AZ31镁合金热轧边裂预判模型研究

采用等压法,通过等温热压缩实验获得了AZ31镁合金变形温度和应变速率分别在473~673 K和0.005~5 s-1条件下对临界断裂应变的影响规律,以及Zener-Hollomon表达式,据此针对AZ31建立了临界断裂应变与变形温度和应变速率间的基本模型;在此基础上,基于镁合金轧制边裂的基本机理,引入CockcroftLatham断裂准则,建立了含有材料变形激活能和基本轧制工艺参数的AZ31镁合金轧制边裂预判模型;并通过相同条件下有限元模拟和热轧试验分别得到沿板宽方向损伤值和边部裂纹深度,以此对所建立的边裂预判模型进行验证,结果显示所建立边裂预判模型的预测值和实测值平均误差为11.3%。     

超声强化2D12铝合金疲劳断裂过程的微观分析

采用超声强化工艺对2D12铝合金进行表面处理,借助金相显微镜和扫描电镜,并结合原位跟踪测量裂纹长度的方法,对强化后的疲劳断裂行为进行了研究。结果表明,超声强化后2D12铝合金表面晶粒得到细化,疲劳源主要产生于试样表面,仅个别向内部转移,强化后疲劳寿命提升了约8倍。其原因一方面是强化过程在试样表面引入了残余压应力,由于裂纹扩展过程倾向于连接裂纹扩展路径上的缺陷,所以残余压应力的存在效降低了裂纹在两缺陷间的扩展速率;另一方面晶粒细化导致晶界密度增加,加强了对裂纹扩展的阻碍,从而有益于提高裂纹扩展寿命。     

SiC_p/Al复合材料薄壁件断裂损伤仿真研究

利用有限元仿真软件ABAQUS对体积分数为56%的Si C_p/Al复合材料薄壁件进行切削加工仿真,研究了切削加工过程中不同切削深度对工件产生裂纹的位置和出现裂纹时工件顶部变形量的影响规律。结果表明:切削深度的改变会对工件产生裂纹的位置和出现裂纹时工件顶部变形量有不同的影响,其中切削深度在1mm左右会使工件上出现裂纹的位置急剧改变,裂纹出现位置从工件的中部变化到工件的下部,增大切削深度后裂纹的位置集中出现在工件下部区域;而切削深度改变,在工件上出现裂纹时,工件顶部的变形量没有太大变化,主要集中于0.7mm左右,这主要是Si Cp/Al复合材料内部Si C颗粒数量多、粒径大使得自身协调变形能力差的缘故。     

基于预期失效分析的伞型专利组合规避设计

寻求专利保护是企业发展的基本专利战略。一类由互补性专利组成的伞型专利组合是通过挖掘同一产品系统存在或潜在的更多技术问题而形成的具有互补技术特征的系列专利方案,通过更大的专利权利保护范围实现对同一产品系统的全方位保护。预期失效分析(Antucupatory failure determination,AFD)作为在发明问题解决算法(Algorithm for inventive-problem solving,ARIZ)中理想解和冲突的基础上发展起来的失效预测分析工具,成为发现失效问题的一种相对成熟的方法,能挖掘现有的产品系统各方面的问题以及预测尚未发现的问题。提出通过伞型专利组合分析、失效分析、TRIZ工具解决问题、专利性评价四个步骤组成的基于AFD的伞型专利组合规避设计,首先通过对规避对象产品系统进行专利组合分析,标定对象产品系统的专利权利范围,为创新设计设定了界限并提供了研究线索;其次,运用AFD进行失效分析和失效预测,确定系统已存及可能的失效点;再次,通过应用TRIZ方法产生出创造性的解决方案;最后,利用专利实质授权条件评价筛选方案并进行专利申请。应用该流程对一种擦窗机器人进行创新案例分析,形成具有自主知识产权的创新设计方案。     

Analysis of internal crack in a six-ton P91 ingot

P91 is a new kind of heat-resistant and high-tensile steel. It can be extruded after ingot casting and can be widely used for different pipes in power plants. However, due to its mushy freezing characteristics, a lack of feeding in the ingot center often generates many defects, such as porosity and crack. A six-ton P91 ingot was cast and sliced, and a representative part of the longitudinal section was inspected in more detail. The morphology of crack-like defects was examined by X-ray high energy industrial CT and reconstructed by 3D software. There are f ive main portions of defects larger than 200 mm~3, four of which are interconnected. These initiated from continuous liquid f ilm, and then were torn apart by excessive tensile stress within the brittle temperature range(BTR). The 3D FEM analysis of thermo-mechanical simulation was carried out to analyze the formation of porosity and internal crack defects. The results of shrinkage porosity and Niyama values revealed that the center of the ingot suffers from inadequate feeding. Several criteria based on thermal and mechanical models were used to evaluate the susceptibility of hot crack formation. The Clyne and Davies' criterion and Katgerman's criterion successfully predicted the high hot crack susceptibility in the ingot center. Six typical locations in the longitudinal section had been chosen for analysis of the stresses and strains evolution during the BTR. Locations in the defects region showed the highest tensile stresses and relative high strain values, while other locations showed either low tensile stresses or low strain values. In conclusion, hot crack develops only when stress and strain exceed a threshold value at the same time during the BTR.     

Modification effect of calcium-magnesia phosphate fertilizer on microstructure and mechanical properties of Mg2Si/Mg-4Si composite

China Foundry - In order to modify in-situ synthesized Mg2Si particles in Mg2Si/Mg-4Si composite, the modification effect of calcium-magnesia phosphate fertilizer on primary Mg2Si phase in...     

Fracture behaviour and numerical study of resistance spot welded joints in high-strength steel sheet

Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out using 980 MPa high strength steel sheet of 1.6 mm thickness. In proportion, as nugget diameter increased from 3√t to 5√t (where t is thickness), cross tension strength (CTS) increased while fracture morphology simultaneously transferred from interface fracture to full plug fracture. In cases of interface fracture, circumferential crack initiation due to separation of the corona bond arose at an early stage of loading. The crack opening process without propagation was recognized until just before fracture and then the crack propagated to the nugget immediately in a brittle manner around CTS. In full plug fracture, main ductile crack initiation from the notch-like part at the end of sheet separation occurred with the sub-crack initiated at an early stage. The ductile crack propagated toward the HAZ and base material to form full plug fracture. The mode I stress intensity factor was considered as a suitable fracture parameter because the circumferential crack behaved pre-crack for brittle fracture in the nugget region at the final stage. Based on the FE analysis, the mode I stress intensity factor was calculated as 116 MPa √m at CTS as fracture toughness for the nugget. With respect to full plug fracture, ductile crack initiation behaviour from the notch-like part was expressed by concentration of equivalent plastic strain. On the assumption that the ductile crack arose in critical value of equivalent plastic strain, the value was calculated as 0.34 by FE analysis. Reasonable interpretation for interface fracture and full plug fracture in the resistance spot welded joint was proposed due to first crack initiation by stress concentration, brittle fracture by using mode I stress intensity factor, and ductile crack initiation by using equivalent plastic strain.     

0Cr17Ni4Cu4Nb钢锻造裂纹分析与控制

针对0Cr17Ni4Cu4Nb不锈钢锻造生产过程中产生的锻造裂纹,对原材料和锻造工艺进行分析发现,产生裂纹的主要原因是由于材料内部组织中的δ铁素体含量超过一定量时,会极大地降低材料的锻造塑性,使得变形抗力增大;当变形量逐渐增加到一定量时,裂纹开始出现,并且随着变形量的增加裂纹越来越严重。研究结果表明,在使用该材料时,需要对原材料进行铁素体含量检查并加以控制,铁素体含量要求20%,符合CB/T 1209—1992 F7级以上标准,保证该材料良好的热加工工艺塑性;毛坯粗糙度要求达到Ra=1.6~0.8μm以上,预热锻造工具,严格控制每一火次的锻造变形量,以满足锻件的设计和质量要求。     

铁道车辆轮轨接触疲劳的影响因素

随着铁路客货运量的增大和列车速度的提高,使得高速铁路的轨道必然比普通线路具有更高的安全性、可靠性和平顺性,为保证轨道结构的这些要求,轨道各部件的力学性能、使用性能和组成为结构的整体性能都比普通轨道部件高,但在高速铁路轨道上仍然存在很多问题。目前,轮轨接触疲劳是最常见的轮轨问题,国内外许多学者对此做了研究。导致机车车辆轮轨接触疲劳的因素有很多,指出了当下一些常见的轮轨接触疲劳损伤形式,同时对滚动接触疲劳的影响因素进行了简要分析。为了研究疲劳损伤产生、发展的机理和影响因素,通过查阅大量资料,了解了各种常见损伤的产生和发展机理,并总结了在该领域的研究方向与热点,同时指出了当下高速铁路建设中存在的技术难题。结合轮轨接触疲劳的失效形式和磨损的特点,从材料对踏面的影响、高速与重载对轮轨接触疲劳的影响、润滑剂对轮轨裂纹增长的影响等方面,进一步提出预防和减缓钢轨接触疲劳损伤的具体措施。     

预充氢马氏体时效钢的氢脆性能研究

目的研究预先存在于试样中的氢对材料力学性能的影响。方法对固溶态和三种时效态18Ni马氏体时效钢,采用双电解槽装置测量了其氢扩散系数,用热分析法获得了材料的氢扩散激活能。采用慢应变速率拉伸法评估了在预充氢后镀镉密封试样的力学性能,并由此评估它们的氢脆敏感性。结果固溶态试样的氢扩散系数最大,为1.40×10~(-8)_ cm~2/s;对时效态试样,当时效温度分别为465、490、530℃时,氢扩散系数分别为6.23×10~(-9)、5.52×10~(-9)、2.84×10~(-9) cm~2/s,即随时效温度升高,扩散系数降低。而扩散激活能正好相反,固溶态的最小,其他的依次逐渐升高。四种试样均显示出氢脆敏感性,且随着预充氢电流密度升高而增大。T465和T490的氢脆敏感性均大于58%,T530的氢脆敏感性小于40%。四种试样的断口形貌均表现为由中心起裂,向周围呈放射状扩展。中心起裂源处为典型的沿晶开裂,扩展区为准解理开裂。结论过时效态样品的抗氢脆性能最好。预先存在于试样中的氢在拉伸过程中向中心富集,造成中心沿晶开裂,与动态充氢拉伸断口相反。