Cracking Susceptibility Index Research Articles

From crack-prone to crack-free: Eliminating cracks in additively manufacturing of high-strength Mg2Si-modified Al-Mg-Si alloys

Large solidification ranges and coarse columnar grains in the additively manufacturing of Al-Mg-Si alloys are normally involved in hot cracks during solidification. In this work, we develop novel crack-free Al-Mg2Si alloys fabricated by laser powder-bed fusion (L-PBF). The results indicate that the eutectic Mg2Si phase possesses a strong ability to reduce crack susceptibility. It can enhance the grain growth restriction factor in the initial stage of solidification and promote eutectic filling in the terminal stage of solidification. The crack-free L-PBFed Al-xMg2Si alloys (x = 6 wt.%, 9 wt.%, and 12 wt.%) exhibit the combination of low crack susceptibility index (CSI), superior ability for liquid filling, and grain refinement. Particularly, the L-PBFed Al-9Mg2Si alloy shows improved mechanical properties (e.g. yield strength of 397 MPa and elongation of 7.3 %). However, the cracks are more likely to occur in the region near the columnar grain boundaries of the L-PBFed Al-3Mg2Si alloy with a large solidification range and low eutectic content for liquid filling. Correspondingly, the L-PBFed Al-3Mg2Si alloy shows poor bearing capacity of mechanical properties. The precise tuning of Mg2Si eutectic content can offer an innovative strategy for eliminating cracks in additively manufactured Al-Mg-Si alloy.

Assessment of process-induced cracks in hot-working operations using crack susceptibility index based on plastic instability criteria

A processing map representing the efficiency of power dissipation as a function of the temperature and strain rate has been used as a guide to depict the hot workability of metallic alloys. However, it has limitations in indicating the local positions where process-induced cracks may occur during hot-working operations. A process-induced crack is formed during hot deformation because the plastic instability associated with crack formation is more favorable than normal flow under a given set of process conditions. This paper proposes a novel approach to assess local cracks using the crack susceptibility index (CSI), which is based on Ziegler's plastic instability criteria. This solution provides useful information regarding the effect of each process parameter on the workability required for high-temperature processes. Lowering the CSI value by controlling process variables, such as the temperature and strain rate, is recommended for a design free from process-induced cracks. More importantly, a coded finite element subroutine coupled with the CSI can help visualize local positions where cracks may be produced during a hot-working operation. For experimental validation, a Gleeble test was performed on Ti6Al4V specimens containing four notches with different depths. The successful application of the CSI to the process design of shop-floor unmanned aircraft components helped confirm the applicability and validity of this study.

High strength Al0.7CoCrFeNi2.4 hypereutectic high entropy alloy fabricated by laser powder bed fusion via triple-nanoprecipitation

Eutectic high-entropy alloys, composed of FCC/B2 phases with a narrow solidification interval and excellent fluidity, have become a new hotspot in additive manufacturing. Nevertheless, their microstructures exhibit significant sensitivity to processing parameters, feedstocks, and composition, ultimately limiting the alloys’ engineering applications. Here, a hypereutectic Al0.7CoCrFeNi2.4 alloy with a low cracking susceptibility index was designed by Thermo-Calc calculation and fabricated by laser powder bed fusion. Results show that the as-printed Al0.7CoCrFeNi2.4 alloy manifests a stable cellular structure, coupled with appreciable ultimate tensile strength (≥1200 MPa) and ductility (≥20%) over a wide range of processing parameters. After aging at 800 °C for 30 min, outstanding strength (1500 MPa) and elongation (15%) were obtained. Considerable mechanical properties after aging stem from a triple strengthening mechanism, i.e., L12 nanoprecipitates and rod-shaped B2 particles within the FCC matrix, along with Cr-enriched spherical nanoparticles in the B2 phase. Meanwhile, hierarchical structure, i.e., FCC dominated matrix, a discontinuous B2 phase, a precipitation-free zone in the B2 phase, and a K-S orientation relationship between FCC and B2, facilitate to maintain excellent plasticity. These results guide designing HEAs by AM with controllable microstructures and outstanding mechanical properties for industrial applications.

CALPHAD-aided design of high-strength Al-Si-Mg alloys for sufficient laser powder bed fusion processability

This study proposes a design concept for an Al-Si-Mg ternary alloy to achieve both high strength and laser-based powder bed fusion (PBF-LB) processability using calculation of phase diagrams (CALPHAD). The thermodynamic calculations using the CALPHAD method evaluated the Al-10Si-4.5Mg (wt%) alloy composition for strengthening by both the Si and Mg2Si phases and high PBF processability by reducing the risk of hot tearing behind the small freezing range of 27 °C and crack susceptibility index (|dT/d(Fs1/2)|) of 77 °C. The PBF-LB Al-10Si-4.5Mg alloy sample (processed by laser power of 128 W and laser scanning velocity of 1000 mm/s) exhibited a fine three-phase microstructure and exceptional hardness of 199 HV. No hot tearing occurred, whereas many cracks preferentially propagated along the melt pool boundaries during the PBF-LB process. The severe lateral cracking resulted in a maximum relative density of 96 % in the Al-10Si-4.5Mg alloy samples. Preferential cracking could be responsible for the localization of the relatively coarse particles of the brittle Mg2Si phase in the cellular microstructures of the melt pool boundaries. Understanding the lateral cracking mechanism can provide new insights into alloy modification using the controlled solidification path to avoid a specific two-phase eutectic reaction (L→α-Al+Mg2Si) to improve the PBF processability. A modified alloy composition of Al-10Si-3Mg (wt%) was assessed using the CALPHAD approach. PBF-LB processing using the modified alloy powder alleviated cracking and improved the maximum relative density to 98 % for the PBF-LB manufactured samples. This result accentuated the validity of the design strategy for Al-Si-Mg ternary alloy utilizing the CALPHAD method.

Experimental investigation and kinetic analysis of Al–Zn–Mg alloy coating

The hot-dip 55 wt%Al–Zn-1.6 wt%Si-(0–3)wt.%Mg alloy coatings were experimentally investigated, and the solidification behaviors and hot cracking susceptibility were simulated by means of CALPHAD (CALculation of PHAse Diagrams) method. The scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA) and glow discharge spectrometer method (GDS) were utilized to determine the microstructures and the distribution of elements of the Al–Zn–Mg alloy coatings with different Mg contents. It is discovered that with the increase of Mg content, the percentage of the eutectic microstructure scales up, and the surface quality of the alloy coating is improved. Meanwhile, the bending properties of Al–Zn–Mg coatings with different Mg contents still requires further improvement according to the present bending test. Subsequently, the equilibrium solidification processes of the coatings were calculated using thermodynamic approach. In general, the calculated results reflect the solidified phases and the precipitated temperatures according to theory of equilibrium solidification. However, there still exist discrepancies between the thermodynamic calculation results and the observed experimental results during the practical galvanizing process, because the cooling rates were not taken fully into consideration. Consequently, the kinetic analysis was carried out to obtain the secondary dendrite arm spacing under different cooling rates. The cracking susceptibility index (CSI) was also calculated to predict the hot workability of the Al–Zn–Mg alloy coating. In summary, the appropriate increase of the cooling rate turns out be the effective approach to benefit the microstructure and the corrosion resistance of the Al–Zn–Mg coatings in the practical galvanizing process, and the coating is not suitable for the hot stamping process.

Laser powder bed fusion of immiscible steel and bronze: A compositional gradient approach for optimum constituent combination

The microstructures and the mechanical properties of a laser powder bed fusion (LPBF) manufactured alloy coupons that were compositionally graded with austenitic stainless steel 316 L and bronze (Cu10Sn), which are immiscible, were investigated. While the microstructure of the pure 316 L contains only columnar γ-Fe grains, the formation of the equiaxed α-Cu, with α-Fe particles embedded in them, at the grain boundaries of the γ-Fe grains was observed upon alloying with Cu10Sn, for up to 50 wt.%. In the graded alloys with > 50 wt.% Cu10Sn, the microstructure inverts and contains α-Fe particle embedded α-Cu matrix along with some scattered γ-Fe grains. The two-dimensional phase-field simulations were employed to reproduced the phase transition and microstructure evolution in different compositions, revealing that the nucleation of γ-Fe occurs synchronously with the spinodal decomposition of liquid and that with further cooling, Cu-rich liquid solidifies and nanoscale spherical γ-Fe precipitates appear. Cross-sections with 10–40 wt.% Cu10Sn were found structurally unreliable owing to the formation of liquation cracks and shrinkage pores, whereas the rest of the build contained few pores and was crack-free. The Scheil solidification simulations together with the crack susceptibility index calculations over the entire compositions range of the CGA, reveal that when with < 50 wt.% Cu10Sn, liquid feeding to interdendritic regions of γ-Fe is limited, which is otherwise necessary to mitigate shrinkage-induced stresses. The hardness, yield and tensile strengths of the compositionally graded alloy in the cross-section with 50 wt.% Cu10Sn are higher than those of the parent constituents and the one with 80 wt.% Cu10Sn has the best combination of strength and ductility. The variations in strength and ductility are attributed to the microstructural and compositional changes in α-Cu matrix that were influenced by the formation of α-Fe particles. Finally, the appropriate proportions and the influence of the LPBF method in successfully mixing Cu10Sn and SS316L was discussed.

Solidification cracking nature and sequence of different stainless steels

For magnesium, nickel, and stainless steel, solidification cracking is a critical failure mode that has been extensively studied for decades. The nature of solidification cracking can be investigated through experiments or theoretical calculations. Different models have been proposed for these calculations, including the cracking susceptibility index (CSI), which was developed in the last decade. Building a CSI map for a group of metals facilitates referencing or comparison when the metal is applied in casting, welding, or developing new alloys. In this study, the CSI of various types of stainless steel was examined, and CSI maps were constructed. For each type of alloy, the calculations include more than ten experimentally determined compositions from commercial alloys. The CSI maps of stainless steel were constructed and compared with those of the Cr and Ni equivalents, as well as their respective phase diagrams. The CSI sequence matched the cracking results of previous studies on austenitic steels. The ferritic type had a lower CSI but a higher cracking rate, which could be attributed to the ductility gap during the austenite-ferrite transformation, as reported in previous studies.

Prediction of the influence of welding metal composition on solidification cracking of laser welded aluminum alloy

The crack length of laser welding 6082 aluminum alloy with three types of 5000 series aluminum alloy welding materials was compared by fishbone test method. It was found that ER5A06 welding material corresponds to the shortest crack length, indicating that the welding material composition has an impact on the welding crack and the crack susceptibility could be controlled by adjusting the welding material composition. Combined with SEM and EBSD, the crack morphology was observed and determined to be solidification cracking. Through EDS, EPMA and thermodynamic calculation, it is found that the main elements affecting solidification cracking are Al, Mg, Si and Cu. The cooling rate of the fusion zone was calculated according to the secondary dendrite arm spacing and welding composition. Compared with casting and arc welding, the laser welding aluminum alloy fusion zone has rapid cooling rate. Due to the rapid cooling rate, the T-fs curves of Al-Mg, Al-Si and Al-Cu binary alloys were calculated by commercial thermodynamic software Thermo-Calc. Furthermore, the cracking susceptibility index (CSI) of binary aluminum alloys were also calculated to determine the element content range of the cracking. To understand the element interaction, both the machine learning and computational thermodynamic methods were employed to predict the influence of multi-component welding metal on solidification cracking susceptibility. Based on the calculation results of solidification cracking susceptibility for binary aluminum alloys, a data set containing 45 groups of aluminum alloy components was established and set as the input dataset. The CSI values corresponding to the compositions in the data set were set as the target and the relationship “welding metal composition - CSI” was founded. Based on the Kriging model, the influence of welding metal composition on solidification cracking susceptibility and the components of most crack susceptible were obtained, which provides a basis for welding materials design.

Effect of Composition on Crack Susceptibility Index of Al-Si and Al-Si-Mg Alloys

In order to reasonably design the composition of Al-Si based alloy and study the effect of alloying elements on hot crck of Al-Si based alloys, the effect of alloying elements on hot cracking of Al-Si based alloy was studied. The influence of different Si contents on the crack susceptibility index (CSI) of Al-Si alloy was calculated by using Pandat, and the influence of Mg addition on the CSI of the Al-Si-Mg alloy was also calculated. The solidification path affecting the CSI of the alloy was analyzed. The results show that when the Si content is below 10% (mass percentage), the CSI is almost non-existent and the Al-Si alloy will not show crack tendency; When the Si content reaches 11~12%, the CSI of the Al-Si alloy is very small. When the Si content reaches the hypereutectic region (Si ≥ 13%), the CSI of the Al-Si alloy begin to increase with increasing of Si content. The crack tendency of the Al-Si alloy in the hypereutectic region is larger. Adding Mg can effectively reduce the CSI of the alloy. Furthermore, with increasing of Mg content, the CSI of Al-Si-Mg alloy gradually decreases.

Microstructure characterization and mechanical properties of crack-free Al-Cu-Mg-Y alloy fabricated by laser powder bed fusion

Additive manufacturing of high-strength Al alloys (such as 2xxx, 6xxx and 7xxx series alloys) by laser powder bed fusion (LPBF) encounters a challenge because of their high susceptibility to solidification crack. The current work develops a crack-free and novel high-strength Al-Cu-Mg-Y alloy fabricated by LPBF using the rare earth yttrium (Y) modified 2024 alloy powders. In the LPBF-fabricated Al-Cu-Mg-Y alloy, the rare earth element Y is revealed as an effective alloying element for the solidification crack elimination, which is mainly attributed to the combination of the narrowed brittle temperature range, the decreased solidification crack susceptibility index and the refined grains. The element Y can also react with Al and Cu to form the Al 8 Cu 4 Y phases during the LPBF process. In the LPBF-fabricated Al-Cu-Mg-Y alloy, the larger number of Al 8 Cu 4 Y phases and the finer grains result in higher compressive yield strength than most LPBF-fabricated Al alloys. Interestingly, the LPBF-fabricated Al-Cu-Mg-Y alloy can be highly deformed without collapse up to a large compressive strain of ~70 %. After T6 heat treatment, the LPBF-fabricated Al-Cu-Mg-Y alloy exhibits a significant increase in the compressive stress after the yield point, corresponding to the homogeneously distributed Ω precipitate in the Al matrix. In addition, the tensile strength is lower than the compressive strength for the LPBF-fabricated and T6 heat-treated Al-Cu-Mg-Y alloys. This difference is related to the internal pores in the two alloys.

Weld Strength and Cracking Susceptibility Analysis of Pulsed TIG Welded Al-Mg-Si Alloy by Experimental Approach

Aluminium is a non ferrous corrosive resistance metal mainly used in automotive coolers, inter coolers and radiators. They are the part of automobile vehicles and made from aluminium alloys. The joints in this application are created by fusion welding process specifically Tungsten Inert Gas (TIG) and Pulsed current TIG (PCTIG) welding process. The working temperature range of different coolers, radiators are 20° C to 300° C and pressure ranges from 2.5 bar to 3.5 bar. During actual working of coolers, the weld joint experiences sudden high level and low level temperature changes. These will create high thermal stresses in the joints. It leads to cracking in the weld region and create failure of the weld joints. Various factors such as mechanical, metallurgical and thermal are responsible for cracking in the weld joints. Range of solidification temperature is one of metallurgical factor, stress generation is mechanical factor and cooling rate of weld metal is thermal factor for cracking. Houldcroft weldability test is employed to identify the cracking susceptibility (CS) of the weld. The current investigation is intended to discover the effect of pulse TIG welding process parameters on the mechanical properties and cracking susceptibility of precipitated aluminium Al-Mg-Si alloy. Diverse pulsed TIG welding process parameters such as peak current (Ip), base current (Ib) and frequency (f) were investigated with the objective of identify the tensile strength, yield strength and cracking susceptibility. The corresponding findings of optimum parameters are 180 peak current (Ip), 60 A base current (Ib) and at 6 Hz frequency (f) with tensile strength of 185.55 MP, yield strength 156.62 MPa. The significant of each parameter for tensile strength are Ip, Ib, Ip*Ib, Ip*f and Ib*f. The corresponding contributions in % are 14.56, 49.49, 12.26, 5.44, 12.15, 5.04, and 1.05 respectively. The statistical method such as Taguchi was employed for experiment design. Weldability test (Fishbone test) was performed on standard specimen and the cracking susceptibility index was identified. The cracking susceptibility index 5.26 % were observed with the value of 180 A of Ip, 80 A of Ib &amp; 2 Hz of frequency (f).The results give an idea about the effect of pulsed TIG welding parameters on mechanical properties and cracking susceptibility.

AZ-Mg Alaşımlarının Katılaşma Çatlama Duyarlılığına Karşı Dolgu Metal ve Alaşım Elamanlarının Etkilerinin Tahmin Edilmesi

Solidification cracking is a concern for welding magnesium (Mg) alloys. An index, the maximum │dT/d(fS)1/2│,was used with the thermodynamic software Pandat to make solidification cracking susceptibility predictions for AZ-Mg arc welds which have the main alloying elements of aluminum and zinc in the magnesium matrix. The effect of AZ101 magnesium filler on reducing crack susceptibility of commercially available AZ31, AZ61 and AZ91 Mg alloys was investigated with the crack susceptibility index. The filler metal AZ101 Mg alloy was found effective to reduce the susceptibility of all the three AZ-Mg alloys to solidification cracking. The influence of the amount of aluminum and zinc in the AZ-Mg alloys on the crack susceptibility was predicted using the cracking index and Pandat based on Scheil solidification model. The predictions based on the index were compared to the experimental crack susceptibility data of the AZ-Mg alloys, and it was seen that the general trend of both predictions and the reported data was consistent with each other. The predictions were explained in the light of the criterion proposed for solidification cracking.

Crack free metal printing using physics informed machine learning

Process parameters and thermophysical and mechanical properties of alloys affect cracking which remains a major challenge in metal printing. Cracks occur because of multiple mechanisms and currently, there is no unified mitigation strategy. Here we evaluate the effects of variables related to the physics of cracking computed by a mechanistic model and independent experimental data using machine learning to prevent cracking. The computed solidification stress, the ratio of the vulnerable and relaxation times, ratio of the temperature gradient to solidification growth rate, and cooling rates and experimental data are used to generate a cracking susceptibility index that predicts crack formation before printing. Computed values of these four variables when used in a decision tree, support vector machines, and logistic regression can predict crack formation with exceptional accuracy. Information gain, information gain ratio, and Gini index-based feature selection calculations provide the same comparative influence of these four variables. Results are presented as easy-to-use cracking susceptibility maps. Our approach can help in process optimization, designing new alloys, and solving problems in manufacturing beyond metal printing.

Back diffusion resisting solidification cracking in austenitic stainless steels

Austenitic stainless steels resist solidification cracking much better if ferrite (δ), not austenite (γ), is the primary solidification phase, but why? The curve of temperature T vs. fraction of solid fS was calculated for 304, which solidifies as primary δ-ferrite, and for 310, which solidifies as primary γ. Back diffusion, being much faster in bcc (δ) than in fcc (γ), flattens the curve much more in 304 than 310. Consistent with quenched 304 mushy-zone microstructure, back diffusion narrows the freezing temperature range, depletes the residual liquid, and lets δ dendrites bond together earlier to better resist cracking. It also reduces the crack susceptibility index |dT/d(fS )1/2| near fS = 1, consistent with the much less cracking observed in 304 than 310.

Study on hot cracking susceptibility of alloy 617M and an analysis on parameters used for its evaluation

Hot cracking study on nickel base superalloy, alloy 617M, a candidate material for Advanced Ultra Supercritical (AUSC) power plant applications, was carried out. The susceptibility of this alloy to solidification cracking was evaluated using varestraint (variable restraint) test set up and parameters like Brittleness Temperature Range (BTR) and Critical Strain rate per Temperature drop (CST) were estimated from the results. The BTR and CST for alloy 617M indicate higher susceptibility to solidification cracking than stabilized and un-stabilized austenitic stainless steels. However, mock-up welding of 400 mm diameter alloy 617M hollow forgings with weld thickness of 95 mm and alloy 617M boiler tubes of ~12 wall thickness made from this alloy did not reveal any significant cracking in the welds (except some isolated micro fissures) and hence, a detailed analysis of the results from varestraint tests was taken up. The study revealed the dual hot cracking behaviour of alloy 617M and the reasons behind such observation is elucidated. The microstructural characterisation of the hot crack tested specimens of alloy 617M revealed eutectics of austenite + carbides rich in Mo and Cr in the backfilled regions of the hot cracks in the fusion zone. The formation of significant fraction of eutectic phases along the interdendritic regions at the end of solidification is the primary reason for the high BTR exhibited by alloy 617M. Further, backfilling of the solidification cracks by this eutectic liquid is enabled under low augmented strain and the same is insufficient to aid backfilling in cracks at high restraints. The mechanism mentioned above was found to be the underlying reason for the dual behaviour revealed by alloy 617M. Hence, it was deduced that BTR and CST determined at saturated strain cannot be effectively used to predict the actual hot cracking behaviour of alloy 617M. A refined criterion-BTR at threshold strain is proposed to be an appropriate solidification cracking susceptibility index for this alloy.

Design, microstructure and thermal stability of a novel heat-resistant Al-Fe-Ni alloy manufactured by selective laser melting

Selective laser melting (SLM) technology is promising to fabricate complex-shaped metal components, especially light-weight aluminum (Al) alloy. Facing the grow requirement of thermal stability for applications in aviation and aerospace at high temperatures, the poor thermal stability of Al alloy needs to be improved. Thus, in this study, we developed a novel heat-resistant eutectic Al-Fe-Ni alloy which is suitable for the SLM process. Firstly, the composition of Al-1.75Fe-1.25Ni (wt%) alloy was designed and confirmed through the calculation of hot crack susceptibility index |dT/d(fs0.5)| and the analysis of the as-cast microstructure. Then, the designed alloy was fabricated using the optimized SLM processing parameters. Finally, the effects of heat treatment on microstructure evolution and phase stability during thermal exposure were investigated. Results show that the SLMed alloy is consisted of α-Al and cellular eutectic Al9FeNi phases. The fine grain structure and nano-sized Al9FeNi phase of the SLMed sample result in high hardness which is more than twice higher than that of the as-cast sample. During long-term thermal exposure at 300 °C, the fine grain structure and cellular eutectic phase remain almost unchanged, and thus, the hardness of the alloy can be kept stable. When the exposure temperature further increased, the fine grains gradually grew up and the cellular Al9FeNi phases were gradually collapsed and coarsened. Heat treatment at such high temperature (above 400 °C) resulted in microstructure coarsening and formation of large-sized rod-like or spherical phases, which significantly degraded the hardness. The novel heat-resistant Al-Fe-Ni alloy together with the alloy composition design method provides new insight into light-weight and complex-shaped components produced by the SLM process for high-temperature applications.

Solidification cracking susceptibility associated with a teardrop-shaped weld pool

For straight columnar dendritic grains growing side by side, |dT/d(fS )1/2| (T: temperature; fS : fraction solid) near their roots can be the crack-susceptibility index as verified against welds made with an elliptical weld pool. This study showed the index is also valid for welds made with a teardrop-shaped pool. In both cases, at the mushy-zone centreline, the tips of such grains grow and finally become the roots as the mushy-zone moves forward. This validity is consistent with: (1) the crack-susceptibility ranking of alloys in arc welding (typically elliptical pool) being similar to that in laser- or electron-beam welding (typically teardrop-shaped pool), and (2) the ranking calculated based on the index being consistent with that observed in laser- or electron-beam welding.

Solidification cracking susceptibility of quaternary aluminium alloys

ABSTRACT The solidification cracking susceptibility was calculated for quaternary Al–Si–Mg–Cu, Al–Zn–Mg–Cu and Al–Li–Mg–Cu alloy systems. It was calculated based on the crack susceptibility index |dT/d(fs ) 1/2 |, where T is temperature and fs the fraction solid. Three different levels of back diffusion were considered: no diffusion, diffusion under a cooling rate of 100 oC/s and diffusion under 20 oC/s. The calculated results were first validated by experimental data previously reported, and then extended to the composition ranges where no experimental data are available. For each alloy system, the crack susceptibility was calculated for 132651 Al alloys, with six crack susceptibility maps covering 15606 Al alloys at each diffusion level. These calculated results can be a database useful for welding, casting and additive manufacturing.

A novel crack-free Ti-modified Al-Cu-Mg alloy designed for selective laser melting

Abstract A novel crack-free Ti-modified Al-Cu-Mg alloy for SLM was developed here, based on the thermodynamic calculations of the crack susceptibility index and growth-restriction factor. We found that the introduction of Ti into the Al-Cu-Mg alloy effectively promoted the grain refinement and columnar-to-equiaxed grain transition as a result of the heterogeneous nucleation provided by Al3Ti precipitates. The hot tearing cracks were eliminated after Ti modification due to the formation of the homogeneous and fine equiaxed microstructure. We created a new high-strength Al-Cu-Mg-Ti alloy with a tensile strength of 426.4 MPa, yield strength of 293.2 MPa and ductility of 9.1%. This novel Ti-modified Al alloy with fine equiaxed grains and highly-enhanced mechanical properties offers a new compositional space for the printable lightweight material categories specifically for the SLM technique.

Hot‐cracking susceptibility of fully austenitic stainless steel using pulsed‐current gas tungsten arc‐welding process

AbstractThe pulsed‐current gas tungsten arc‐welding process (PCGTAW) has recently been recommended as an approach to eliminate the high susceptibility of austenitic stainless steel to hot cracking. Within a wide range of variables, in the present study, the effects of pulse frequency, time ratio, and pulse current on the cracking susceptibility of austenitic stainless‐steel thin sheets are investigated. A fan‐shaped cracking test specimen is adopted, and suitable final dimensions are obtained by conducting some preliminary experiments. A good correlation is found between the maximum crack length, which is taken as a cracking susceptibility index, and the pulse form variables. The results are found to be significantly improved when compared with those of the continuous‐current process, which indicates that the PCGTAW process has an effective role in reducing the hot‐cracking susceptibility. The optimal conditions of the pulse form variables based on the hot‐cracking susceptibility are determined. The optimal values of the time ratio are found to be less than 50%, together with a pulse frequency of less than 1 Hz. Some new microstructure measures, such as columnar structure ratio, grain orientation angle, puddle diversion angle, and overlapping ratio, which are originally defined in this work, in addition to grain size and puddle form factor (aspect ratio), are used to interpret the hot‐cracking behavior. All are found to have a direct impact on the hot‐cracking susceptibility. The results obtained from microstructure examinations showed that the cracks formed are either intergranular or transgranular cracks.