Modelling of electron energy absorption and backscattering during EBW

A three-dimensional heat flux model for deep-penetrating electron beam welding (EBW) is established to mathematically describe the physical heat generation process during interaction between electrons and the dynamic molten pool free surface. Monte Carlo method is used to determine the electron-target interaction, and random distribution of initial electrons, progressive trajectory tracing and electron backscattering models are used to describe the spatial distribution of electrons absorption. The model is verified in preset keyholes and applied in the simulation on electron beam welding process, and the calculated bead shape shows a good consistency with experimental results.     

选区激光熔化成形AlSiMg3合金

基于选区激光熔化（SLM）技术熔体快速冷却的特点，通过提高Al-Si-Mg合金中Mg的含量，设计获得SLM技术专用AlSiMg3合金。系统研究了不同工艺参数和时效处理条件对SLM成形AlSiMg3合金组织和硬度的影响。结果表明，SLM成形样品均由α-Al、Si和Mg2Si相构成。高激光能量密度有利于增加粉末样品的成形性，当激光功率为160 W，扫描速度为200 mm/s时，样品具有最低孔隙率0.07%。随着激光扫描速度的增加，样品中富Si组织的比例逐渐升高，Mg元素在α-Al中固溶量逐渐增大，使得SLM成形样品的硬度逐渐升高，最大值为194±3 HV。样品经150 ℃时效处理后，由于α-Al内部纳米颗粒的析出，导致样品硬度增大，最大值为210±2 HV，远高于现有报道的SLM成形Al-Si和Al-Si-Mg铝合金。本研究报道了成形性和力学性能优异的SLM专用Al-Si-Mg合金。     

Effects of welding parameters and welding sequence on residual stress and distortion in Al6061-T6 aluminum alloy for T-shaped welded joint

The distribution of temperature and then the distribution of residual stress and distortion in the stiffened aluminum alloy Al6061-T6 plates under the metal inert gas (MIG) welding process were investigated by three dimensional thermo-mechanical coupled finite element model using Ansys software. The properties of materials were considered temperature-dependent and the filler metal was added to the workpiece by the element birth and death technique. In three modes of current, two different speeds and two various sequences, the distribution of residual stress and distortion were calculated and analyzed. The results showed that increase in welding speed decreased the vertical deflection in the plate, transverse shrinkage and angular distortion of plate and the lateral deflection of stiffener, but increased the maximum longitudinal tensile stress in the plate and stiffener. Furthermore, increase in current increased the residual stress and deformation in the plate and stiffener, and the change in the welding sequence changed the distribution of the distortion in the plate and the stiffener without significant change in the distribution of the longitudinal residual stress.     

Microstructure and properties of WC-12Co composite coatings prepared by laser cladding

A comprehensive study of the phase composition, microstructure evolution, microhardness and wear performance of WC-12Co composite coatings fabricated by laser cladding using coaxial powder-feed mode was presented. It was shown that a combination of high scan speed and high laser energy density made WC on the edge of WC-12Co composite powders partially melt in liquid Co and 304 stainless steel matrix, and then new carbides consisting of lamellar WC and herringbone M₃W₃C (M=Fe, Co) were formed. Meanwhile, WC-12Co composite coatings with no porosity, cracks and drawbacks like decarburization were obtained, showing high densification and good metallurgical bonding with the substrate. Furthermore, a considerably high microhardness of HV_0.3 1500-1600, low coefficient of friction of 0.55 and wear rate of (2.15±0.31)×10^-7 mm³/(N·m) were achieved owing to the synergistic effect of excellent metallurgical bonding and fine microstructures of composite coating under laser power of 1500 W.     

Local inhomogeneity of mechanical properties in stir zone of friction stir welded AA1050 aluminum alloy

The local inhomogeneity of the stir zone in friction stir welded face-centered cubic metal was investigated, which has multiple activated slip systems during plastic deformation, by selecting commercial AA1050 aluminum alloy as an ideal experimental material. The local inhomogeneity was evaluated by uniaxial tensile tests using small samples with a 1 mm gauge length. The corresponding microstructural parameters such as grain size, misorientation angle distribution, and micro-texture, were quantified by the backscattered electron diffraction technique. A comprehensive model was used to reveal the microstructure−mechanical property relationship. The experimental results showed that the uniaxial tensile property changes significantly across the weld. The maximum ultimate tensile strength (UTS) occurred in the center of the stir zone, which was 99.0 MPa. The weakest regions were located at the two sides of the stir zone. The largest difference value in UTS reached 14.9 MPa, accounting for 15% of the maximum UTS. The analysis on the structure−mechanical property relationship suggests that the micro-texture change with the location formed during the rotational material flow is the main reason for the local inhomogeneity.     

Microstructure and tensile properties of rapid-cooling friction-stir-welded AZ31B Mg alloy along thickness direction

Rapid-cooling friction-stir-welding (FSW) was used to join AZ31B magnesium alloy plates of 6 mm in thickness. The microstructure and mechanical properties in thickness direction were carefully investigated with electron backscattered diffractometer, and transmission electron microscope. The obtained results showed that ultrafine grains with high dislocation density were obtained in the top region of the weld due to liquid CO₂ cooling. A large number of twins and second-phase particles were also induced in these refined grains. The basal texture intensity was significantly reduced due to the appearance of twins. The top region showed the higher strength and elongation compared with the bottom region, and the welding efficiency reached 93%. This work provided a simple and efficient strategy for manufacturing a gradient structure in the FSW Mg alloy joint.     

Al3Zr/Al基原位复合材料等离子体焊接区的组织及表面腐蚀行为

摘要： 本文以5wt%Al3Zr/铝基复合材料为研究对象，以Ar气体为离子气体，Al-Ti-B为丝材进行等离子焊接。用电化学综合测试仪测定了焊接区在3.5%NaCl溶液中的腐蚀极化曲线。采用扫描电子显微镜（SEM）、X射线衍射仪（XRD）对复合材料及其焊缝的组织和相组成进行了分析和表征，研究了Al3Zr/Al基复合材料等离子焊接后在3.5%NaCl溶液中的腐蚀行为和腐蚀机理。SEM分析结果表明，等离子弧焊接可获得较好的焊缝组织，焊缝组织主要由Al3Ti相和Al基体组成，Al3Ti相呈条状和球形。电化学极化曲线表明，材料在熔核处的耐蚀性略低于母材，焊缝处浸泡腐蚀机理为Al3Ti相和基体相的电极电位不同形成原电池反应造成。     

2195铝锂合金电子束焊接头的焊后热处理

采用光学显微镜、扫描电镜、能谱仪及X射线衍射仪等微观分析手段,研究不同热处理工艺条件下2195铝锂合金电子束焊接头焊缝区的显微组织演变,探讨接头的焊后热处理强化机制。结果表明,焊后热处理可显著改善接头区域的显微组织,促进强化相的析出,有利于提高接头的力学性能。经过焊后固溶+双级时效热处理,焊态下接头熔合线附近存在的等轴细晶区消失,β′、θ′和T₁等强化相在接头焊缝区析出,与单级时效处理工艺相比,双级时效处理的析出强化效果更为显著。力学性能测试表明,经过双级时效热处理后,接头的抗拉强度达到492.5 MPa,为母材强度的90.4%。接头拉伸断口表面存在许多小韧窝,并伴随出现解理面,接头呈韧-脆混合型断裂特征。     

Al2O3 eliminating defects in the Al coating by laser cladding to improve corrosion and wear resistance of Mg alloy

Although Al produces a solid metallurgical bonding with Mg alloy substrates, micropores or crevices in the Al coating can reduce the resistance of Mg alloy to corrosion. In this study, a composite coating with a defect-free microstructure was prepared on the AZ31 Mg alloy substrate by introducing Al₂O₃ into the Al matrix via the method of laser cladding. On the one hand, Al₂O₃ with thermal insulation had a low thermal expansion coefficient and was not very prone to voids during laser melting. On the other hand, Al₂O₃ particles with a small size acted as the filler in the micropores or crevices. The Al/Al₂O₃ coating exhibited a smaller current density (2.1 × 10⁻⁶ A/cm²) in comparison with those of bare substrate and Al coating (158.4 × 10⁻⁶ and 3.1 × 10⁻⁶ A/cm², respectively), which was mainly ascribed to the pore-free microstructure and high resistance to corrosion of Al₂O₃ phase. A favorable microhardness value of 95.3 HV was achieved for Al/Al₂O₃ coating, approximately 1.8 times higher than that of Al coating (52.8 V), which was mainly ascribed to the dispersion hardening of Al₂O₃ phase. Meanwhile, the Al/Al₂O₃ coating significantly reduced wear volume from 2.8 mm³/m of Al coating to 0.4 mm³/m, showing great potential for weight reduction applications.     

双光束激光焊匙孔动态特征分析

文中研究了高功率单光束激光焊与双光束激光焊过程中匙孔动态特征的差别. 结果表明，单光束激光焊及双光束激光焊接过程中匙孔均处于从产生到湮灭的剧烈波动的过程，不同于单光束激光焊匙孔的形成、长大、维持、缩小、湮灭过程，双光束激光焊的匙孔还存在分离长大及合并缩小的过程；在相同的焊接参数及焊缝具有相同熔深的条件下，双光束激光焊匙孔的波动频率约为单光束激光焊的2/3，单光束激光焊匙孔的开口面积均值约为双光束激光焊匙孔开口面积的1/2，开口面积的波动变异系数约为单光束激光的2倍，即双光束激光焊过程中匙孔较单光束激光焊的具有较高稳定性.