查看更多>>摘要：Electron beam welding of Nb/GH3128 dissimilar alloys was conducted in this research. Microstructure characterization and finite element simulation were utilized to uncover the cracking mechanism of the joint. The obtained results revealed that a brittle reaction layer, composed of Ni6Nb7/Laves phase beside the Nb substrate, was a critical cracking factor triggering the cracking along the reaction layer. The generation of cracks was affected by a certain thermal-mechanical effect. The longitudinal residual stress fluctuated dramatically in the Nb/FZ interface region, resulting in different welding deformation. High transverse residual stress was also detected in the vicinity of Nb/FZ interface, increasing the cracking propensity of Nb/GH3128 joint. This high residual stress combined with the brittle reaction layer illustrated the intrinsic cause of welding crack. Crack-free welding of Nb/GH3128 dissimilar alloys was realized via adopting the beam offset process. The thickness of the brittle reaction layer was dramatically decreased and the brittle Laves phase within FZ was replaced by the gamma phase.
查看更多>>摘要：In ultra-precision grinding of the wafer, most existing studies on wafer shape control only consider the kinematic relationship between grinding wheel and wafer, but the influence of grinding residual height on wafer grinding shape cannot be ignored. In this paper, the formula for the distance between the grinding marks formed by adjacent grains was established. Afterward the model of residual height and the model of wafer shape consid-ering the influence of residual height were established. Methods to enhance the flatness of the ground wafer were put forward by changing the process parameters, and changing the inclination angles of the grinding wheel shaft. The residual height under different parameters and the inclination angles needed to offset the influence of re-sidual height on the wafer shape were simulated and analyzed. Finally, the trial experiments were conducted to verify the proposed methods and the wafer shape model. The study outcomes will provide helpful instruction for ameliorating the flatness of the wafer.
查看更多>>摘要：This study investigates the dynamic temperature behaviour around a melt pool in metal-based powder bed fusion using a laser beam (PBF-LB/M) to clarify the influence of the associated morphological changes of the metal powder experimentally. Gas-atomized 18Ni (300-grade) maraging steel powders were processed by PBF-LB/M while high-speed photography with a two-colour radiometric thermal imaging system that was employed to correlate the temperature with melt pool behaviour. In addition, the cooling rate of the melt pool was measured directly using the dynamic temperature distribution. The temperature distribution of the melt pool was influenced by the morphological changes of the metal powder induced by physical and thermal interactions, and the melt pool exhibited an asymmetric temperature distribution in the direction parallel to the laser scan. The significant factors were droplet cohesion at low melt pool temperatures, remaining heat energy from previous laser beam irradiation, and the heat conduction inside the melt pool. The laser beam incident on the metal powder was primarily characterized by two modes: direct heating induced by laser beam irradiation and heat conduction through the single track, droplets, and substrate. In addition, the dynamic temperature behaviour provided a direct explanation for the cooling rate, the values of which ranged from 0.1 to 0.9 x 10(6) K/s owing to the self cooling induced by PBF-LB/M.
Maina, Martin RuthandiOkamoto, YasuhiroHamada, KazukiOkada, Akira...
13页
查看更多>>摘要：Unstable and low absorption of laser energy is experienced in copper welding at around 1000 nm wavelength. At 532 nm wavelength, there is stable and high laser absorption by copper. Past researches have shown that transitional processing condition between keyhole and heat conduction welding results in a stable micro-welding process characterized by good surface quality and deep penetration. In order to adapt laser welding to copper using pulsed Nd:YAG lasers, investigations of welding quality and efficiency were addressed. Processing was done under transitional processing condition between heat conduction and keyhole welding. Copper C1020 specimens were processed using superposed laser wavelengths of 1064 nm and 532 nm. Effects of irradiation delay and power density on the process were clarified by taking measurements of molten volumes, and by analyzing the weld beads. In addition, the dynamics of molten area and keyhole formation were investigated through three-dimensional FEM analysis. A stabilized laser absorption and increased molten volume was achieved by superposition using 532 nm laser of an appropriate high power density coupled with a short irradiation delay for the 1064 nm laser, which resulted in high-efficiency welding of copper.
Brown, Jennifer ShafferWilson, CalebBohlen, CharlesChoi, Hongseok...
10页
查看更多>>摘要：While useful for optics and solar energy applications, solder-glass joints are difficult to fabricate due to poor wettability and adhesion, but reliable joints can be achieved with the use of ultrasonic-assisted soldering (UAS). In this study, an automated UAS system was used to investigate the effects of processing parameters on the solder joint quality so that the technology can be more suitably scaled for future widespread use in industry. Through mechanical shear strength testing of soldered joints and microstructure failure surface analysis with XRD, a more comprehensive understanding of the relationships between processing parameters and joint quality was developed. Two novel failure surface types with distinct failure behavior were observed which could be predicted based on soldering tip speeds. The differences between the two failure surfaces appear to be due to the growth of a thin surface oxide layer on the solder lines from prolonged sonication, as determined from EDS and XRD microstructure analysis. These observations and analysis highlight a complex relationship that determines the quality of the solder-glass joint and suggest further research is necessary to refine the UAS process specifically for glass substrates and their varied applications in industry to prevent premature failure.
Riquelme, A.Candela, C. Sanchez de RojasRodrigo, P.Rams, J....
13页
查看更多>>摘要：Highly reinforced 316 L / SiCp metal matrix composites (MMC) walls have been additively manufactured by Direct Laser Deposition (DLD) using mixtures with different percentages of 316 L stainless steel powder and SiC particles (SiCp). The effect of the percentages of SiCp of up to 80 wt.% and DLD conditions on the geometry, microstructure, and hardness of the fabricated structures have been analyzed.
查看更多>>摘要：Advanced high-strength steel (AHSS) has been widely used for lightweight body-in-white. However, the fatigue strength of welded joints is not improved even employed AHSS. In this research, the advanced post-weld cold working (PWCW) experiment was proposed to improve the fatigue strength of DP980 spot welded joints. X-ray diffraction (XRD) method was employed to measure the welding residual stress. A numerical model was established to analyze the PWCW induced residual stress as the first try. The influence of the diameter of the cylindric tool used in PWCW on the residual stress of welded joints was also discussed in detail. The hardness distribution on weld joint was increased after PWCW, which represented the plastic strain induced work hardening in DP980 steel. The predicted PWCW induced residual stress had a reasonable agreement with XRD measurement. The PWCW introduced a compressive stress in the weld nugget and its heat affected zone with a value of more than-400 MPa at the edge of joined zone. The strengthened material and compressive stress were beneficial to suppress the generation and propagation of fatigue crack, thereby enhance the fatigue strength of welded joints. Besides, the effect of PWCW continuously strengthened with a larger diameter of tool, especially when the tool diameter was larger than 10 mm. The fatigue strength was almost doubled compared with the as welded condition. The smaller tool provided a critical requirement for precise positioning. The operation of PWCW was also simple enough to realize the automation, which indicated its potential in manufacturing automotive units.
查看更多>>摘要：Porosity is an inherent feature of additively manufactured components that impairs the mechanical properties. Since porosity depends not only on the process parameters but also on the component geometry, it is necessary to investigate this dependency. This study reveals relationships between porosity, process parameters and component geometry in AlSi10Mg specimens. Micro-computed tomography images of different geometries built with varying parameters were analysed for pore concentrations. The quantity and distribution of pores at overhangs depends on the laser scan speed and the width of the overhang. Similarly, the scan speed influences the effective bead width and thus the pore formation at thin structures. This relationship is revealed by an indepth analysis of the laser path together with the mu CT images. The investigations show that the process parameters can only be optimal for a specific geometry. The presented interdependencies can be used to derive locally adjusted scanning strategies and machine parameters.
查看更多>>摘要：Dissimilar material joints or multilayered metals have become inevitable in the manufacturing industry due to the increasing demand for multifunctional materials with variable mechanical, thermal, or electrical characteristics in a single assembly. Lattice mismatch of materials at the interface of dissimilar materials leads to inferior mechanical characteristics. In particular, the mismatch in elastic properties indicated by a large initial elastic deformation is critical to determine the extent of variation in stress. However, nanoindentation, the most common and accepted technique to measure elastic modulus, is destructive, time-consuming, and can only examine mechanical properties within a limited area. A non-invasive elastographic mapping technique evaluates the mechanical properties using ultrasonic elastography to study incompressibility. The dissimilar joint between steel and copper was obtained via friction stir welding. The variation of the stress developed at the welded joint of the two different metals was evaluated from the dynamic bulk modulus map. A tensile test of the involved workpiece confirmed a good agreement with our analysis based on the dynamic bulk modulus elastographic mapping results. This study provides a rapid and non-invasive technique for the bulk metallurgic elastic modulus inspection to overcome the limitations of conventional methods.
Bowen, Daniel T.Russo, Iacopo M.Cleaver, Christopher J.Allwood, Julian M....
18页
查看更多>>摘要：The traditional metal smith has the remarkable capability to form a variety of part shapes from flat sheets using only a few universal tools. Such versatility is increasingly appealing to manufacturers who now seek to diversify part catalogues and reduce tooling costs. Despite this utility, the laborious, manual nature of these traditional techniques preclude them from meeting modern-day, high-volume demand. However, some techniques have served as starting points for the development of new flexible metal forming processes, either through creating new processes that closely replicate the traditional techniques or by automating the manual process. Here, we look closely at some of the techniques used by the traditional smith to form sheets and review automated adaptations of these processes. We find that decision-making elements of these adapted processes are not yet as capable as their manual counterparts, suggesting there is still a lot we can learn from the traditional smith. As such, we look both within and beyond the domain of metal forming at the technologies and the methods that can be used to capture the skilled actions of the smith and how the resulting data can be used to enhance the design and operation of mechanised variants.
NSTL
Elsevier