Microstructural modification and mechanical property improvement in friction stir zone of thixo-molded AE42 Mg alloy

Thixo-molded AE42 Mg alloy was friction stir welded, and the soundness of joints was evaluated, together with the microstructure evolution and mechanical properties in friction stir zones. According to X-ray radiography, the optimum FSW condition range of AE42 alloy exists between AZ61 and AZ31 alloys, and it seems that the optimum welding condition range increases with decreasing Al content in the Mg alloys. There are mainly two kinds of compounds in the thixo-molded AE42 alloy, and FSW has little influence on the grainy Al₁₀RE₂Mn₇ compound, but it has great influence on Al₁₁RE₃ phase, which is changed from lamellar eutectic to small particles after welding. Furthermore, the average diameter of Al₁₁RE₃ particles in SZ decreases with increasing the traveling speed at constant rotation speed due to less heat input. The hardness in SZ is higher than that in BM, and tensile strength and elongation are both improved after welding because the stirring refines and uniforms the microstructure and intermetallic compounds.     

Regional valuation of infrastructure and transport attributes for Swedish road freight

Is it possible to identify regional differences among shippers in their valuation of infrastructure improvements? In this study, the question is analysed using a random utility approach, where parameters are estimated by a logit model. Data consist of a Swedish stated-preference study from 1992. The results indicate that regional differences exist, but that a considerable heterogeneity in the empirical data means that in some cases the results are not robust. However, when industrial mix, shipping distance, and goods values are held constant, the analysis still indicates the existence of regional differences. Independently of the limitations in the results, the study has implications for any infrastructure benefit analysis where parameters from spatial averages are used. The results are based on short-term decisions, and one should recognise that parameters may vary in the mid- and long-term. Received: May 1999/Accepted: March 2001     

Do vertical tax externalities lead to tax rates being too high? A note

When both vertical and horizontal externalities are at work in a federation, they generally distort levels of taxation in opposite directions. This article is an extension of the Keen and Kotsogiannis (Am Econ Rev March: 363–370, 2002) model. It focuses on state tax policy when states provide an industrial public good increasing firms’ marginal productivity rather than a residential public good intended to households. We find that some results related to whether equilibrium state taxes are likely to be too high or too low are much more complex when state-provided industrial public goods are considered. First, we show that in the presence of industrial public goods, a dominant vertical externality may result in state tax rates being too low (and not too high as it is the case with residential public goods). Second, we find that under certain conditions, horizontal and vertical externalities may point in the same direction and reinforce each other towards tax rates being too high or too low.     

回火温度对42CrMo钢组织和力学性能的影响

通过显微组织观察和力学性能检测,分析了42CrMo钢在不同回火温度下微观组织形貌和力学性能的变化。通过三维原子探针(3DAP)技术分析500 ℃回火温度下42CrMo钢中元素分布情况,研究了Cr、Mn、Mo等合金元素对钢性能的影响。结果表明,42CrMo钢水淬后在450 ℃回火时显微组织为回火屈氏体,在500~650 ℃区间回火时显微组织均为回火索氏体,随着回火温度的增加,颗粒状碳化物增多;抗拉强度和规定塑性延伸强度降低,-40 ℃低温冲击性能升高。在500 ℃回火可达到12.9级螺栓力学指标(R_m≥1200 MPa,KV₂≥27 J),力学性能最佳,且满足低温环境下螺栓用钢的使用要求。3DAP结果表明,钢中的合金元素通过固溶强化和沉淀强化提高了钢的性能。     

深冷处理对YG8硬质合金/42CrMo钢钎焊接头残余应力的影响

通过ANSYS仿真软件建立了YG8硬质合金/42CrMo钢钎焊接头的焊接工艺模型和深冷处理工艺模型,分析了接头表面残余应力分布,并与经深冷+回火处理后的焊接接头表面残余应力进行对比。结果表明,深冷处理可使焊接接头42CrMo钢侧形成残余压应力,且深冷温度对残余压应力的影响较深冷次数更明显。最优深冷工艺为-160 ℃深冷3次,且仿真结果与试验测量值相差不大,表明所建立的模型具有一定的可靠性。     

工艺参数对42CrMo钢激光淬火效果的影响

对42CrMo钢进行不同参数下的激光淬火处理,研究了激光淬火功率和扫描速度对42CrMo钢表面淬火硬度和淬火深度的影响规律,分析了在激光功率密度为定值时不同激光淬火参数下硬化性能的变化趋势。研究结果表明,激光功率的增大和扫描速度的减小均可以提高淬火层的深度和硬度;结合激光功率和扫描速度计算所得的能量密度参数与淬火效果存在较强相关性,在能量密度相同时,高功率高速度参数可以增加淬火硬度和深度,并提高加工效率。     

回火温度对大尺寸锻态35CrMo钢微观组织和力学性能的影响

研究了回火温度对大尺寸锻态35CrMo钢调质处理后组织特性及力学性能的影响,特别针对回火过程中渗碳体的析出动力学及大尺寸试块的组织性能不均匀性展开了分析。结果表明,锻坯热处理前的铁素体、珠光体带状组织经调质处理后完全消除,最终组织为含有大量渗碳体析出的回火马氏体,同时试块心部包含少量分布于原奥氏体晶界的回火贝氏体。回火过程中渗碳体的析出分为C扩散控制的快速长大阶段和Cr扩散控制的尺寸稳定阶段。锻态35CrMo钢经调质处理后仍存在力学性能各向异性,随着回火温度的升高,试验钢横、纵向强度下降,塑性和韧性同步提升。经综合考虑,当回火温度为570 ℃时,其强度、塑性和韧性具有最优匹配。     

TiB2/42CrMo连续梯度材料力学分析与结构设计

在以往超重力场合成梯度材料的基础上优化制备的工艺参数,制备出TiB₂-TiC-Fe组分呈连续梯度变化的TiB2/42CrMo复合材料。经XRD、SEM观察其梯度层间陶瓷/合金相界呈连续梯度变化。通过硬度测试,得出硬度自陶瓷部分至金属部分呈梯度递减变化,中间梯度层的弹性模量变化形式更趋近于三角函数。在相关参数提取研究的基础上,建立层状强度解析模型。之后采取ANSYS有限元仿真的方法,模拟分析了在施加外部位移载荷的条件下,层状模型的正应力与切应力分布,得出了梯度材料底部金属所承受的最大拉应力为造成该型材料损伤失效的主要原因,与实际材料的三点弯曲实验现象相对应。最后通过研究材料长厚比对应力分布的影响,得出在一定的尺度范围内,梯度材料的力学性质不会随长厚比的增大而线性无限增大,对下一步工程材料设计具有一定指导意义。     

精炼渣成分对42CrMo钢洁净度的影响

为了研究不同精炼渣对42CrMo钢中洁净度和夹杂物数量、尺寸的影响,采用渣-钢平衡试验和FactSage热力学理论研究了CaO-Al₂O₃-SiO₂-MgO四元渣系对42CrMo钢中洁净度和夹杂物控制的影响规律。结果表明,当初渣碱度(CaO/SiO₂的质量比)和钙铝比(CaO/Al₂O₃的质量比)分别为5和1.8时,能将钢中T.[O]质量分数降至7.5×10-6,同时夹杂物尺寸控制良好,均小于8 μm,在该渣系条件下,42CrMo钢的精炼效果最好;研究还表明,钢中夹杂物成分受渣系影响较大,特别是钢中夹杂物Al₂O₃含量受渣中Al₂O₃活度影响较大,Al₂O₃活度高的精炼渣渣系去除夹杂物Al₂O₃的能力弱,导致此精炼渣系下钢中夹杂物Al₂O₃含量高。     

15CrMo钢的蠕变-疲劳试验及蠕变回复行为

试验测试了500℃时峰值保持时间分别为0,2,5,10和20 min条件下15CrMo钢的蠕变疲劳性能,重点分析了材料在蠕变疲劳载荷下的变形行为和蠕变回复行为。结果表明,随着峰值保持时间的延长,材料的蠕变-棘轮总变形显著增加;当疲劳-蠕变存在峰值保持时间时,前一个循环中峰值保持结束时的应变值大于再次加载达到峰值时的应变值,存在一定的蠕变回复行为;峰值保持时间越长,15CrMo钢蠕变应变回复量越大,且相同保持时间下的蠕变应变回复量总体上保持恒定。