Effect of metallurgical evolution during post-weld aging treatment on localised corrosion of resistance spot welding joints of A286 superalloy

The combined effect of resistance spot welding and precipitation hardening on the localised corrosion of A286 superalloy is studied. The specimens tested by double loop electrochemical potentiokinetic reactivation were welded in the solution treated condition, and then subjected to different precipitation hardening treatments. For both base metal and weld nugget, the maximum localised corrosion is reached when η phase is clearly observable. The fact that the localised corrosion resistance of weld nugget is different from that shown by base metal may be explained by the segregation of Ni and Ti towards the interdendritic region of weld nugget (studied by using scanning electron microscope/energy dispersive X-ray analysis).     

12CrNi3钢外锁止套局部表面激光淬火工艺

利用半导体激光宽带对12CrNi3外锁止套局部进行表面淬火处理,并对处理后的零件进行外观检验、显微组织观察、硬度检测分析和摩擦磨损试验。试验结果表明:采取定光斑方式,激光头到工件表面扫描区域中心点距离375 mm(离焦量为0),工件倾斜22°,激光头倾斜18°,温度1350 ℃,扫描速度9 mm/s,单道扫描后表面平整性最好。表面激光淬火硬化层为750～1000 μm,最浅处为443.1 μm,硬化区组织为极细小马氏体组织,硬度达600～700 HV0.2,是基体硬度的2倍左右, 约为渗碳+淬火态硬度的1.3倍,且相变硬化区耐磨性能明显提高。     

Strain rate and orientation-dependent strain hardening of Mg–9Li–Al using crystal plasticity

A crystal plasticity finite element model was proposed considering slip and twinning interactions. The grain morphology and crystallographic orientations were introduced into the model to describe the microstructure of duplex polycrystalline Mg–9Li–Al. The activation of the slip systems and the strain localisation with respect to initial grain orientations were investigated. In addition, the effects of phase distributions and volume fractions on the macroscopic responses and on strain hardening rates were analysed. The results show that the strain hardening is rate-dependent but the texture is less sensitive to strain rate. The distribution of a phase and its volume fraction play primary roles in governing the mechanical response.     

节镍型奥氏体不锈钢的组织性能对比

针对200系J5A、J5两种牌号节镍型奥氏体不锈钢冷轧压延性能的差异,对比分析J5A、J5钢成分设计、组织、力学性能和马氏体含量,研究冷加工性能差异产生的机理。结果表明:由于J5A钢成分设计不合理,镍当量较低,奥氏体稳定性较差,马氏体转变点M_d(30/50)高,在轧制过程中产生大量冷加工变形诱变马氏体,加工硬化程度严重,压延性能差。     

Strain hardening behaviors and mechanisms of polyurethane under various strain rate loading

The uniaxial tension experiments are performed on thermoplastic polyurethane to investigate its mechanical behaviors and related potential mechanisms, and the loading strain rate is designing to be wide ranging from 0.0001 to 1 s⁻¹. It is found that the polyurethane presents an obvious rate-dependence, and the stress strain curves share distinct strain hardening characteristics under the investigated strain rates. Furthermore, the strain hardening ratios are sharing nearly same trends and appear to be influenced by both strain rate and the induced adiabatic heating. Besides, the ratio is also strain-dependent on previous loading history. Then, a two-dimension unit cell model is built to investigate potential equivalent mechanisms, of which the hard phase as inclusion is equivalent with crystallization zone, crosslinking sites, and so forth. The simulation results facilitate to explain the distinct strain hardening ratios, even for the matrix from the extrapolated curves under super-low strain rate loading. Finally, the analogic mechanisms of equivalent hard inclusions are proposed, which can reasonably explain the strain rate- and strain-dependence characteristics of polyurethane mechanical behaviors.     

Grain size effect on cyclic deformation behavior and springback prediction of Ni-based superalloy foil

In order to clarify the influence of grain size on cyclic deformation response of superalloy sheets and springback behavior, cyclic loading–unloading and shearing tests were performed on the superalloy foils with 0.2 mm in thickness and diverse grain sizes. The results show that, the decline ratio of elastic modulus is weakened with increasing grain size, and the Bauschinger effect becomes evident with decreasing grain size. Meanwhile, U-bending test results determine that the springback is diminished with increasing grain size. The Chaboche, Anisotropic Nonlinear Kinematic (ANK) and Yoshida-Uemori (Y-U) models were utilized to fit the shear stress–strain curves of specimens. It is found that Y-U model is sufficient of predicting the springback. However, the prediction accuracy is degraded with increasing grain size.     

Effect of aging time on discontinuous precipitates,continuous precipitates and mechanical properties of AZ80A magnesium alloy

The evolution of precipitates and mechanical properties of AZ80A magnesium alloy with aging time was studied by in situ observation with SEM, TEM and tensile testing. The results show that the continuous precipitation (CP) phases near the reaction front (RF) are replaced by the discontinuous precipitation (DP) phases at the early aging stage. In DP regions, the elliptical phases coarsen obviously with the increase of aging time, which results in a slightly slow reduction of the intracrystalline hardness of DP regions. In CP regions, some small plate phases reprecipitate simultaneously with the growth of the initial precipitates, which contributes to a slight increase in the intracrystalline hardness in CP regions at the later aging stage. The aging hardening of DP regions is faster and stronger than that of CP regions. However, the age strengthening of CP regions not only compensates for the overaging softening of DP regions but also improves the strength of the alloy.     

5754铝合金板材冲压成形材料模型研究与应用

建立准确的材料模型是冲压数值模拟的基础,通过3个方向的单向拉伸试验、以及单向压缩试验和成形极限试验,获得5754铝合金材料性能数据,基于Voce硬化模型、Barlat89屈服准则和成形极限,建立5754铝合金成形用材料模型。利用Pamstamp-2G软件,对5754铝合金汽车大梁的冲压成形进行数值模拟,并与冲压试验结果进行对比。结果表明:数值模拟获得的应变数据与试验测量获得的应变数据比较接近,且各区域最大减薄率误差在±10%以内,验证数值模拟的可靠性。     

Effect of Cu addition on microstructural evolution and hardening of mechanically alloyed Al−Ti−O in-situ composite

The microstructure formation and strengthening of an Al−5wt.%TiO₂ composites with additions of 5 wt.% Cu and 2 wt.% stearic acid (as a process control agent, PCA) during mechanical alloying and subsequent thermal exposure were studied. The powder composites were prepared by high-energy ball milling for up to 10 h. Single line tracks of the powders were laser melted. Optical and scanning electron microscopy, XRD analysis and differential scanning calorimetry were used to study microstructural evolution. The results showed that the Cu addition promotes an effective mechanical alloying of aluminum with TiO₂ from the start of milling, resulting in higher microhardness (up to HV 290), while the PCA, on the contrary, postpones this process. In both cases, the composite granules with uniform distribution of TiO₂ particles were formed. Subsequent heating of mechanically alloyed materials causes the activation of an exothermic reaction of TiO₂ reduction with aluminum, the start temperature of which, in the case of Cu addition, shifts to lower values, that is, the transformation begins in the solid state. Besides, the Cu-added material after laser melting demonstrates a more dispersed and uniform structure which positively affects its microhardness.     

4320H钢风电机齿轮的渗碳淬火

4320H风电机齿轮需进行渗碳、淬火,达到表面硬度58~62HRC、心部硬度33~44HRC、有效硬化层(至515HV0.5)深度2.0~3.0mm,有非马氏体组织的深度≤0.05mm,马氏体1~4级,心部铁素体1~3级,但无连续网状碳化物。采用UBE密封箱式炉对4320H钢齿轮进行了渗碳、淬火。通过热处理工艺的调整,最终达到了上述质量要求。