【作者】 李路；

【导师】 周杰；

【作者基本信息】 重庆大学 ， 材料加工工程， 2010， 博士

【摘要】 课题来源于重庆大江信达铸锻有限公司项目“重车转向节复合成形工艺与提高挤压模具寿命的研究”。本文以生产实际的客观要求为根本出发点,在充分研究挤压和模锻材料变形特点和工艺特性的基础上,对挤压-模锻这一新的转向节复合成形工艺进行了实践及跟踪。通过生产观察发现:转向节热挤压质量的不稳定性以及挤压模具寿命低下已经成为制约该复合成形工艺广泛应用和推广的主要问题。如何提高转向节热挤压的工艺性,显著提高热挤压模具寿命已经成为转向节复合成形工艺的关键技术。本文立足这两个问题开展了以下三个方面的研究:①转向节热挤压工艺性研究首先对某典型重车转向节的立式挤压模锻复合制造工艺进行了分析和研究。对生产中普遍存在的杆部及主销臂叉形头部填充不足,侧壁折叠等问题进行了有限元分析,找到了缺陷产生的主因。首先提出在挤压工序前增加成形镦粗工序这一转向节立式挤压中行之有效的工艺优化手段,同时对成形镦粗高度和挤压圆角半径进行了一系列的优化对比分析,显著提高了转向节立式挤压的工艺性。进一步,以该转向节不同热挤压预制坯方案为研究对象。创造性的提出了镦挤拍扁后卧挤的转向节热挤压精密预制坯方案。并将其与其他三种预制坯方案从挤压件的杆部填充性能,飞边铺展均匀性,流线分布状态等方面进行了综合对比研究,解析了不同预制坯状态下材料的流动特性及流线分布机理。研究证明,镦挤拍扁后卧挤的转向节热挤压精密预制坯方案具有最优的综合工艺性能。最后,将镦挤拍扁后卧挤的转向节热挤压精密预制坯方案推广到Hummer和Steyr重车转向节的挤压—模锻复合成形制造中并获得了成功。证明该新式预制坯方案具有优越的工艺性能和广泛的可推广性。②转向节热挤压系统的解析和控制本部分的研究重点不再局限于重型汽车转向节的工艺性控制方面,而是从更宏观的角度来综合考察转向节热挤压过程中物质、能量的流动特点以及坯料、模具、设备和工艺参数等诸多因素对整个转向节热挤压系统稳定性和耐久性的影响规律,使得本课题的研究更具一般性。首先,依照系统工程的相关理念提出了“转向节热挤压系统”的概念,对转向节热挤压系统的特点进行了分析和概括。并将该系统的子系统进行了归纳和划分,明确了物质、能量在各子系统中的流动过程和特点。然后,采用“全面质量管理”中的相关方法研究了该系统稳定性和耐久性差的根本原因,得到了主因分析的石川图,并结合生产实际的统计数据对主因按发生频率和破坏程度进行了排序。从提高系统稳定性和耐久性的角度,明确了“归根结底需要从热挤压系统受力和受热这两方面去综合研究”的思路。随后,采用有限元数值模拟方法重点研究了不同成形速度、坯料初始温度、摩擦系数、模具初始硬度这四个主要的可控工艺参数对热挤压系统力(最大成形载荷,凸凹模应力分布)、热(挤压件温度场,模具表层温度分布)两方面的影响规律。并运用最小二乘法拟合得到了以上工艺参数关于最大挤压力、凹模应力集中区最大主应力、挤压锻件最高温度、模具表层最高温度的影响规律曲线。最后,根据以上研究成果提出了针对转向节热挤压系统的工艺参数综合工程指导规范。③转向节热挤压模具结构研究及其参数优化解析转向节挤压—模锻复合制造工艺中热挤压模具常见的失效形式、部位和机理,首次将阻力墙结构引入转向节热挤压模具结构中以改善其整体的强受载性质,以期获得填充性和模具寿命的平衡和匹配。采用有限元模拟方法分别对闭式挤压结构、开式飞边槽结构和阻力墙结构进行了关于受力、受热和填充能力三个方面的对比研究,指出阻力墙结构的实质,得到阻力墙结构具有最优综合性能的评价。进一步,采用部分析因试验设计的方法,针对转向节热挤压加工中常用的三个评价指标:填充能力、最大成形载荷、杆部圆角区最大磨损量进行主参数效应的筛选和效应显著性排序,得到了主参数效应的排序表。随后采用径向基函数近似模型方法结合遗传算法(RBF-GA)作为组合优化策略对转向节热挤压模具上的阻力墙结构参数开展多目标优化研究,以期找到最优的结构参数组合,充分发挥阻力墙结构在填充性和模具寿命方面的综合性能潜力。首先通过拉丁超立方抽样试验设计,选择了适当的设计参数样本。在对相关核函数拟合效果综合对比的基础上,构建了具有较高精度的以对数路径为核函数的径向基函数近似模型。然后将遗传算法引入近似模型进行多目标优化,并结合工程实际评价了目标函数的权重效应,将相关的权重因子引入多目标评价中,采用线性加权的方法重构目标函数并进行寻优操作,最终得到了最优的阻力墙参数取值。最后将研究结论应用到大江铸锻公司相关转向节热挤压模具上进行了试验和跟踪。统计发现五套跟踪的转向节热挤压模具寿命平均为5187件,是原闭式结构挤压模具寿命的2.66倍,是参数优化前阻力墙结构挤压模具寿命的1.48倍。阻力墙结构在转向节热挤压模具上获得了成功,在保证锻件质量的前提下,挤压模具增寿的效果显著。更多还原

【Abstract】 This topic is originated from the Dajiang Xinda Vehicle Stock Limited Company which project is studying on the extrusion process and extrusion die life for knuckle of heavy truck compound process.In this paper, with fully taking into account the objective requirements of actual production, based on the intensive study for the characteristics of extrusion deformation, the new forming method, i.e. combined extrusion and forging processes for knuckle of heavy truck is put into practice and tracked. It found that the instability of forming quality and the lower service life of extrusion die are the major problems to restrain the widely application for the new compound process. This paper carried out the following three research points based on the up two issues:①Hot extrusion process study on knuckle of heavy truckFirstly, the traditional extrusion process about typical knuckle of heavy truck is studied. The reasons about fold and underfill in the process were analyzed. According the results of analysis, added the upset process with a fillet before extrusion to the original process, which could prevent the offset and the defects effectively. Then the heights of the billet in the upset process and the radius size of fillets are optimized,“Table about different upset heights of billet versus qualities of extruding part”and“Curve about radius of fillets versus extrusion ratio”are given. It significantly improve the manufacturability of steering knuckle with vertical extrusion process.For further study, different processes about knuckle hot extrusion are comparative analyzed by FEM simulation, which focus on filling capacity, deformation uniformity and flow lines distribution. The results show that the process of upsetting-extrusion and press has the best integrated process performance.At last, the precision perform process of upsetting-extrusion and press is introduced to the deformation of Hummer and Steyr knuckles, which obtained satisfactory results. It proves that the new program has a superior process performance and the wide range of replicability.②Analyze and control of the knuckle hot-extrusion systemIn this part, we pay more attention to the characteristics of matter and energy in the knuckle hot-extrusion system other than the extrusion process,which focus on the influences of system components such as billet,equipment, die and process parameters upon the stability and durability of the system. First of all, in accordance with the idea of“System Engineering”, a new concept called "Knuckle hot-extrusion system" is put forward and the characteristics of it and its subsystems are analyzed and summarized. Then the Ishikawa diagram which comes from the method about“Total Quality Management”is proposed to study the defects of the system and find out the root causes finally. According to the statistical data of actual production, the sequences about root causes are lined up based on the frequency and extent of damage. It draws a conclusion that in order to improve the stability and durability of the system, we must study on the system by the force and heat. The effects of various forming velocities,initial temperature of billets, friction coefficient and the initial hardness of die on knuckle hot extrusion system about pressure(maximum extruding force and maximum principal stress) and temperature(maximum part temperature and maximum die surface layer temperature) are studied by FEM simulation. The curves about process parameters are obtained by least square method. Finally, based on the findings, the engineering guide specifications about process parameters are proposed which are suitable for knuckle hot-extrusion system.③Study on structure of knuckle hot extrusion die and parameters optimizationBased on analyzing the failure modes, locations and mechanisms which commonly appear in knuckle hot extrusion dies, a new flash structure which is named resistance wall is introduced, expecting to improve the severe service conditions and balance the needs between the better filling capacity and longer die life. Focus on filling capacity, the maximum deformation load and the highest die surface-layer temperature, different die structures, such as open-flash cave structure, traditional close die structure and resistance wall structure, are comparatively analyzed by FEM simulation. Then identify the essence of the new structure and draw a conclusion that it has the best comprehensive performance. After that, the fractional factorial experimental design method is used to evaluate main effects of the structure parameters on filling capacity, the maximum deformation load and the maximum wear in bar fillet area. A rank-order table about main parameters effects is obtained. In order to fulfill potential of resistance wall in filling capacity and extend die life, it’s very important to find the optimal combination of structural parameters. The multi-objective optimization is studied using combinatorial optimization strategy which is radial basis function approximation model methods (RBF) combined with genetic algorithm (GA). Latin hypercube sampling design is applied to select the appropriate design parameters for the FEM experiments. The RBF meta-models are obtained after comprehensive comparing the fitting accuracy of five different kernel functions. Then the GA is introduced into meta-models to carry out multi-objective optimization. The weighting coefficients of the objective functions are determined according to engineering practice. The optimal parameter values of resistance wall are got by weighting function optimization. Finally, the conclusion of the study applied to five sets of knuckle hot extrusion dies of Dajiang Company. Statistics find that the average service life of the five sets of dies is 5187, which is 2.66 times of the life of original close-die and 1.48 times of the life of die with unoptimized resistance wall structure更多还原