慢走丝电火花线切割以其加工精度高、表面质量好和不受工件材料限制的加工特点成为机械加工领域不可替代的加工手段,但也存在加工效率低的弊端.钛合金TC4因具有高温力学性能优异、比强度高和失稳临界值高等优点而广泛应用于航空航天领域,但其强度高、导热性差和易变形特点使其成为一种典型难加工材料[1].
慢走丝电火花线切割加工钢、铜和硬质合金等材料已相当成熟,但对于其他材料的线切割研究相对较少[2].目前慢走丝电火花线切割机床系统中没有针对TC4材料的加工参数.本文在试验基础上,利用灰色关联分析和信噪比的方法探究各电参数和非电参数对慢走丝电火花线切割TC4的影响规律,从而得到兼顾加工效率、加工稳定性和加工精度的多目标工艺参数最优组合,并用试验验证这一最优组合的正确性.为提高慢走丝电火花线切割机床的加工效率和其他型号钛合金材料加工参数的选择提供参考依据.
1 试验设计 1.1 试验原理、设备与材料慢走丝电火花线切割是利用脉冲放电去除多余材料以达到对零件尺寸和加工形状要求的非接触式加工[3].在微观上大致分为去离子水介质击穿和形成放电通道、在TC4试件和黄铜丝间进行能量转换和传递、两极材料抛出和极间介质消电离等几个阶段.本文所用机床为阿奇夏米尔CA20,见图 1.厚度为(10±0.01)mm的TC4试件接正极,黄铜丝(直径d=0.2 mm)接负极并以一定速度(30~330 mm/min)沿着电极丝轴线方向根据预定的切割轨迹进行单向移动,不断进入和离开TC4试件窄缝内的放电加工区.工作液为去离子水,有利于排除熔融的蚀除物,从而提高加工速度[4].加工时上导丝嘴与工件上表面距离为0.1 mm.
切缝宽度决定慢走丝线切割的加工尺度范围和最终加工尺寸的精度,其受电极丝直径、电极丝横向振动和两侧放电间隙的影响[5].采用基恩士公司的超景深三维立体显微系统VHX-1000E对加工后的切缝宽度进行测量,为准确测量出切缝宽度,本文进行二维测量,见图 2a,首次提出三维面间距的方法更准确测量出切缝宽度,见图 2b.
在冲液压力为0.8 MPa,脉冲间隔为25 μs的条件下,以峰值电流(I)、脉冲宽度(ti)、开路电压(UHP)、走丝速度(vAW)和丝张力(FW)为慢走丝线切割加工TC4的5个工艺参数,每个参数选择4个水平,见表 1,以材料加工时间、表面粗糙度和切缝宽度为工艺指标,正交试验结果如表 2所示.
慢走丝线切割加工后的表面不同于传统加工的表面,由无数个无方向性的凸边和凹坑组成.采用基于白光干涉原理的法国STIL三维轮廓仪对TC4试件表面进行非接触测量,得出表面粗糙度(Rs),图 3为表面形貌和表面粗糙度值在线检测.
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