit is narrow and shallow, precise alignment with a joint and melting/wetting both sides of the joint can be difficult. For example, a butt joint exists in the circular-patch test [24] and the Murex hot cracking test [25]. An inverse-T joint exists in the VDR test [26] and the T-joint weldability test [27]. A lap joint exists in the Transverse-Tension Weldability test [28].
(3)
Transverse tension during welding: The workpiece is typically greater in length than width. So, transverse tension, e.g., in the Controlled-Tension Weldability test [29], requires a heavier horizontal tensile testing machine than longitudinal tension.
(4)
Instantaneous tension during welding: The widely used Varestraint test [30] and the transverse Varestraint test [31] are examples. Since the liquid pool in arc welding travels slowly, the tension applied instantaneously has risen to its full level before the pool travels significantly. However, since the liquid pool in AM travels fast, the pool has already moved significantly while the tension is still rising, i.e., the applied tension varies along the deposit.
(5)
Automatic crack initiation at specimen’s leading edge: A weld pool fully penetrating through the specimen at its leading edge may initiate cracks automatically when the pool travels inward, e.g., in the Houldcroft test [32]. In AM, the liquid pool can be too shallow to penetrate through the thickness of the specimen.
(6)
Automatic crack initiation in a pre-stressed specimen: When a rectangular specimen is pre-stressed in the longitudinal direction [33,34] before welding, cracks may be initiated near a weld pool that penetrates through the thickness of the specimen. Examples included a specimen thickness of 2.3 mm in arc welding [33] and 2.0 or 5.7 mm in laser welding (in the so-called U-Type Hot Cracking test [34]). Since the liquid pool is shallow in AM, cracks may not be initiated.
The present study identified one weldability test that does not have any of the six requirements mentioned above. It was a German weldability test called the PVR test [35,36,37], which in German stands for the Programmierter (Programmable) Verformungs (Deformation) Riss (Crack) test. Figure 1 shows an example of the PVR test [37], with a rectangular plate as the specimen, 300 mm long, 40 mm wide and 3.5 mm thick. The specimen is joined to a grip at each end. One end is fixed, and the other is pulled as in tensile testing to induce cracking near the weld pool while the specimen is welded along its centerline at the welding speed Vweld. Pulling is accelerated linearly with time. The pulling speed Vpull at which the first crack forms represents the minimum pulling speed required for cracking, i.e., the critical pulling speed Vcr, which can be determined as follows:
Vcr = a (L1st/Vweld)
where a is acceleration and L1st is the distance between the first crack and the starting end of the weld. The ratio L1st/Vweld represents the time available for acceleration. Vcr can be considered as an index for the cracking susceptibility. The lower Vcr is, the higher the susceptibility to cracking.
The second step was to check the reliability of the PVR test. According to the review by Kannengiesser and Boellinghaus [8], “The PVR-test is a convenient tool principally for arc welding, laser beam welding, and hybrid welding with or without filler material. It has been standardized in ISO17641 [8] owing to its high repeat accuracy and ease of handling.” and “The PVR test can be used to investigate all types of hot cracks.” So, the PVR test is reliable and potentially can be modified into a susceptibility test for solidification cracking and liquation cracking in AM.

2.2. The AM-PVR Test for Hot Cracking Susceptibility in AM

The third step was to modify the PVR test as a test for evaluating the cracking susceptibility in AM. The tensile testing machine selected was compact enough to fit in an Ar-filled chamber designed for AM. The workpiece (i.e., the specimen) traveled because the heat source (i.e., the laser beam) was stationary. So, the tensile test machine selected was light enough to travel quickly with the base that supported it and the specimen. A slender specimen was designed to allow pulling by a compact light tensile test machine. Lastly, metal powder was fed into the stationary laser beam. For the purpose of discussion, the proposed test method is called the “AM-PVR test” as illustrated in Figure 2 [38].
As shown in Figure 2, the substrate was in the form of a slender tensile test specimen (details described later), mounted on a base that traveled at the speed Vbase under a stationary laser beam, to which metal powder was fed for deposition along the specimen centerline. With one end of the specimen fixed on the base, the other end was pulled at the speed Vpull relative to the base by a preprogrammed servomotor. This test can be extended to study the effect of substrate preheating on cracking by heating the specimen from below.
The specimen was a pin-loaded specimen, with two end sections and a slender gauge section in between. The end sections were similar to those in a flat specimen for tensile testing. The dimensions shown in Figure 2 were chosen to be similar to those of Kotkunde et al. [39] for the ASTM E8/E8M-11 [40] sub-size standard specimen, but the length of the gauge section was extended to 365 mm in order to accommodate very fast deposition speeds (e.g., 100 mm/s) if needed. However, this length could be reduced significantly as the typical travel speed in laser-directed energy deposition can be much slower. The use of a pin-loaded tensile test specimen allowed a specimen to be mounted and removed easily from outside the Ar-filled chamber.
The distance between the first crack and the starting end of the deposit L1st was measured after the test. The quantity (L1st/Vbase) represents the available deposition time at constant acceleration from zero travel speed to Vbase. Since the travel speed Vbase is much greater than Vpull, the critical pulling speed velocity can be determined as follows:
Vcr = a (L1st/Vbase)
As in the PVR test, Vcr can be considered as an index for the cracking susceptibility, i.e., the lower Vcr is, the higher the cracking susceptibility.
6061 Al alloy was selected in view of its high susceptibility to solidification cracking in welding [28,41,42]. Spherical gas-atomized 6061 Al powder, supplied by Valimet, Inc., Stockton, CA, USA, was used for directed energy deposition by laser. So, in AM by laser directed energy deposition, the susceptibility of 6061 Al to solidification cracking was tested by depositing 6061 Al powder on a 6061 Al substrate. Since solidification cracking occurs in the region where the substrate is melted completely, whether the substrate is a wrought 6061 Al or an AM-prepared 6061 Al does not matter. What matters is the composition of the deposit. Thus, for convenience, wrought 6061 Al was used as the substrate.
6061 Al powder was also deposited on a 7075 Al substrate in view of the high susceptibility of 7075 Al to liquation cracking in welding [43,44]. For convenience, wrought 7075 Al was used as the substrate. This represented the case of a PMZ in the substrate at the start of AM. To consider a PMZ in a previous pass or layer next to the liquid pool during AM, a groove can be prepared in a wrought 7075 Al sheet and filled with laser-deposited 7075 Al powder. The groove can be about 3 mm wide and 2 mm deep (wider and deeper than the deposit expected in a cracking-susceptibility test) and longer than the expected L1st. Waterjet-cutting the sheet and milling the top surface flat can help prepare a specimen with laser-deposited 7075 Al along its centerline. Table 1 shows the compositions of the alloys.
Experiments were conducted inside a LENS® chamber (Optomec MR-7®, Albuquerque, NM, USA) [45]. It was a lab-scale unit for laser directed-energy deposition in an Ar-filled chamber, with an X-Y table for specimen motion. Two long rubber gloves on the transparent front panel allowed specimen manipulation in the chamber. The powder was filled in one of the four hoppers connected to the system. Ar flow gas was used to bring the powder from the hoppers into the laser path to be melted and deposited on the specimen. A kW Nd:YAG fiber laser (IPG Photonics—Materials Processing Systems, Minneapolis, MN, USA) was used, with a 1070 nm wavelength and a 600 µm spot size. The experimental parameters are shown in Table 2. The energy per unit length of deposit (J/mm) is defined as the laser power (W, i.e., J/s) divided by the travel speed (mm/s). The powder feed rate was set at 10 rotations per minute (rpm) on the LENS hopper rotors, which amounted to 6.8 gm/min of the powder feed rate. Only one specimen was tested under each condition due to the very limited time allocated for using the highly demanded LENS chamber (Albuquerque, USA).
Each specimen was visually inspected for cracks after testing. If cracking occurred, the Vcr value was determined by Equation (2) for the deposition and testing conditions used in the test. The fracture surfaces were examined using scanning electron microscopy (SEM, Zeiss LEO 1550VP FESEM/EDS, ZEISS, Oberkochen, Germany) to confirm the type of cracking.
Similarly to welding [2], if a group of Al alloys of different compositions needs to be ranked in the cracking susceptibility, they should all be tested under identical conditions so that their cracking susceptibility can be compared. In the present study, 6061 Al alone was tested for the susceptibility to solidification cracking. However, since several different testing conditions were tried in the course of the present study, the effect of the test conditions on the value of Vcr can be discussed subsequently. The laser power and the travel speed were selected to keep the deposit as continuous as possible but not two wide (<2 mm width, which is one-third of the specimen width). Below 350 W at a travel speed above 6 mm/s, the melted powder was more likely to freeze on the Al substrate as a discontinuous deposit initially. This is because Al alloys have a high thermal conductivity. Acceleration above 0.070 mm/s2 encouraged the deposition to deviate from the centerline of the gauge section. Increasing the pulling force could increase specimen deformation at the pins and hence deposit deviation. Acceleration below 0.020 mm/s2 seemed insufficient to cause cracking. Before incorporating the AM-PVR test, deposits were made at various laser power levels and travel speeds. The deposition was of similar quality to that in the test. No cracks were observed. There was no issue in alignment since the only moving part was the stage.

3. Results

The experimental results are summarized in Table 2 (shaded area); “6061” indicates specimens with 6061 Al as the substrate, and “7075” indicates specimens with 7075 Al as the substrate. As mentioned previously, the powder was 6061 Al in both cases, fed at 6.8 g/min.

3.1. Solidification Cracking Susceptibility

Specimen 6061-P2 was tested by depositing 6061 Al powder on a 6061 Al substrate at laser power P = 350 W, Vbase = 7.5 mm/s and a = 0.020 mm/s2. No solidification or liquation cracking occurred. The deposit was narrow (<1 mm width) and discontinuous initially, suggesting 350 W was too low for a 7.5 mm/s travel speed of 6061 Al substrate. A higher laser power, e.g., 400 W, might be needed.
Specimen 6061-P3, shown in Figure 3, was tested by depositing 6061 Al powder on a 6061 Al substrate at P = 400 W, Vbase = 7.5 mm/s and a = 0.020 mm/s2. The energy per unit length of the deposit = 53.3 J/mm (i.e., 400 W ÷ 7.5 mm/s). As can be seen in Figure 3, no solidification or liquation cracking is visible.
Two comments can be made as follows. First, incompletely melted metal powder is visible at the top surface of the deposit. However, as will be shown subsequently by the transverse cross-section of the specimen, incompletely melted metal powder existed only as a thin layer at the top surface of the deposit and did not affect the validity of the test.
Second, the deposit deviates from a straight-line path. During pulling, local deformation of the 6061 Al specimen at the pins (Figure 2a) caused the gauge section of the tensile specimen to shift slightly in the lateral direction. This deformation seems consistent with the excellent extrudability of 6061 Al through dies in the production of 6061 Al channels and pipes. Increasing the pin diameters or the thickness of the end sections may decrease the deformation and improve the straightness of the deposit.
To increase the chance of solidification cracking, Specimen 6061-P9 was made at P = 400 W, Vbase = 7.0 mm/s and a significantly higher acceleration of a = 0.050 mm/s2. So, the energy per unit length of the deposit = 57.1 J/mm (i.e., 400 W ÷ 7.0 mm/s). As shown in Figure 4, solidification cracking is now visible in the deposit. The crack at 97 mm (from the starting point of the deposit) is the first one that runs across the width of the deposit (i.e., fusion zone) and is taken as the first solidification crack. According to Equation (2), the critical pulling speed Vcr = a (L1st/Vbase) = (0.05 mm/s2) (97 mm/7.0 mm/s) = 0.69 mm/s. An alternative is to consider the shorter cracks earlier as well. That is, the first solidification crack is located at 91 to 97 mm (i.e., 94 ± 3 mm), corresponding to Vcr = 0.67 ± 0.02 mm/s.
Figure 5 shows solidification cracking in Specimen 6061-P5. 6061 Al powder was deposited on a 6061 Al substrate at P = 350 W, Vbase = 6.5 mm/s and a = 0.050 mm/s2. The energy per unit length of the deposit was 53.8 J/mm (i.e., 350 W ÷ 6.5 mm/s). As shown in Figure 5, L1st = 82–93 mm or 87.5 ± 5.5 mm. So, Vcr = a (L1st/Vbase) = (0.050 mm/s2) (87.5 ± 5.5 mm/6.5 mm/s) = 0.67 ± 0.04 mm/s.

3.2. Liquation Cracking Susceptibility

Figure 6 shows Specimen 7075-P3, which was prepared by depositing 6061 Al powder on a 7075 Al substrate at P = 400 W, Vbase = 6.0 mm/s and a = 0.010 mm/s2. The energy per unit length of the deposit = 66.7 J/mm (i.e., 400 W ÷ 6.0 mm/s). The deposit is 114 mm long (a section about 40 mm long was cut off from the specimen for metallographic examination), and no solidification or liquation cracking is visible.
To increase the chance for liquation cracking, Specimen 7075-P4, shown in Figure 7, was prepared still at P = 400 W and Vbase = 6.0 mm/s but with a faster acceleration of a = 0.025 mm/s2. The thinner deposit in the 1st 10 mm was the 1st deposit made at a lower laser power. After the first deposit had cooled down, a second deposit was made. So, the energy per unit length of the new deposit was still 66.7 J/mm (i.e., 400 W ÷ 6.0 mm/s). Liquation cracking caused the specimen to break into two pieces. The first liquation crack occurred at 77 mm, and it ran across the entire width of the specimen, including the fusion zone. According to Equation (2), the critical pulling speed Vcr = a (L1st/Vbase) = (0.025 mm/s2) (77 mm/6.0 mm/s) = 0.32 mm/s.
Figure 8 shows liquation cracking in Specimen 7075-P6, in which 6061 Al powder was deposited on a 7075 Al substrate at P = 350 W, Vbase = 6.0 mm/s and a = 0.030 mm/s2. The crack was normal to the specimen, and it ran across the specimen, breaking it into two pieces. The portion of the specimen indicated by the rectangular box in Figure 8a was cut off for examination of the fracture surfaces. The energy per unit length of the deposit = 58.3 J/mm (i.e., 350 W ÷ 6 mm/s). Figure 8a shows L1st = 86 mm. So, Vcr = a (L1st/Vbase) = (0.030 mm/s2) (86 mm/6.0 mm/s) = 0.43 mm/s. This is higher than the Vcr = 0.32 mm/s in Specimen 7075-P4 (Figure 7), thus suggesting a lower susceptibility to liquation cracking. Again, liquation cracking propagated through the specimen, breaking it into two pieces. Figure 8b shows Specimen 7075-P6 at a higher magnification. The deposit came off at a few spots during clamping and cutting of the specimen for SEM examination of the fracture surfaces.

4. Discussion

4.1. Fracture Surfaces

To help the discussion of the test results, the fracture surfaces are shown first as follows. Figure 9 shows the SEM images of the fracture surface of Specimen 7075-P6 that was prepared at P = 350 W, Vbase = 6.0 mm/s and a = 0.030 mm/s2. The image is rotated 90° counterclockwise from its horizontal position. In the higher-magnification image, the deposit is on the left side and the substrate on the right. Columnar grains grew epitaxially from the substrate into the deposit. Some metal particles did not melt completely at the top surface. However, complete melting occurred during deposition within the area occupied by the columnar grains. Thus, the incomplete melting of some particles at the top surface of the deposit could not have affected the test result significantly.
Figure 10a enlarges a portion of the fracture surface near the interface between the deposit, which is dendritic, and the substrate, which is nondendritic. The dendritic fracture surface on the deposit side can be seen more clearly in Figure 10b. Thus, liquation cracking occurred in the substrate, and it propagated as solidification cracking through the deposit, which is a common phenomenon in welding [2,46].
As will be explained subsequently, the extent of liquation can become significantly higher if the distance between the deposit and the edge of the specimen is shortened. This can cause more significant liquation along the grain boundaries in the substrate next to the deposit, and liquation cracking can occur prematurely. Figure 11 shows Specimen 6061-P7 prepared by depositing 6061 Al powder on a 6061 Al substrate at P = 400 W, Vbase = 6.5 mm/s and a = 0.050 mm/s2. As shown, liquation cracking occurred prematurely when the distance between the deposit and the specimen edge was shortened suddenly due to the deformation of 6061 Al at the pins during pulling. Liquation cracking occurred at 82 mm from the starting point of the deposit. This and many other liquation cracks propagated as solidification cracks into the deposit. On the opposite side of the deposit, liquation cracks were very short or not visible because there was much more substrate material to act as a bigger heat sink to reduce liquation. The liquation crack at 148 mm initiated a fracture, and the specimen bent significantly before rupture. This is because the substrate between the deposit and the opposite specimen edge was much less liquated, still able to deform during pulling.
Figure 12a shows the fracture surface of Specimen 6061-P7. Figure 12b enlarges an area near the interface between the deposit and the substrate. As shown, cracking is intergranular on both sides of the interface. It is dendritic in the deposit, thus confirming solidification cracking. It is nondendritic in the substrate near the deposit, on the other hand, thus confirming liquation cracking.
Figure 12c enlarges the columnar dendrites in the deposit. The primary dendrite arm spacing was about 0.7 mm, measured across several dendrites as indicated by the broken rectangle. As shown in Figure 13, this corresponds to a cooling rate of about 500 K/s based on the data for alloy Al-3Cu-1Li as an approximation [47].
Figure 14 shows the SEM image of Specimen 6061-P7. It shows the microstructure of the deposit in a horizontal longitudinal plane near the top surface of the substrate. The pulling direction is from left to right. Thin eutectic (light etching) is visible along grain boundaries. Cracks are intergranular, consistent with solidification cracking. No significant deformation in the pulling direction is visible to show significant resistance to solidification cracking.

4.2. Solidification Cracking Susceptibility

As mentioned previously, different Al alloys can be tested under identical conditions to compare the effect of the alloy composition on the susceptibility to solidification cracking [2]. In the present study, 6061 Al alone was tested for the susceptibility to solidification cracking. In addition to 6061 Al, the authors intended to test 2219 Al (~Al-6.3Cu), that is, depositing 2219 Al powder on 2219 Al specimens. 2219 Al has been shown by various weldability tests to be much more resistant to solidification cracking than 6061 Al [28,41,42]. By testing both 6061 Al and 2219 Al, the validity of the AM-PVR test can be checked easily. If desired, the crack density in 6061 Al can also be compared with that in 2219 Al under identical testing conditions. Unfortunately, even though 2219 Al is a common material, 2219 Al powder was not commercially available at the time the present study was conducted.
As shown in Table 2, solidification cracking occurred in two specimens, and they happened to have the same cracking susceptibility (Vcr)SC = 0.67 mm/s. The authors do not believe that this necessarily suggests a restricted sensitivity of the method to alterations in process parameters. Their similarity in both the acceleration and the energy per unit length of deposit might have contributed to their similarity in the solidification cracking susceptibility. The acceleration was 0.050 mm/s2 for both specimens. The energy per unit length of deposit was 53.8 J/mm (=350 W ÷ 6.5 mm/s) for Specimen 6061-P5 and 57.1 J/mm (=400 W ÷ 7.0 mm/s) for Specimen 6061-P9, only 6 % higher than 53.8 J/mm.
Slyvinsky et al. [48] conducted the PVR test on the susceptibility of a Ni-base alloy NiCr25FeAlY to solidification cracking during gas-tungsten arc welding at a constant arc power. It was shown that the susceptibility decreased, i.e., (Vcr)SC increased, when the travel speed increased. This was attributed to fast travel encouraging the formation of finer equiaxed grains in the fusion zone. However, the effect of process variables on the solidification cracking susceptibility may not always be straightforward. For instance, different alloys tested by different weldability tests have shown that, when the arc power remains constant and the travel speed increases, the solidification cracking susceptibility can change in different ways [49]. It can remain unchanged [50], decrease [51,52] or increase [53,54]. It remains to be seen if the effect of various microstructure features on the solidification cracking susceptibility can be revealed by the AM-PVR test or any in situ sensing technologies employing optical, acoustic, and thermal in situ sensors.

4.3. Liquation Cracking Susceptibility

For Specimen 7075-P6, the energy per unit length of the deposit was 58.3 J/mm, and the critical pulling speed (Vcr)LC = 0.43 mm/s. For Specimen 7075-P4, the energy per unit length of the deposit was 14 % higher at 66.7 J/mm, and the critical pulling speed was lower at (Vcr)LC = 0.32 mm/s. The lower energy in 7075-P6 suggests less liquation, i.e., less liquid formation along grain boundaries to weaken them and make them less able to resist tension. The transverse cross-section of the substrate was much greater than that of the deposit, suggesting the substrate bore almost all the tensile load during testing. Thus, under the same tensile loading, the liquation cracking susceptibility of an alloy can be expected to be higher to a greater extent of liquation caused by a higher energy per unit length of the deposit. The acceleration was slightly faster for 7075-P6 (0.030 mm/s2) than for 7075-P4 (0.025 mm/s2), but either one was well above the 0.010 mm/s2 acceleration that failed to cause liquation cracking (in 7075-P3 and 7075-P5). Thus, 7075-P6 can be expected to have a higher (Vcr)LC and hence a lower liquation cracking susceptibility. This is consistent with the susceptibility to liquation cracking increasing with increasing energy per unit length of deposit in laser directed energy deposition of Inconel 718 [55]. In arc welding, a lower energy per unit length of weld suggests less liquation and hence less liquation cracking [2,56].
Since 6061 Al powder was deposited on 7075 Al as well as 6061 Al, the effect of the substrate composition on the susceptibility to liquation cracking can be revealed. For Specimen 6061-P9, the energy per unit length of deposit was 57.1 J/mm (i.e., 400 W ÷ 7.0 mm/s) and the acceleration was 0.050 mm/s2. No liquation cracking occurred (solidification cracking occurred at 91–97 mm). As for Specimen 7075-P6, the energy was 58.3 J/mm (i.e., 350 W ÷ 6.0 mm/s), very close to the 57.1 J/mm of 6061-P9, and the acceleration a = 0.030 mm/s2 was significantly lower than the a = 0.050 mm/s2 for 6061-P9. Yet, liquation cracking occurred in 7075-P6 but not 6061-P9. This suggests 7075 Al is much more susceptible to liquation cracking than 6061 Al, just like in arc welding [44]. Thus, more liquation cracking can be expected in building a 7075 Al part from 7075 Al powder than in building a 6061 Al part from 6061 Al powder under similar AM conditions.

4.4. Effect of Substrate and Deposit Location

Figure 15 explains the effect of the substrate material and the deposit location on the susceptibility to liquation cracking. For illustration, grain boundaries are shown to be normal to the pulling direction (similar to Figure 10). Liquation cracking might be easier to occur when grains are normal to the pulling direction [2].
As mentioned previously, 7075 Al (Figure 15a) is more susceptible to liquation cracking than 6061 Al (Figure 15b). The difference can be explained based on the curves of temperature T vs. fraction of solid fS of 6061 Al and 7075 Al shown in Figure 16. The curves were calculated based on their compositions shown in Table 1, using the thermodynamic software Pandat [57] and aluminum database PanAl [58] of CompuTherm, LLC (Middleton, WI, USA) based on the Scheil-Gulliver solidification model [1]. The portion of the T-fS curve near the end of solidification, i.e., the eutectic temperature, is much wider and lower for 7075 Al than 6061 Al. The fraction of eutectic could have been somewhat reduced in production by heat treating after casting. Upon heating during welding or AM, however, the eutectic reaction occurs again. During heating, the reaction can be expected to occur earlier in 7075 Al and form significantly more liquid along grain boundaries to weaken them. This can explain why 7075 Al is more susceptible to liquation cracking than 6061 Al.
If the deposit comes close to the one edge of the 6061 Al substrate (Figure 15c), liquation can become much more severe in the PMZ close to that edge because much less substrate material is available to act as a heat sink, i.e., the edge behaves like thermal insulation. Consequently, liquation cracking can be promoted (Figure 11).
It is worth mentioning that the test can be improved in future studies. The first improvement is to reduce the deviation of the deposit from a straight-line path. Increasing the pin diameter or end-section thickness (Figure 2) can be tried. The second improvement is to melt the surface of the deposit completely. Optimizing the laser power and the base travel speed can be tried. The third improvement is to melt the substrate surface from the starting point of the deposition. Laser heating the substrate briefly before the base starts traveling can be tried.
As already mentioned, the AM-PVR test can evaluate the cracking susceptibility based on a single-pass, single-layer deposit in which cracks are visible from the outside. The AM-PVR test may complement the cracking susceptibility based on measuring the total crack length in authentic multi-pass, multi-layer assemblies. For instance, it can be checked if Vcr can be correlated with the total crack length measured in the assemblies first. If so, the AM-PVR test can be used along with the correlation.
Research in AM has been increasingly employing optical, acoustic, and thermal in situ sensors for detecting cracks. As compared to these sensing technologies, the AM-PVR test is simple and requires no advanced monitoring/sensing equipment and holding time.

5. Conclusions

(1)
The PVR weldability test has been selected, modified and used as a test for evaluating the susceptibility to solidification cracking and liquation cracking in AM. In this AM-PVR test, a single-pass, single-layer deposit is made along a slender specimen that is pulled, like in tensile testing, but with acceleration.
(2)
Solidification cracks are visible on the surface of the deposit. Liquation cracks are visible in the substrate near the deposit, and they can propagate through the specimen. The fracture surfaces are readily accessible for examination by SEM. Dendritic fracture surfaces of the deposits have confirmed solidification cracking. Nondendritic fracture surfaces of the substrates showing intergranular cracking have confirmed liquation cracking.
(3)
The critical pulling speed for solidification cracking can be determined as an index for the cracking susceptibility under the conditions of deposition and testing used in the test, so can that for liquation cracking. In either type of cracking, the lower the critical pulling speed, the higher the cracking susceptibility.
(4)
The results of the present study can be considered as a proof of concept for this AM-PVR test.

6. Patents

Kou, S. Device and Method for Evaluating the Susceptibility of Hot Cracking in Additive Manufacturing. U.S. Patent 12,442,739 B2, 14 October 2025.

Author Contributions

Conceptualization, S.K.; methodology, S.K.; Investigation, S.D. and S.K.; writing—original draft preparation, S.K.; writing—review and editing, S.D. and D.T.; supervision, S.K. and D.T.; project administration, S.K.; funding acquisition, S.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Science Foundation of the United States under Grant No. DMR 1904503.

Data Availability Statement

The data presented in this study are openly available in FigShare at doi:10.6084/m9.figshare.30369202.

Acknowledgments

The authors thank Phalgun Nelaturu and Mythili Thevamaran for their help in conducting the laser directed-energy deposition experiments. Soumyadeep Dasgupta, former Graduate Student at the University of Wisconsin-Madison supervised by Sindo Kou, is now Graduate Student at the University of Michigan-Ann Arbor.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
AMAdditive Manufacturing
PMZPartially Melted Zone
PVRProgrammierter (Programmable) Verformungs (Deformation) Riss (Crack)
SEMScanning Electron Microscopy

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Figure 1. The PVR test for cracking susceptibility in welding: (a) longitudinal cross-section along specimen centerline; (b) top view. Adapted from Yushchenko et al. [37].
Figure 1. The PVR test for cracking susceptibility in welding: (a) longitudinal cross-section along specimen centerline; (b) top view. Adapted from Yushchenko et al. [37].
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Figure 2. The AM-PVR test proposed in the present study: (a) longitudinal cross-section along specimen centerline; (b) top view of specimen; (c) critical pulling speed for solidification cracking, (Vcr)SC; (d) critical pulling speed for liquation cracking (Vcr)LC. The lower the critical pulling speed, the higher the cracking susceptibility.
Figure 2. The AM-PVR test proposed in the present study: (a) longitudinal cross-section along specimen centerline; (b) top view of specimen; (c) critical pulling speed for solidification cracking, (Vcr)SC; (d) critical pulling speed for liquation cracking (Vcr)LC. The lower the critical pulling speed, the higher the cracking susceptibility.
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Figure 3. Specimen 6061-P3 showing no solidification cracking. The deposit starts at 0 mm and ends at 223 mm.
Figure 3. Specimen 6061-P3 showing no solidification cracking. The deposit starts at 0 mm and ends at 223 mm.
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Figure 4. Specimen 6061-P9 showing solidification cracking: (a) lower magnification; (b) higher magnification showing the first solidification crack at (L1st)SC = 91–97 mm from the starting point of the deposit. Arrows indicate cracks.
Figure 4. Specimen 6061-P9 showing solidification cracking: (a) lower magnification; (b) higher magnification showing the first solidification crack at (L1st)SC = 91–97 mm from the starting point of the deposit. Arrows indicate cracks.
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Figure 5. Specimen 6061-P5 showing solidification cracking: (a) lower magnification; (b) higher magnification showing the first solidification crack at (L1st)SC = 82–93 mm from the starting point of the deposit. Arrows indicate cracks. Red dots were marked for identifying location to be photographed.
Figure 5. Specimen 6061-P5 showing solidification cracking: (a) lower magnification; (b) higher magnification showing the first solidification crack at (L1st)SC = 82–93 mm from the starting point of the deposit. Arrows indicate cracks. Red dots were marked for identifying location to be photographed.
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Figure 6. Specimen 7075-P3 showing no liquation or solidification cracking: (a) lower magnification; (b) higher magnification. A section of the specimen about 40 mm long was cut off for metallography (not included in the photos).
Figure 6. Specimen 7075-P3 showing no liquation or solidification cracking: (a) lower magnification; (b) higher magnification. A section of the specimen about 40 mm long was cut off for metallography (not included in the photos).
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Figure 7. Specimen 7075-P4 showing liquation cracking (LC) propagating through the specimen, including the deposit as solidification cracking (SC): (a,b) lower magnifications; (c) higher magnification showing the first liquation crack at (L1st)SC = 77 mm from the starting point of the 2nd deposit at 0 mm. In (a,b) the 2nd deposit was made after the 1st deposit (the deposit to the left of 0 mm) had cooled down. In (c) the arrows indicate cracks.
Figure 7. Specimen 7075-P4 showing liquation cracking (LC) propagating through the specimen, including the deposit as solidification cracking (SC): (a,b) lower magnifications; (c) higher magnification showing the first liquation crack at (L1st)SC = 77 mm from the starting point of the 2nd deposit at 0 mm. In (a,b) the 2nd deposit was made after the 1st deposit (the deposit to the left of 0 mm) had cooled down. In (c) the arrows indicate cracks.
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Figure 8. Specimen 7075-P6 showing liquation cracking (LC) propagating through the specimen, including the deposit as solidification cracking (SC): (a) lower magnification showing first liquation crack at (L1st)SC = 86 mm from the starting point of the deposit; (b) higher magnification. The material in the rectangular box was cut off for fracture-surface examination. While clamping and cutting, the deposit broke off the specimen at several spots.
Figure 8. Specimen 7075-P6 showing liquation cracking (LC) propagating through the specimen, including the deposit as solidification cracking (SC): (a) lower magnification showing first liquation crack at (L1st)SC = 86 mm from the starting point of the deposit; (b) higher magnification. The material in the rectangular box was cut off for fracture-surface examination. While clamping and cutting, the deposit broke off the specimen at several spots.
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Figure 9. SEM image of fracture surface of Specimen 7075-P6 (350 W, 6.0 mm/s and 0.030 mm/s2) showing liquation cracking in the substrate propagating through the deposit (as solidification cracking) and breaking the specimen into two pieces: (a) lower magnification; (b) higher magnification. The specimen is rotated 90 degrees counterclockwise, so its top surface is on the left side of the SEM image and its bottom surface on the right.
Figure 9. SEM image of fracture surface of Specimen 7075-P6 (350 W, 6.0 mm/s and 0.030 mm/s2) showing liquation cracking in the substrate propagating through the deposit (as solidification cracking) and breaking the specimen into two pieces: (a) lower magnification; (b) higher magnification. The specimen is rotated 90 degrees counterclockwise, so its top surface is on the left side of the SEM image and its bottom surface on the right.
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Figure 10. SEM image of fracture surface of Specimen 7075-P6 showing: (a) liquation cracking (LC) in the substrate (right side; intergranular nondendritic grains aligning in the rolling direction) and solidification cracking (SC) in the deposit (left side; intergranular dendritic grains); (b) intergranular dendritic grains in the deposit (SC).
Figure 10. SEM image of fracture surface of Specimen 7075-P6 showing: (a) liquation cracking (LC) in the substrate (right side; intergranular nondendritic grains aligning in the rolling direction) and solidification cracking (SC) in the deposit (left side; intergranular dendritic grains); (b) intergranular dendritic grains in the deposit (SC).
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Figure 11. Specimen 6061-P7 showing liquation cracking (LC) in the substrate propagating through the deposit (as solidification cracking SC): (a) lower magnification; (b) higher magnification. The arrows indicate cracks.
Figure 11. Specimen 6061-P7 showing liquation cracking (LC) in the substrate propagating through the deposit (as solidification cracking SC): (a) lower magnification; (b) higher magnification. The arrows indicate cracks.
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Figure 12. SEM image of fracture surface of Specimen 6061-P7 (6061 Al deposited with 6061): (a) Specimen rotated 90 degrees counterclockwise, so its top surface is on the left side of SEM image and its bottom surface on the right; (b) microstructure in boxed area in (a), which is dendritic in the deposit (solidification cracking SC) and nondendritic in the nearby substrate (liquation cracking LC); (c) microstructure in boxed area in (b). Primary dendrite arm spacing was measured in the area inside the white dotted rectangle in (c).
Figure 12. SEM image of fracture surface of Specimen 6061-P7 (6061 Al deposited with 6061): (a) Specimen rotated 90 degrees counterclockwise, so its top surface is on the left side of SEM image and its bottom surface on the right; (b) microstructure in boxed area in (a), which is dendritic in the deposit (solidification cracking SC) and nondendritic in the nearby substrate (liquation cracking LC); (c) microstructure in boxed area in (b). Primary dendrite arm spacing was measured in the area inside the white dotted rectangle in (c).
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Figure 13. Primary dendrite arm spacing vs. cooling rate of Al-3Cu-1Li alloy as an approximation for 6061 Al. Adapted from Santos et al. [47].
Figure 13. Primary dendrite arm spacing vs. cooling rate of Al-3Cu-1Li alloy as an approximation for 6061 Al. Adapted from Santos et al. [47].
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Figure 14. SEM image of Specimen 6061-P7 showing the microstructure of the deposit in a horizontal longitudinal plane near the top surface of the substrate. Thin eutectic (light etching) is visible along grain boundaries. Cracking is intergranular. No significant deformation caused by resistance to solidification cracking is visible.
Figure 14. SEM image of Specimen 6061-P7 showing the microstructure of the deposit in a horizontal longitudinal plane near the top surface of the substrate. Thin eutectic (light etching) is visible along grain boundaries. Cracking is intergranular. No significant deformation caused by resistance to solidification cracking is visible.
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Figure 15. Effect of substrate and deposit location on liquation: (a) 7075 Al substrate; (b) less liquation in 6061 Al substrate; (c) liquation in 6061 Al substrate worsens if deposit comes close to the substrate edge. For illustration, grain boundaries are shown normal to the pulling direction (similar to Figure 10).
Figure 15. Effect of substrate and deposit location on liquation: (a) 7075 Al substrate; (b) less liquation in 6061 Al substrate; (c) liquation in 6061 Al substrate worsens if deposit comes close to the substrate edge. For illustration, grain boundaries are shown normal to the pulling direction (similar to Figure 10).
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Figure 16. Curves of temperature vs. fraction of solid for 6061 Al and 7075 Al.
Figure 16. Curves of temperature vs. fraction of solid for 6061 Al and 7075 Al.
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Table 1. Compositions of Al-6061 and Al-7075 specimens.
Table 1. Compositions of Al-6061 and Al-7075 specimens.
Wt% CrCuFeMgMnSiTiZnAl
60610.200.330.61.00.080.720.020.1balance
70750.191.50.172.40.030.070.035.7balance
Table 2. Deposition parameters, solidification cracking (SC) and liquation cracking (LC) *.
Table 2. Deposition parameters, solidification cracking (SC) and liquation cracking (LC) *.
CrackingSpecimenLaser Power (W)Travel Speed, Vbase (mm/s)Energy per Unit Length (J/mm)Acceleration, a (mm/s2)Deposit Length at 1st SC, L1st (mm) Critical Pulling Speed Vcr for SC (mm/s)Deposit Length at 1st LC, L1st (mm)Critical Pulling Speed Vcr for LC (mm/s)
No6061-P23507.546.70.020
6061-P34007.553.30.020
SC6061-P53506.553.80.05082–930.67 ± 0.04
6061-P94007.057.10.05091–970.67 ± 0.02
LC6061-P74006.561.50.050 82 0.63
No7075-P34006.066.70.010
7075-P53506.058.30.010
LC7075-P44006.066.70.025 770.32
7075-P63506.058.30.030 860.43
* Shaded area indicates test results.
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Dasgupta, S.; Thoma, D.; Kou, S. A Test for Susceptibility to Solidification Cracking and Liquation Cracking in Additive Manufacturing. Metals 2025, 15, 1147. https://doi.org/10.3390/met15101147

AMA Style

Dasgupta S, Thoma D, Kou S. A Test for Susceptibility to Solidification Cracking and Liquation Cracking in Additive Manufacturing. Metals. 2025; 15(10):1147. https://doi.org/10.3390/met15101147

Chicago/Turabian Style

Dasgupta, Soumyadeep, Dan Thoma, and Sindo Kou. 2025. "A Test for Susceptibility to Solidification Cracking and Liquation Cracking in Additive Manufacturing" Metals 15, no. 10: 1147. https://doi.org/10.3390/met15101147

APA Style

Dasgupta, S., Thoma, D., & Kou, S. (2025). A Test for Susceptibility to Solidification Cracking and Liquation Cracking in Additive Manufacturing. Metals, 15(10), 1147. https://doi.org/10.3390/met15101147

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