Search

Purpose High temperature gradient induces high residual stress, producing an important effect on the part manufacturing during laser powder bed fusion (LPBF). The purpose of this study is to investigate the effect of the molten pool mode on the thermal stress of Ti-6Al-4V alloy during different deposition processes. Design/methodology/approach A coupled thermal-mechanical finite element model was built. The developed model was validated by comparing the numerical results with the experimental data in the maximum molten pool temperature, the molten pool dimension and the residual stress described in the previous work. Findings For the single-track process, the keyhole mode caused an increase in both the maximum stress and the high-stress area compared with the conduction mode. For the multitrack process, a lower tensile stress around the scanning track and a higher compressive stress below the scanning track were found in the keyhole mode. For the multilayer process, the stress along the scanning direction at the middle of the part changed from tensile stress to compressive stress with the increase in the deposition layer number. As the powder layer number increased, the stress along the scanning direction near the top surface of the part decreased while the stress along the deposition direction obviously increased, indicating that the stress along the deposition direction became the dominant stress. The keyhole mode can reduce the residual stress near the top of the part, and the conduction mode was more likely to produce a low residual stress near the bottom of the part. Originality/value The results provide a systematic understanding of thermal stress during the LPBF process.

Laser powder bed fusion (LPBF) can be directly used to fabricate the metal components with complex structures and high performances. However, the extremely high temperature gradient induces the high residual stress, which leads to deformation or even cracking during the process. A layer-by-layer model was developed to investigate the residual stress of the typical part fabricated by LPBF. The maximum stress was found at the position where the part corner joined with the substrate. The second-largest stress occurred at the top center for the solid parts while occurred at the top center of the long side for the hollow and semi-hollow parts. The residual stress levels of the hollow and semi-hollow parts were obviously less than that of the solid parts, and the residual stress at the middle of the inclined-wall part is lower than that of the thin-walled part. Using the rounded corner instead of the sharp corner and using the arc structure instead of the straight-line structure were helpful in reducing the residual stress in LPBF. The simulated results were in good agreement with the experimental data obtained by the hole-drilling method. The investigated results of this paper can be used to reduce the residual stress in LPBF.

The large thermal stress caused by the high temperature gradient leads to part deformations which are the important factor restricting the rapid development and wide application of the laser powder bed fusion (LPBF) technology. In the present study, a layer-by-layer model based on the finite element method was developed to study the thin-walled part deformation and the effect caused by the part dimension during LPBF of Ti–6Al–4V alloy. The deformation along the length direction dominated for the thin-walled parts, and the maximum deformation occurred at the middle of the thin-walled part. A mechanism built in the present study revealed the deformation evolution of the thin-walled part. With the increase of the part height and length, the maximum deformation increased first and then tended to be stable. However, the maximum deformation decreased with an increase of the part thickness. Furthermore, a method of compensation design was used to effectively reduce the deformation of the thin-walled part. The simulated results and the deformation control method were validated by the experimental data. The investigated results of this research provide a significant understanding of the part deformation during the LPBF process.

The influence of the defocusing distance on melt pool morphology, defect, microstructure and mechanical properties of the laser powder bed fusion (L-PBF) of high strength Al–Cu–Mg–Mn alloy was stud...

Selective laser melting (SLM) was employed to fabricate the aluminum alloy AlCu5MnCdVA in this work. The microstructure, mechanical behavior, anisotropy, and strengthening mechanisms were investigated. The results show that the main phases consist of α-Al and Al2Cu. In addition, the supersaturated solid solution with fine columnar grain, high dislocation density and weak texture has been obtained. There is little difference in mechanical properties when the scanning velocity changes if dense samples are obtained. The yield strength, the ultimate strength (UTS) and the elongation are 145.5–157.6 MPa, 309.1–319.7 MPa, 11.44%–14.34%, respectively. The anisotropy is weak, which results from the weak texture. The horizontal specimens show a little higher yield strength and lower elongation. The UTSs of the samples are nearly the same. The strongest anisotropy results from the lowest scanning velocity. The solid solution strengthening, grain boundary strengthening, dislocation strengthening, precipitate and dispersoid strengthening contribute to the strength of the specimens. The quantitative analyses show that all the four strengthening mechanisms dominant in the SLMed AlCu5MnCdVA alloys.

Purpose Selective laser melting (SLM) is a promising additive manufacturing technology in the field of complex parts’ fabrication. High temperature gradient and residual stress are vital problems for the development of SLM technology. The purpose of this paper is to investigate the influence of substrate characteristics on the residual stress of SLMed Inconel 718 (IN718). Design/methodology/approach The SLMed IN718 samples were fabricated on the substrates with different characteristics, including pre-compression stress, materials and pre-heating. The residual stress at the center of the top surface was measured and compared through Vickers micro-indentation. Findings The results indicate that the residual stress reduces when the substrate contains pre-compression stress before the SLM process starts. Both substrate thermal expansion coefficient and thermal conductivity affect the residual stress. In addition to reducing the difference of thermal expansion coefficient between the substrate and the deposited material, the substrate with low thermal conductivity can also decrease the residual stress. Substrate pre-heating at 150°C reduces nearly 42.6 per cent residual stress because of the reduction of the temperature gradient. Originality/value The influence of substrate characteristics on the residual stress has been studied. The investigation results can help to control the residual stress generated in SLM processing.

Laser remelting (LR) is often used during selective laser melting (SLM) processes to improve the densification degree and top surface quality of the products. However, researches regarding its effects on side surface quality, residual stress, microstructure, and mechanical property are still quite lacking. The influence of the repeated usage of LR on a solidified layer is also not very clear. To address these issues, LR treatments with the cyclic numbers of 1–3 on every solidified layer have been employed during the SLM processes of a Ti-5Al-2.5Sn alloy in this study and their influences on deposition quality, residual stress, microstructure, and mechanical property were researched. The results first indicated that although the improvement in top surface quality and densification degree would be more and more notable with the increase of LR cycles, the side surface quality could not be improved through all the LR treatments. Then, it was shown by the hole-drilling tests on the surface centers of several 15 × 15 × 3 mm3 thin-plate specimens that when 1 cycle of LR was conducted on every solidified layer, the principal residual stress at the hole bottom of the SLM sample could be far beyond that of the non-treated one. In contrast, when 2 or 3 cycles of LR were conducted, the measured principal residual stresses could be reduced to lower than that of the non-treated ones. It was also proved that unlike the non-treated sample which presented a martensite-dominated microstructure with a very weak texture, all the LR-treated samples exhibited two kinds of preferential orientations. Finally, tensile properties of the stress-relieved samples were tested and it was seen that under the combined influence of the densification and the microstructure factors, the elongations of the LR-treated samples became slightly higher than that of the non-treated one whereas the tensile strengths were nearly identical. In summary, this paper demonstrates that there is still limitation in using LR as an auxiliary process of the SLM production of titanium products. The possible increase in residual stress and the formation of strong textures should be particularly considered.

Selective laser melting (SLM) produces a large amount of residual stress which limits its applicability. Despite the increasing number of studies to understand the relationship between the process parameters and the residual stress, the underlying complex mechanisms are yet to be understood. In this work, a 3D finite element model (FEM) is established to investigate the relationship between the process parameters and the residual stress during SLM. When the process parameters are in different ranges, the residual stress behavior is different. The complex relationship between the residual stress and the process parameters could be the reason for the contradictory parametric effects reported in the literature. The magnitude of the parametric effect on the residual stress is obtained. The influence of the scanning speed is stronger than that of the laser power, which is in turn stronger than that of the hatch spacing. These results can bring insights into the understanding of the residual stress during SLM.

A 3D thermo-mechanical finite element model was developed to study the effect of the laser jump speed on the temperature distribution and thermal stress in laser powder bed fusion (LPBF) of Ti6Al4V alloy. Gradient grid and gradient time step were used to further improve solution efficiency. It was found that the temperature of the scanned region rapidly increased as the laser jump speed increased from 100 mm/s to 1000 mm/s. The temperature distribution reached a stable state when the laser jump speed was above 5000 mm/s. The pre-heat peak, the heat peak and the post-heat peak increased with the laser jump speed. The effect of the laser jump speed on the post-heat peak was greater than the pre-heat peak. Compared with the long scan track, the thermal stress distribution varies more sharply with the laser jump speed when using the short scan track. The influence of the laser jump speed on the stress perpendicular to the scan direction was greater than that along the scan direction, whether in a short scan track or in a long scan track. To reduce the residual stress, the length of the scan track should be reduced while the laser jump speed should be increased as much as possible to make the post-heat peak close to the fusion temperature of the material. The simulated results were well validated by the experimental data. The findings of this research help to reduce the thermal stress in laser powder bed fusion.

The multi-laser powder bed fusion (MLPBF) has attracted more and more attention due to high productivity and large area. However, thus far the distribution and evolution of the thermal stress with different laser beams are rarely investigated. A block-by-block numerical model considering the laser scanning strategy and part-scale was developed to study the residual stress in MLPBF. As the laser beam number increased from 1 to 4, the residual stress at the top surface of the part decreased but the internal residual stress at the bottom center of the part significantly increased. The average and maximum stresses increased with the laser beam number, indicating that the cracks were more likely to occur with an increase of the laser beam number. The residual stress in the multi-laser overlap area was reduced, resulting in an important influence on the residual stress distribution. More frequent tension-compression stress transitions occurred during MLPBF, and the compressive stress along the deposition direction had a more important influence when the laser beam number increased. The simulated results agreed well with the experimental data. The results of the study provide a significant understanding of the residual stress and are helpful to the process optimization in MLPBF.

Rescanning has been introduced as a new method to improve the performance of selective laser melting (SLM) in recent years. However, many of the related works were done on the investigating of the rescanning parameters and only one rescanning cycle was used. In this work, the influences of rescanning cycle on the relative density, microstructure, residual stress and mechanical properties of SLMed Ti6Al4V alloy were investigated. The results show that as the rescanning cycles increase from 0 to 4, the relative density and residual stress of the samples increase at first and then decrease, the highest relative density and residual stress are found when the number of rescanning cycle is one. The number of rescanning cycle has little effect on the microstructure. The microstructures all consist of the martensitic α′ laths which grow inside the columnar prior β grains. The ultimate tensile strength (UTS), yield strength, micro-hardness and micro-strain of the samples all increase as the rescanning cycles increase from 0 to 3, but decrease when the samples are rescanned fourth times. This research provides an insight in the influence of rescanning cycles on characteristics of SLM process and can be used to optimize the process parameters of SLM.

Selective laser melting (SLM) can be directly used to fabricate high-performance three-dimensional (3D) parts with complex structures. However, the high residual stress caused by the extremely uneven temperature distribution is detrimental to formability during the process. A 3D finite element model based on the sequentially coupled thermal-structural method was developed to predict the effect of an overlap region on the residual stress induced by the island scanning strategy. The existence of the overlap region had the most influence on the X-component of stress in comparison with the Y and Z components of stress when using the unidirectional pattern. The X-component of stress and equivalent stress decreased first and then increased with the increase of the overlap rate, which primarily relied on the combined action of the laser re-scanning, pre-heating effect from the previous island and scanning length. In terms of the maximum X-component of stress and equivalent stress, an overlap rate of 25%–50% was recommended for the generally utilized island size. The laser vertical re-scanning the overlap region with a short scanning length was beneficial in reducing the residual stress. The simulated results were in good agreement with the experimental data, including the molten pool dimension, residual stress distribution and the effect of overlap rate on the residual stress. The findings of this study will improve the understanding of stress distribution during the SLM process and provide effective methods to reduce residual stress.

In this work, the effect of the pouring temperature of magnesium melt, the preheating temperature of the barrel of the screw mixer and the shear rate, on the solidified microstructures of semi-solid slurry was investigated by the mechanical stirring method. The appropriate processing parameters of slurry preparation were obtained. The mold filling for a thin-walled casting was examined. The results indicated that the solid volume fraction of non-dendritic structure increased with decrease in the pouring temperature of the magnesium melt and in the barrel preheating temperature of the screw mixer. Also, the grain size of the primary α-phase was reduced. Furthermore, the solid volume fraction of semi-solid non-dendritic structure decreased with increase in the shear rate. At the same time, the grain size of the primary α-phase changed slightly. A fine round granular solidified structure of 30–50 μm of prepared AZ91D magnesium semi-solid slurry was obtained. Finally, it was possible to successfully fabricate a 1.0 mm extremely thin-walled casting with a clear contour and good soundness.