•According to the characteristics of pulsed laser, the application of pulsed laser in alloy welding was summarized.•By discussing the crack sensitivity criterion, the mechanism of pulse laser welding ...to solve welding crack was analyzed.•The outlook for the future trends of pulsed laser welding and crack criterion was presented.
There are mainly two kinds of laser used in welding of materials, continuous laser beam welding (CLBW) and pulsed laser beam welding (PLBW). The two kinds of laser welding have each welding characteristics. Compared with CLBW, the heat input of PLBW can be controlled lower, and has been widely used in the connection of thin plate materials. Since the pulse shaping can be edited to realize the preheating and slow cooling of each welding process, pulsed laser has been studied to suppress hot cracks and other defects in the field of aluminum alloy, magnesium alloy and other materials welding. This article describes the application of pulsed laser in alloy welding. The main welding problems and prevention methods were summarized. By discussing the index had been proposed to predict the sensitivity of solidification cracks, the formation and growth processes of hot cracks in pulsed laser welding was more comprehensively analyzed. Finally, the outlook for the future trends of pulsed laser welding is presented.
The formation of undesirable coarse columnar grains and periodic hot cracks has been a long-standing issue in the additive manufacturing (AM) of unweldable high-strength aluminum alloys and their ...nanocomposites. Herein, the LPBF-printability of AA2024-based nanocomposites is improved by TiC-induced in-situ reinforcement and nanoparticle-enabled grain refinement during laser powder bed fusion (LPBF). The TiC-triggered in-situ reaction during LPBF overcomes the poisoning of the TiC grain refiner that occurs during conventional solidification. It not only results in fine equiaxed grains with a minimum average size of ∼1.6 μm but also suppresses hot crack formation. Powerful L12-Al3Ti nucleants are incorporated in-situ into AA2024 through TiC nanoparticle addition during LPBF, promoting the heterogeneous nucleation of α-Al. Moreover, the growth restriction effect induced by residual TiC nanoparticles and lamellar Al4C3 nanoparticles through pinning behavior along grain boundaries also contributes to grain refinement. Furthermore, these particles serve as effective reinforcement particles as well as grain refiners. The as-built AA2024-based nanocomposites tailored by nano-TiC show a maximum ultimate tensile strength (UTS) of ∼388 MPa, a yield strength (YS) of ∼332 MPa and an elongation (El) of ∼10.2%. After T6 heat treatment, the nanocomposites exhibit an outstanding UTS of ∼507 MPa, a YS of ∼456 MPa and an El of ∼6.6%. This nano-TiC-induced microstructural control method provides new insights into successful AM of hot-crack-sensitive high-strength aluminum alloys and their composites.
Lightweight materials are of paramount importance to reduce energy consumption and emissions in today's society. For materials to qualify for widespread use in lightweight structural assembly, they ...must be weldable or joinable, which has been a long-standing issue for high strength aluminum alloys, such as 7075 (AA7075) due to their hot crack susceptibility during fusion welding. Here, we show that AA7075 can be safely arc welded without hot cracks by introducing nanoparticle-enabled phase control during welding. Joints welded with an AA7075 filler rod containing TiC nanoparticles not only exhibit fine globular grains and a modified secondary phase, both which intrinsically eliminate the materials hot crack susceptibility, but moreover show exceptional tensile strength in both as-welded and post-weld heat-treated conditions. This rather simple twist to the filler material of a fusion weld could be generally applied to a wide range of hot crack susceptible materials.
•The hot cracks are eliminated in the LPBF-processed ZK60.•The grain size is significantly reduced after the powders modified by Y.•The grain boundaries are strengthened by the I - phase.•The cracks ...in LPBF-processed ZK60 samples are attributed to the Mg7Zn3.
The hot cracks of laser powder bed fusion (LPBF) processed ZK60 components were eliminated by adding rare earth element Y. The cause of the cracks in the samples and the suppression mechanism were analyzed in detail. The cracks in ZK60 LPBF samples were mainly caused by the low-melting eutectic phase Mg7Zn3 at the grain boundaries. The average grain size decreased from 7.2 μm to 2.0 μm after adding Y. The I-phase with excellent thermal stability strengthened the grain boundaries. The elimination of cracks was mainly attributed to the I-phase at the grain boundary and the grain refinement.
Hastelloy X is the trademark for a nickel-based, high-temperature superalloy that is increasingly applied in gas turbine engines because of its exceptional combination of oxidation resistance and ...high-temperature strength. The superalloy suffers from hot cracking susceptibility, however, particularly when processed using additive manufacturing and laser powder bed fusion (LPBF). This paper systematically studies for the first time the effect of post-treatment hot isostatic processing (HIP) on the microstructure and mechanical properties of LPBF-fabricated Hastelloy X, with an emphasis on fatigue performance. The experimental results demonstrate that despite the very small number of remaining gas-filled micropores due to pressure counteraction, the high temperature and high pressure during the HIP process promote recrystallisation and closing of the internal microcracks and gas-free pores. The HIP-processed specimens are shown to be roughly 130 MPa and 60 MPa weaker than the non-processed specimens in yield strength and ultimate tensile strength, respectively. The HIP-processed Hastelloy X exhibits significant improvements in fatigue life, however: the effect of the HIP processing is apparent once the applied stress decreases. This improvement in fatigue performance is attributable to the reduction in stress concentration and residual stress release caused by the HIP process. The paper also studies the hot cracking mechanism and finds that intergranular microcracks generally occur along high angle grain boundaries; the interdendritic liquid pressure drop between dendrite tip and root is found to be a significant factor in the hot crack mechanism. The significance of this research is in developing a comprehensive understanding of HIP processing on the fatigue behaviour of the LPBF-fabricated Hastelloy X. The insights on the cracking mechanism, which presents a significant step towards using additive manufacturing to produce complex crack-free parts from this superalloy.
A TiC nanoparticle modified non-weldable nickel-based superalloy Inconel 738LC has been fabricated via laser powder bed fusion. The formation of refined grains with a larger high angle grain boundary ...length density and zigzag grain boundaries inhibits the initiation and propagation of hot cracks, leading to a 97% reduction in cracking and improved mechanical properties.
Selective laser melting (SLM) is an additive manufacturing process, forming the desired geometry by selective layer fusion of powder material. Unlike conventional manufacturing processes, highly ...complex parts can be manufactured with high accuracy and little post processing. Currently, different steel, aluminium, titanium and nickel-based alloys have been successfully processed; however, high strength aluminium alloy EN AW 7075 has not been processed with satisfying quality. The main focus of the investigation is to develop the SLM process for the wide used aluminium alloy EN AW 7075. Before process development, the gas-atomized powder material was characterized in terms of statistical distribution: size and shape. A wide range of process parameters were selected to optimize the process in terms of optimum volume density. The investigations resulted in a relative density of over 99%. However, all laser-melted parts exhibit hot cracks which typically appear in aluminium alloy EN AW 7075 during the welding process. Furthermore the influence of processing parameters on the chemical composition of the selected alloy was determined.
Hastelloy X (HX) is a Ni-based superalloy which suffers from high crack susceptibility during the laser powder bed fusion (LPBF) process. In this work, the microstructure of as-built HX samples was ...rigorously investigated to understand the main mechanisms leading to crack formation. The microstructural features of as-built HX samples consisted of very fine dendrite architectures with dimensions typically less than 1 µm, coupled with the formation of sub-micrometric carbides, the largest ones were mainly distributed along the interdendritic regions and grain boundaries. From the microstructural analyses, it appeared that the formation of intergranular carbides provided weaker zones, which combined with high thermal residual stresses resulted in hot cracks formation along the grain boundaries. The carbides were extracted from the austenitic matrix and characterized by combining different techniques, showing the formation of various types of Mo-rich carbides, classified as M₆C, M
C and M
C
type. The first two types of carbides are typically found in HX alloy, whereas the last one is a metastable carbide probably generated by the very high cooling rates of the process.
•The crack-free AA-7075 specimen is fabricated by LPBF.•The grain size is significantly reduced with the addition of Zr50.7Cu28A112.3Ni9.•The excellent tensile strength of LPBF fabricated specimen ...(488.7 MPa) is obtained.
The inclusion of 2 wt% Zr50.7Cu28Al12.3Ni9 metallic glass (MG) powders successfully eliminated the hot cracks in AA-7075 specimens fabricated using laser powder bed fusion (LPBF). The microstructure analysis revealed a significant enhancement in grain refinement within the AA-7075/MG specimen as a result of the formation of Al3Zr precipitates. The average grain size reduced from 9.26 μm to 1.6 μm, and the previously coarser columnar grains transformed into finer equiaxed grains with random orientation. Moreover, the AA-7075/MG specimen demonstrated a noteworthy microhardness exceeding 160 HV. With an elongation of about 7.2%, the highest ultimate tensile stress and yield strength are 488.7 MPa and 460.8 MPa, respectively.
Hot cracking has been a long-term challenge for the solidification processing of many high-performance aluminum alloys, such as 7075, 6061, and 2024 alloys. Nano-treating, by adding a low loading of ...ceramic nanoparticles into a metal matrix, can effectively reduce the hot cracking susceptibility of aluminum alloys during solidification processes such as casting, welding, and additive manufacturing. While previous studies have shown that ceramic nanoparticles enhance heterogeneous nucleation, inhibit grain growth, and modify secondary phases during solidification of various alloys, no systematic study has been conducted to investigate the underlying mechanisms of hot cracking elimination by different nanoparticles on different alloy systems. In this work, TiC and TiB
2
nanoparticles have been incorporated into hot crack susceptible aluminum alloys 7075, 6061, and 2024, and a detailed thermal analysis and microstructure study were carried out to investigate the nanoparticle-enabled principal mechanisms of hot cracking elimination. It is discovered that the underlying mechanism is attributed to the unusual modification of both grains and intermetallic phases as well as a much higher liquid fraction at the final stage of solidification. More specifically, nanoparticles enable faster nucleation with a gradual latent heat release, an effective growth restriction of spherical aluminum
α
-grains (especially by TiC nanoparticles), a significant modification of the intermetallic phases, and a higher liquid fraction of non-equilibrium eutectic due to nanoparticle-induced diffusion blockage at the terminal stages of solidification.
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