Hot Cracking Research Articles

An optimal control problem for single-spot pulsed laser welding

We consider an optimal control problem for a single-spot pulsed laser welding problem. The distribution of thermal energy is described by a quasilinear heat equation. Our emphasis is on materials which tend to suffer from hot cracking when welded, such as aluminum alloys. A simple precursor for the occurrence of hot cracks is the velocity of the solidification front. We therefore formulate an optimal control problem whose objective contains a term which penalizes excessive solidification velocities. The control function to be optimized is the laser power over time, subject to pointwise lower and upper bounds. We describe the finite element discretization of the problem and a projected gradient scheme for its solution. Numerical experiments for material data representing the EN AW 6082-T6 aluminum alloy exhibit interesting laser pulse patterns which perform significantly better than standard ramp-down patterns.

Hot cracking susceptibility prediction from quantitative multi-phase field simulations with grain boundary effects

Hot cracks are notorious defects in casting, welding, and additive manufacturing. Grain boundaries play a key role in hot cracking formation. Still, their effects are often neglected in hot cracking susceptibility predictions, and thus a quantitative picture of hot cracking susceptibility and liquid fracture is still lacking. In this work, we construct a multi-order parameter phase-field model for binary alloy solidification to simulate attractive, neutral, and repulsive grain boundaries in the thin-interface limit. Phase-field simulations of different combinations of grain boundary and bi-crystal growth types are used to obtain solidification path data, which is then used as the input into the Rappaz, Drezet and Gremaud (RDG) model (Rappaz et al., 1999). We study the so-called Λ-shape variation of hot cracking susceptibility as a function of solute concentration, showing that the magnitude of the Λ curve peak shifts to higher values and to lower concentrations as grain boundary energy increases. Meanwhile, the peak magnitude becomes higher and shifts to a higher value and higher concentrations when convergent grain growth is applied. Furthermore, in all cases examined, the corresponding pressure drops predicted for hot cracking exceed the theoretical liquid rupture stress, consistent with experimental observations. These results demonstrate that quantitative phase-field simulations, when coupled with the RDG model, are capable of relatively accurately predicting liquid rapture states relevant to hot cracking since the large disparity between the predicted pressure drop and critical stress of liquid film is eliminated through the consideration of grain boundary effects in the solidification path calculation.

Crack-free and high-strength AA2024 alloy obtained by additive manufacturing with controlled columnar-equiaxed-transition

Laser powder bed fusion (LPBF) of high-strength Al alloys is challenging due to the formation of both hot and cold cracks. In the present work, highly dense and crack-free AA2024 samples could be additively manufactured via inoculation treatment of Zr-based metallic glass (MG) powders. The columnar grains in the LPBF-fabricated AA2024 alloy were significantly refined and almost completely transformed to the equiaxed grains with a bimodal grain size distribution consisting of ultrafine grains with a size smaller than 1 μm and relatively coarser grains. The grain refinement can be associated with the formation of Al3Zr particles, serving as the heterogeneous nucleation sites for the α-Al matrix. Complete routes for columnar-equiaxed-transition (CET) have been revealed by tailoring the concentration of nucleation particles and solidification conditions. CET occurs both at the melt pool boundary due to the sufficiently high concentration of Al3Zr particles and at the topmost of the melt pool due to the heterogeneous nucleation driven by constitutional undercooling. Between these two regions, columnar grains epitaxially grow with orientations determined by the thermal gradient. The as-built Zr-based MG inoculated AA2024 specimens are robust in healing hot cracks due to a more tortuous propagation path of cracks for equiaxed grains. The as-fabricated AA2024/5% MG specimens exhibit a high ultimate tensile strength of 531 MPa due to crack elimination and grain refinement, surpassing most of the reported values for the LPBF-fabricated AA2024 alloy inoculated with other inoculated powders. The present work could provide a novel inoculation agent to fabricate high-strength Al alloys and the CET can be used to precisely control the grain morphology.

Crack formation of camshaft castings: Hot tearing susceptibility and root causes

As a widely used cyclical motion component, the camshaft plays a key role in the accuracy and stability of the whole machine. Micro hot cracks in the camshaft casting process are an important cause of failure in subsequent machining and operation. In this paper, the hot tearing susceptibility of the camshaft in the casting process was studied using the alloy composition (carbon equivalent and Mn/S ratio) and mold preheating temperature. The formation mechanism of the hot tear source was analyzed through the microscopic morphology of the fracture-sensitive cross-section. Using the improved Clyne-Davies model and the newly proposed hot tearing susceptibility (HTS) model, the results show that as the carbon equivalent and Mn/S ratio increase, the hot tearing susceptibility of the camshaft first decreases and then increases. The hot tearing tendency of the casting can be reduced by appropriately increasing the mold preheating temperature. The microscopic morphology indicates that the initiation and propagation of the crack source are caused by the excessively serious non-equilibrium solidification of the camshaft metal fluid. Finally, potential countermeasures are proposed to decrease micro hot cracks in camshaft castings, including the adjustment of alloy composition, optimization of casting process parameters, and improvement of system solidification conditions.

The role of TiH2 on microstructure and mechanical properties of Al–Zn–Mg–Cu alloy fabricated by laser powder bed fusion

In this study, a crack-free 7075 aluminum alloy composed of fine, randomly-oriented and equiaxed grains ∼1 μm in size was successfully prepared by laser powder bed fusion (L-PBF). TiH2 particles were introduced to achieve significant grain refinement, serving as a critical method to relieve the high residual stress and prohibit inherent hot cracking during L-PBF manufacturing. Multiscale microstructural characterizations were applied to reveal the mechanisms of grain refinement and elimination of hot cracking. On the one hand, in-situ produced L12-structured Al3Ti particles stimulated heterogeneous nucleation of α-Al. On the other hand, Ti solute atoms played an important role in restricting grain growth during solidification. A conventional T6 treatment was conducted to modify the microstructures and mechanical properties of the alloy. E-Al18Mg3(Ti,Cr)2 dispersoids were precipitated, playing a role in strengthening through pinning grain boundaries. Thanks to grain boundary strengthening and Orowan dislocation strengthening arising from η' precipitates, Al3Ti particles and E dispersoids, the yield strength, ultimate tensile strength and elongation of T6 treated 7075-Ti alloy are 496 ± 10.8 MPa, 537 ± 7.6 MPa and 7.2 ± 0.3 %, respectively. These findings provide a broader insight into the use of TiH2 to improve the microstructure and mechanical properties of L-PBF-fabricated high-strength aluminum alloy.

Study on nano-treating of Al-Mg-Si-Cu alloys with TiC nanoparticles

The 6xxx series wrought aluminum alloys (Al-Mg-Si-Cu) provide good formability and high strength potential, especially with high Cu additions. However, the large freezing ranges of these alloys often cause hot cracking susceptibility in solidification-based processes, such as casting, welding, and 3D printing. In this work, nano-treating, which is to tune the manufacturability and microstructure of metals by a small addition of nanoparticles, was applied to Al-Mg-Si-Cu alloys during casting. This study shows that nano-treating can effectively refine microstructure, eliminate hot cracking, and promote the dissolution of alloying elements. Tensile tests showed that nanoparticles enable strengthening. An extensive study on the TiC nanoparticle effect on Al-Mg-Si-Cu with various Mg and Si contents was conducted. This work provides valuable insights for rational design of nano-treated Al-Mg-Si-Cu alloys to achieve optimal combination of strength and ductility.

Processing of Haynes® 282® Alloy by Direct Energy Deposition with Arc and Wire

Direct energy deposition with arc and wire (DED-AW) is a versatile, low-cost, and energy-efficient technology for additive manufacturing of medium- and large-sized metallic components. In this study, the effects of arc energy and shielding gas in cold metal transfer (CMT) welding of walls and blocks on cooling time, mechanical properties, and macro- and microstructure have been studied using precipitation-hardenable Ni-based superalloy Haynes® 282®. The arc energy and consequently the cooling rate were varied by changing the wire feed rate and the travel speed. As expected, increasing the arc energy leads to higher cooling times for the walls. Due to the 2D thermal conduction, the thin walls cool down much slower than multi-layer welded blocks, but this reduces the strength values only very slightly. While the walls have no sensitivity to the occurrence of unacceptable seam irregularities, the multi-layer blocks show isolated seam defects, such as hot cracks or lack of fusion. Despite shielding gas variation, the as-welded blocks show acceptable strength properties at room temperatures (RT) and impact values at RT and -196 °C. However, the use of an N-containing shielding gas results in lower elongation and notched bar impact energy. Precipitation-hardened specimens tested at 871 °C exhibit a similar strength level to transverse tensile specimens of gas metal arc welding (GMAW) welded joints on 12.7 mm thick plates with fracture in the weld metal.

Effect of Process Parameters on Arc Behavior and Weld Formation in Weaving Gas Tungsten Arc Welding

The arc welding with weaving has been used widely to obtain better weld quality by avoiding lack of side wall fusion and improve the weld efficiency by obtaining the wide weld. But the effect of weaving process parameter on the weld is not clear. The aim of this work is to study the effect of process parameters on arc behavior and weld formation in Weaving-Gas Tungsten Arc Welding (W-GTAW), those parameters include welding current, tungsten electrode height from the electrode tip to upper surface of workpiece, weave angle, weave speed, and weave stop time on the left and right sides. The instantaneous arc shape and electrical signal data were collected by high-speed camera and electrical signal acquisition system, respectively. Furthermore, the weld morphology was also systematically analyzed. This result shows that the bottom surface radius of the arc changed with weaving in the W-GTAW. When the weave speed increased to 0.40 × 10−1 rad/s, the change of the radius was the least, with only 0.10 mm drift, and the difference between the arc forces in the middle and the two sides of the molten pool was smallest in all experiments. Compared with the welding stability of each process, decreasing the tungsten electrode height, weave angle and speed could significantly enhance the welding stability. The forming coefficient of weld with a weave angle of 1.9° was 3.11. It shows that increasing reasonably the weave angle and speed can increase the weld penetration and might help reduce stress concentration and hot crack tendency of the weld. When the welding current is reduced to 130 A for W-GTAW welding, the hardness transition from base metal (BM) zone to weld zone (WZ) is more gentle. Furthermore, the W-GTAW technology shows great application potential in weld forming control by adjusting process parameters.

On the optimal process window for powder-based laser-directed energy deposition of AA7050 under different robot programs and scanning strategies

Powder-based laser-directed energy deposition (L-DED) has drawn lots of attention during the last years because of the multiple application areas, such as coating, repairing, and building 3D structures. One challenge in L-DED is the limited variety of applicable materials, especially in the case of aluminum alloys. To broaden the material spectrum, the processability and the optimal process window of high-strength Al-alloy AA7050 are investigated in the present study. Besides the process parameters, special emphasis was paid to the scanning strategy and the robot program controlling. The present results show a strong effect of scanning strategy and robot program on buildability, geometrical accuracy, melt pool visibility, porosity level, and crack initiation. Hot cracks can be reduced or eliminated by choosing an appropriate combination of scanning strategy and robot program. Meanwhile, the geometrical accuracy of specimens with any height was well maintained. Based on the systematic experimental study, appropriate L-DED process parameters were identified for the deposition of structures up to 24 layers with a lower porosity level (about 3.7 ± 0.8 %) and lower amount of hot cracks, which are comparable with that in the 12-layer structure.

Effect of the Laser Cladding Parameters on the Crack Formation and Microstructure during Nickel Superalloy Gas Turbine Engines Repair

Cracking of nickel superalloys with a high content of γ’-phase remains an unresolved problem, including in technologies for repairing gas turbine engines blades. Laser cladding is a method of material deposition used to repair parts exposed to aggressive environment and surface wear. Cladding parameters have a high influence on cracking susceptibility nickel superalloys. Alloy ZhS32 has a high propensity for hot cracking when exposed to laser radiation. In this work, the study of the structural and phase features of ZhS32 alloy was carried out. A high tendency to form segregation of refractory elements and carbides in the intergranular areas was found. The features of the structure and phase composition of the material for different cladding parameters were studied. The main contribution of technological parameters to the formation of cracks is shown.

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.

New alloy design approach to inhibiting hot cracking in laser additive manufactured nickel-based superalloys

Avoiding the formation of cracks to ensure a reliable printability and a good stability is crucial in the laser additive manufacturing of alloys. Contrary to previous studies that have generally tried to decrease the liquid film and solidification range, in this work, we innovatively utilized segregation engineering and abundant cell boundaries to introduce liquid backfilling as well as a network of segregation phases to alleviate thermal stress, consequently eliminating hot cracking. More specifically, zirconium was introduced into a nickel-based superalloy to form a continuous interdendritic liquid film during the laser additive manufacturing process. It was found that the continuous intermetallic Ni11Zr9 segregation phase decorated the cell boundaries, and cracks were completely eliminated in the as-printed Haynes 230 alloys when their Zr content reached 1 wt.%. Moreover, this continuous Ni11Zr9 network layer was able to act as a “skeleton” to significantly improve the yield strength of the as-printed samples. Following appropriate heat treatment, these Zr-modified Haynes 230 alloys exhibited an extraordinary combination of strength and plasticity, which were superior to those of the previously reported Haynes 230 alloy. These findings provide a new alloy design route for the laser additive manufacturing of crack-free alloys with excellent mechanical properties.

Fabrication of high-performance CoCrNi medium entropy alloy by laser powder bed fusion: The effect of grain boundary segregation

A CoCrNi composite material with 3 wt% TiC was successfully fabricated by laser powder bed fusion (LPBF). With increasing content of TiC particles, C gradually segregated along the grain boundaries (GBs). The segregation of C improved the grain coalescence, intensified the attraction of the columnar grains, and increased the consumption of the liquid film. Furthermore, the segregation of C led to the formation of Cr23C6 and core–shell structure (TiO2/TiC) nanoprecipitates along the GBs. These precipitates reduced the mobility of the boundaries and restricted the grain growth along the direction of the maximum thermal gradient. Reinforcing the CoCrNi microstructure with 3 wt% TiC yielded optimal results, as the resultant composite microstructure was free of hot cracks, consisted of small grains, and exhibited a more random texture. The high density of wide stacking faults and Lomer–Cottrell locks hindered cross-slip during further deformation at ambient temperature, and twins dominated at elevated temperatures. These nanoprecipitates hindered dislocation propagation at all temperatures. Therefore, the synergistic strengthening effects from the different types of dislocations and nanoprecipitates endowed the CCN-3M alloy with excellent mechanical properties. Ultimate tensile strengths of approximately 1.34 and 0.88 GPa, yield strengths of approximately 1.02 and 0.75 GPa, and elongations of approximately 15.6% and 25.3% were observed at ambient and elevated temperatures (700 °C), respectively. These results indicate that GB segregation is an effective method for suppressing the evolution of hot cracks, thus expanding the scope of LPBF-fabricated components for industrial applications.

The strengthening mechanism synergy of heat-treated 3D printed Al-Sc alloy

ABSTRACT According to the material nature, aluminium alloys are widely applied in aerospace, construction and automotive applications due to their characteristics, such as lightweight, good formability and good corrosion resistance. Among the aluminium alloys, scalmalloy (Al-4.49Mg-0.71Sc-0.51Mn-0.27Zr-0.07Fe-0.03Si alloy) was developed to overcome the hot crack issue during the laser powder bed fusion (LPBF) process. Hence, the degree of lightweight can be further improved by introducing this high-specific strength material with a structure of the lightweight design. However, the strengthening mechanism of the heat-treated 3D printed scalmalloy has not been sufficiently explored. In this study, the synergistic effect of the strengthening mechanisms is explored through detailed microstructure analysis. The grain size, size and spacing of the precipitate and coherent phase contribute to the strengthening of scalmalloy. Through the observation of the microstructure feature, the theoretical strength of the heat-treated 3D printed scalmalloy can thus be calculated by three strengthening mechanisms and match the experimental results perfectly.

Influence of heat input on hot cracking sensitivity of the EA395-9 filler metal

Abstract Microcracks are always present in the deposited metal of 10MnNi2MoVA steel weld seam, which affects the safety of the power plant steam pipeline. It is important to investigate the effect of heat input on the hot cracking sensitivity of the EA395-9 weld metal. In this work, the deposition of the weld metal of EA395-9 was prepared, then the hot cracking tendency of the weld with the different heat input was analyzed and observed. The results show that the moderate welding heat input can help to avoid hot cracking.

Increasing the build rate of high-strength aluminium alloys produced by laser powder bed fusion

Aluminium alloys from the 2xxx series represent important structural materials due to their optimal combination of strength and corrosion resistance. However, their production by means of laser powder bed fusion (PBF-LB/M) is hampered by the presence of detrimental pores and hot cracks. These defects can be mitigated with appropriate process parameters investigations, often at the expense of the build rate.In this study, a combination of modelling and experimental work is adopted to predict the consolidation behaviour of AA2024, a well-known high-strength Al alloy, in regimes characterised by a progressive increase of build rate. The melt pool geometries are predicted as a function of various process parameters using the Rosenthal solution and variable values of absorptivity. This leads to a facile production of near-full dense crack-free parts at an increasing build rate. The analysis of the specimens produced with such processing parameters reveals that the microstructural features and mechanical properties of AA2024 are largely preserved when build rate is increased.Our approach can be easily implemented in standard PBF-LB/M systems as it does not rely on machine adaptations. Therefore, our new proposed approach might result in a practical solution to increase the productivity of other high-strength Al-alloys in industrial settings.

Design high-strength Al–Mg–Si alloy fabricated by laser powder bed fusion: Cracking suppression and strengthening mechanism

Considerable researches on laser powder bed fusion (LPBF) have proved the high crack susceptibility of aluminum alloys and the necessity to increase the processability of LPBF for broader applications. To eliminate the hot cracking and achieve high performance, the present work devised a novel component with the advantages of superior processability, refined microstructure, and high strength based on theoretical calculation of crack susceptibility. The addition of 4.4 wt% Mg and 3.1 wt% Si was capable of obtaining the relative density of 99.6% and eliminating the hot cracks under the processing parameters of 300 W-1000mm/s-0.10 mm. This LPBF fabricated alloy formed granular Mg2Si and rod-shaped Si phases, and an ultimate tensile strength of 460 MPa, yield strength of 385 MPa, and elongation of 3.2% were obtained. In this work, hot cracks could be solved by composition adjustment without adding any nano nucleating agent. Detailed strengthening mechanism analysis revealed the source of high strength corresponding to refined microstructure, strengthening phase, and composition design. This work guides for defects suppression and composition design in additively manufactured high-strength Al–Mg–Si alloys and other aluminum alloys.

On the Si-induced microstructure evolution, solidification cracking healing and strengthening behavior of laser powder bed fusion additive manufactured Al-Cu-Mg/Si alloys

Severe hot-cracking behavior is a crucial challenge for the further development of high-strength aluminum alloys processed by laser powder bed fusion (LPBF). In the present study, compositional modification with addition of 2.04 wt% Si to Al-Cu-Mg alloy was performed to avoid hot cracking and enhance tensile performance. The effect of Si on hot-cracking resistance, microstructure evolution, interactional behavior with Cu and Mg, and precipitation behavior was explored. The LPBF processing window of the Al-Cu-Mg alloy is enlarged after introducing 2.04 wt% Si, and the threshold laser scanning speed for hot cracking in the Si-modified Al-Cu-Mg alloy increases to 400 mm/s. The trapping behavior of Cu in α-Al matrix and segregation of Mg into interdendritic regions occur in the solidification process because of the Si-induced variation in kCuR and kMgR. The addition of Si refines the columnar grains by increasing the value of growth restriction factor (Q) and promotes the formation of Mg2Si at grain boundaries, both of which are conducive to improving the hot-cracking resistance. For the Si-modified Al-Cu-Mg alloy, a high volume fraction of S′ phase because of the accelerated precipitation kinetics and novel Q′ phase obviously enhanced the tensile performance. Moreover, the trade-off behavior of the Al-4.41Cu-2.42Mg alloy was broken once 2.04 wt% Si was added, with the yield strength (YS) and elongation (EI) increasing from 234.7 ± 9.6 MPa, and 2.6 ± 0.1% to 320.1 ± 14.3 MPa, and 4.1 ± 0.7%, respectively.

Influence of surface roughness on fatigue performace of additively manufactured metal matrix composite Al2024–RAM2 alloy

Numerous studies on Laser Powder Bed Fusion (L-PBF) have demonstrated the necessity to refine microstructure of additive manufactured (AM) parts to eliminate hot cracking and anisotropy to improve L-PBF processability. It has been shown that ceramic nanoparticles based on Ti can be extremely effective inoculant resulting in fine grains structure for high strength aluminum alloys without hot cracking. The company ELEMENTUM 3D has patented production process of reactive additive manufacturing (RAM) with a low content of ceramic nanoparticles as Al2024-RAM2. This contribution presents results of an experimental determination of high cycle fatigue life of L-PBF Al2024-RAM2 alloy with as-built (i.e., rough) and polished surface. Experiments were done on miniature specimens printed on a SLM 280 HL system with a layer thickness of 60 µm. The specimens were tested in cyclic plane bending at a load ratio of R = 0 at a frequency of 25 Hz. Moreover, the microstructure was investigated by means of scanning and transmission electron microscopy.

Efficiency of using heat-insulating mixtures to reduce defects of critical parts

A study was conducted to reduce defects in especially critical cast large-sized parts of freight car bogies. The side frame was used as the part under study. The main factors influencing the formation of hot cracks in large steel castings are presented. To avoid the rejection of steel castings, the use of heat-insulating mixtures to reduce hot cracks has been investigated. The temperature distribution of the melt along the height of the ladle is modeled. An analysis of the influence of the order of pouring molds and pouring temperature on the number of hot cracks in the side frames is presented.