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作者简介:

陈燕,女,1963年出生,教授,博士。研究方向为精密加工与特种加工。E-mail:laochen412@gmail.com

中图分类号:TG356

文献标识码:A

DOI:10.11933/j.issn.1007-9289.20210309002

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目录contents

    摘要

    为了去除选区激光熔化技术成型的零件表面缺陷和降低表面粗糙度,并寻求最佳的加工参数。 从理论上解析电解辅助磁粒研磨的加工机理,利用仿真软件模拟加工区域的磁感应强度分布,设计 Box-Behnken 试验方案,先对材料为 Ti6Al4V 的钛合金工件表面进行电解钝化,后进行机械磁粒研磨,根据试验结果建立表面粗糙度的二次响应回归方程并对建立的数学模型进行方差分析,最后用响应面分析法分析主轴转速、磨料粒径、电解温度和电解电压对表面粗糙度的影响规律,得到最佳的加工参数,在最佳工艺参数下对磁粒研磨和电解辅助磁粒研磨的加工效果进行比较和分析。 建立的回归方程调整后的拟合优度为 92. 14%,经过优化后的电解辅助磁粒研磨最佳加工参数如下:电解液为浓度 16%的硝酸钠溶液,电解温度 28 ℃ ,电解电压 12 V,磨料粒径 180 μm,主轴转速 1100 r/ min,使用磁粒研磨加工 60 min 后,工件表面粗糙度由原始的 Ra 10. 7 μm 降为 Ra 0. 52 μm,使用电解辅助磁粒研磨加工 60 min 后,工件表面粗糙度由原始的 Ra 10. 7 μm 降为 Ra 0. 354 μm。 使用电解辅助磁粒研磨可以有效去除选区激光熔化技术成型零件型腔内表面的缺陷,并降低零件的表面粗糙度,通过响应面分析法可以有效优化加工参数,使用电解辅助磁粒研磨加工比单一磁粒研磨加工的加工效果好,加工效率高。

    Abstract

    In order to remove the surface defects and reduce the surface roughness of parts formed by selective laser melting technology, and seek the best processing parameters. The mechanism of electrolysis assisted magnetic abrasive finishing was analyzed theoretically, and the magnetic induction distribution in the machining area was simulated by using the simulation software. The box Behnken test scheme was designed. Firstly, the surface of titanium alloy workpiece with Ti6Al4V was electrolytically passivated, and then mechanical magnetic abrasive grinding was carried out. According to the test results, the quadratic response regression equation of surface roughness was established, and the established mathematical model was analyzed by variance. Finally, the influence of spindle speed, abrasive particle size, electrolysis temperature, and electrolysis voltage on the surface roughness was analyzed by the response surface method, and the best machining parameters were obtained. Under the optimal process parameters, the machining effects of magnetic abrasive finishing and electrolytic assisted magnetic abrasive finishing were compared. The goodness of fit of the established regression equation was 92. 14%, and the optimized parameters were as follows: electrolyte was 16% sodium nitrate solution, electrolysis temperature was 28 ℃ , electrolysis voltage was 12 V, abrasive particle size was 180 μm, spindle speed was 1100 r/ min, The surface roughness of the workpiece was reduced to Ra 0. 52 μm by magnetic abrasive finishing and Ra 0. 354 μm by electrolysis assisted magnetic abrasive finishing. Electrolysis assisted magnetic abrasive finishing can effectively remove the surface defects and reduce the surface roughness of parts formed by selective laser melting technology. The processing parameters can be optimized effectively by response surface analysis. The results show that the machining effect of electrolysis assisted magnetic abrasive finishing is better than that of single magnetic abrasive finishing.

  • 0 前言

  • 目前,增材制造技术已经成为衡量一国制造业发展水平的重要标志。作为增材制造技术的一个重要分支,选区激光熔化技术( Selective laser melting, SLM)在工业生产中的作用越来越大,SLM与传统减材制造的方法相比,可以快速成型复杂的零件,自动化程度较高, 大大降低了零件制造的成本和时间[1-2]。但由于成型过程中存在热交互和飞溅以及阶梯效应等现象,生产出来的零件表面质量较差,无法满足应用时的表面粗糙度要求,因此常常需要对成型后的零件进行表面光整加工以消除部分缺陷和降低其表面粗糙度[3-5]

  • 常见的对SLM成型零件后处理的方法有热处理、喷丸、喷砂、机械研磨和超声滚压等加工方法。江庆红[6]研究了机械喷丸和激光喷丸两种不同的处理方式对SLM成型合金的影响,机械喷丸处理后的工件表面的缺陷基本去除,但易对表面造成损伤且表面粗糙度仍然较高,激光喷丸可以起到细化表面晶粒的作用,延缓疲劳裂纹的扩展,但对工件表面的强化作用不明显且作用范围较窄,不适用于缺陷较多的零件。王震[7] 利用超声滚压加工技术对SLM成型的零件表面进行后处理加工,有效降低了表面粗糙度并起到了一定强化工件表面的作用,但该方法仅对平面类零件加工效果较好,不适用于曲面和阶梯面的加工。

  • 磁粒研磨加工( Magnetic abrasive finishing, MAF)是一种常用于工件表面研磨和抛光的特种加工技术,具有自锐性好、柔性和自适应性等诸多优点,可以适用于不同类型零件的加工,但MAF属于微量切削加工,材料去除效率低,尤其是对硬度较高的合金金属进行加工工时,磁性磨粒上的切削刃极易磨钝进而导致磨削性能下降,所以在加工硬度较高的工件表面时,单纯的MAF往往无法达到较为理想的研磨效果[8-10]

  • 电解辅助磁粒研磨( Electrolysis assisted magnetic abrasive finishing,EMAF)是在MAF的基础上将电解加工技术融入进去进而形成的一种复合加工技术,EMAF不仅具有MAF的优点,而且由于电解后形成了表面钝化膜,使MAF研磨的对象由硬度较高的工件基体变为硬度较软的金属钝化膜,可以极大地降低MAF受金属工件硬度约束的影响,加工效率也比单纯的MAF要高,特别适用于对工件表面的后处理加工[11-15]。为了去除SLM零件表面缺陷和降低零件表面粗糙度,探究电解加工和MAF加工相结合后产生的加工效果,文中使用EMAF的后处理加工方法对材料为Ti6Al4V(钛六铝四钒)的SLM成型零件型腔内表面进行加工,先进行电解,后进行机械磁粒研磨,使用Box-Behnken设计进行试验方案的制定,根据试验结果建立了二次回归预测模型, 并用方差分析验证了模型的合理性,然后通过响应面分析法确定了最佳加工参数,并在此参数下设计了对比试验验证使用EMAF和MAF两种加工方法的加工效果,最终通过测定加工后的表面粗糙度和观测表面形貌发现EMAF的加工效果要优于MAF的加工效果。

  • 1 加工机理

  • 1.1 EMAF加工过程中的电化学反应原理

  • EMAF中的电化学反应是一种氧化还原反应, 其反应原理如图1所示。工件与电源正极相连,成为电化学反应的阳极,空心石墨电极与电源负极相连,成为电化学反应的阴极,试验中选用硝酸钠溶液作为电解质溶液。整个电解反应的过程实际上是电解水的过程,通过电解水产生氢离子和氢氧根离子, 电源通电后,工件表面的难加工金属元素(如Ti、Al和V等)发生氧化反应,由原子状态变为离子状态, 然后与电解质溶液中的氢氧根离子结合形成不溶于水的氢氧化物,与此同时,溶液之中的氢离子由于电荷之间的相互吸引而逐渐向空心石墨电极处聚集, 最终与阴极处的电子结合形成氢气[16-19]。电解辅助磁粒研磨加工过程中的主要电化学反应如式(1)~(4)所示

  • Ti4++4OH-Ti(OH)4
    (1)
  • 图1 EMAF加工过程中电化学反应原理图

  • Fig.1 Schematic diagram of electrochemical reaction in EMAF process

  • Al3++3OH-Al(OH)3
    (2)
  • V3++3OH-V(OH)3
    (3)
  • 2H++2e-H2
    (4)
  • 1.2 EMAF加工过程中的MAF加工机理

  • MAF加工机理如图2所示,待加工工件型腔内表面为不规则曲面,根据曲面的形状设计一种仿形组合磁极,磁极仿形压附于工件表面并与工件表面保持一定的距离。在磁极和工件之间1~2mm间隙填放磁性磨粒,在磁场中,磁性磨粒被磁化沿磁力线方向有序地排列聚集形成具有一定刚度的柔性磁粒刷,通过对磁极施加旋转运动,使磁极带动磁粒刷与工件表面做相对运动,此时工件被固定保持不动,从而实现对工件表面的光整加工[20-21]。单一磁性磨粒在磁场中所受的合力 F 可分解为沿磁力线方向的分力F x 和沿磁等势线方向的分力F y。各个力的表达如式(5)~(6)所示

  • Fx=Vχ0HHx
    (5)
  • Fy=Wχ0HHy
    (6)
  • F=Fx2+Fy2
    (7)
  • 式中,F x 为沿磁力线方向的分力,F y 为沿磁等势线方向的分力,F 为单一磁粒所受合力,V 为磁粒的体积, χ 为磁粒的磁化率, μ0 为真空磁导率,H 为磁场强度,( H/x) 和(H/y) 分别为磁场强度在磁力线和磁等势线方向上的梯度分量。

  • 通过对加工区域进行磁场仿真可以更加直观地分析磁粒刷的状态,利用MAXWELL软件建立2D仿真模型,组合磁极的材料设定为N35(NdFeB),工件材料设置为不导磁材料,对建立完的模型进行仿真分析,加工区域的磁感应强度分布云图如图3所示。从仿真结果可以直观地看到磁极边缘处的磁感应强度较强,说明磁极边缘处的磁力线较密集,更有利于在加工区域形成稳定的磁粒刷,进而对工件表面进行有效的光整加工。

  • 图2 MAF加工机理示意图

  • Fig.2 Schematic diagram of MAF processing mechanism

  • 图3 加工区域磁感应强度分布云图

  • Fig.3 Cloud map of magnetic induction intensity distribution in processing area

  • 2 试验

  • 2.1 试验装置与加工过程

  • 图4 为试验装置的外观图,试验装置主要由小型数控机床、仿形磁极、蠕动泵、直流电源和空心石墨电极等组成,电解辅助磁粒研磨加工示意图如图5所示,工件被固定在可以移动的机床工作台上,直流电源的正极通过导线与工件相连,负极则与空心石墨电极相连,仿形磁极由两个 Φ35mm×5mm的磁极和一个 Φ25mm×5mm的磁极组合而成。整个加工过程分为两个工位完成,第一个工位上发生的是电化学反应,电解液经过过滤器过滤后,在蠕动泵的驱动下,通过软管喷洒到待加工工件的表面,直流电源通电后,工件作为阳极发生氧化反应,在工件表面形成一层质地较软的钝化膜,然后将工件移动到第二个工位上,利用磁粒研磨加工的方法,去除工件表面上硬度较低的钝化膜。

  • 图4 EMAF加工装置示意图

  • Fig.4 Schematic diagram of EMAF processing device

  • 图5 EMAF加工过程示意图

  • Fig.5 Schematic diagram of EMAF process

  • 2.2 试验条件与加工参数

  • 电解辅助磁粒研磨加工的主要恒定参数如表1所示,工件是通过选区激光熔化技术直接成型得到, 其材料为Ti6Al4V,工件示意图如图6所示,其形状为不规则形状,内表面为较为复杂的曲面组合,采用JB-8E触针式表面粗糙度仪对工件表面进行表面粗糙度测量,每次检测的最大区域长度为4mm,工件表面的初始表面粗糙度在Ra 10.5 μm和Ra 11.0 μm之间,电解液是影响电解效果的基本因素,中性电解液中常见的电解质有NaCl、NaNO3 和NaClO3, NaCl溶液的成本较低但易对工件和设备产生腐蚀作用,NaClO3 溶液腐蚀作用较弱,但其成本较为昂贵,且加工过程中ClO3-离子容易损失,需要不断地补充电解质溶液,所以综合考虑,选择浓度为16%的NaNO3 溶液作为电解质溶液,在进行MAF加工之前在第一工位上提前进行电解,电解时间为5min,随后在第二工位上进行MAF加工,加工时间为60min,磁极与工件之间的加工间隙设置为2mm。

  • 表1 EMAF加工过程中的主要恒定参数

  • Table1 Main constant parameters of EMAF process

  • 图6 工件示意图

  • Fig.6 Schematic diagram of workpiece

  • 影响EMAF最终加工效果的参数有很多,通过文献调研,将主轴转速、磨料粒径、电解温度、电解电压和电解液浓度确定为主要影响因素[14,22-24],由于在电解过程中,电解液在不断地流动与损耗,其浓度也在不断地变化,无法维持稳定的状态,因此无法将电解液浓度作为独立的参数水平设计因素水平试验。其他主要影响因素均设置三个水平,独立参数水平表如表2所示。主轴转速受机床性能的影响, 当主轴转速低于1 000r/min时,磁粒刷不能有效地去除工件表面的钝化膜,而转速高于1 200r/min时,磁粒刷往往会不稳定,磁粒链易断裂,从而降低加工效率,因此将主轴转速的范围设置为1 000~1 200r/min。磁性磨料是通过烧结法制备的,将铁粉和Al2O3 粉末以2 ∶ 1的比例均匀混合后,经过制坯、压制、烧结、粉碎和筛分等步骤制备得到,选用粒径分别为150 μm、180 μm和250 μm的磁性磨料作为研究对象。电解温度和电解电压都是影响电化学反应的重要因素,在配置电解液时通过温控装置控制电解液的温度,并将温度保持在20℃、30℃ 和40℃3个水平,同时,调节直流电源将电解电压设置为5、10和15V3个水平。

  • 表2 独立参数水平

  • Table2 Levels of independent factors

  • 3 结果与讨论

  • 3.1 试验结果

  • 运用Box-Behnken设计进行响应曲面试验设计,Box-Behnken设计无需进行多次连续试验,可以大大减少试验的次数,并且可以有效评估加工因素的非线性影响[25],每隔60min测一次零件表面的粗糙度度值,并且记录在表格中,所设计的试验方案及试验结果如表3所示,然后利用软件MINITAB进行数学建模,所建立的多元二次回归模型如式(8) 所示

  • Ra=5.811-0.0637X3+0.0115X4-0.02938X2-0.006225X1+0.000967X3X3+0.004067X4X4+0.000207X2X2+0.000003X1X1+0.000100X3X4-0.000025X3X2+0.000009X3X1-0.000375X4X2-0.000075X4X1+0.000001X2X1
    (8)
  • 式中,X1 为磁极转速,X2 为磨料粒径,X3 为电解温度,X4 为电解电压。

  • 用方差分析的方法对所建立的回归模型的有效性和合理性进行分析,分析结果如表4所示,该模型的总自由度DF为26,拟合优度 R 2 为96.37%,远远大于85%, 且调整后的拟合优度 R 2( adj) 为92.14%,与调整前的拟合优度差别较小,故说明该回归模型的拟合效果较好,所拟合的模型具有良好的有效性。另外,观察回归模型的残差图,从图中可以直观地看出残差图上的点在残差值为0的线上下呈无规律随机均匀分布,说明回归模型响应的随机性部分合理,进一步验证了该回归模型的合理性,回归模型的残差图如图7所示。

  • 3.2 加工参数对表面粗糙度的影响

  • 电解温度对表面粗糙度的影响如图8a所示,随着电解温度的升高,工件表面粗糙度值呈现先减小后增大的趋势,这是因为在温度相对较低时,增加电解温度可以促进金属元素(如Ti、Al和Fe等)向离子态的转变,并且温度越高,越容易形成稳定的高价态金属离子,有利于钝化膜的形成,从而在第二工位进行加工时能够有效去除钝化膜和降低表面粗糙度,而在电解温度较高时,随着电解温度的增加,电流效率不断降低,所以无法及时有效地形成钝化膜, 在第二工位进行磁粒研磨时,易对工件基体产生过磨,最终使工件表面的粗糙度值升高。

  • 表3 Box Behnken试验设计及结果

  • Table3 Box Behnken experimental design and results

  • 表4 响应面方差分析

  • Table4 ANOVA for response surface

  • 图7 回归模型的残差图

  • Fig.7 Residual graph of regression model

  • 电解电压对表面粗糙度的影响如图8b所示,随着电解电压的升高,工件表面的粗糙度值先减小后增大,出现这种现象的原因是因为在低电压时,电解的电压越高,电子的移动速度越快,所以工件表面上钝化膜的形成速率较快,但当电解电压过高时,易出现过电解的现象,所形成的钝化膜较厚,以至于在相同的加工时间内,钝化膜未被完全去除,最终工件表面的粗糙度值反而会增大。

  • 图8 加工参数对表面粗糙度的影响

  • Fig.8 Influence of processing parameters on surface roughness

  • 磨料粒径对表面粗糙度的影响如图8c所示,一方面,磨料的粒径较大时,在相同的研磨时间内,去除钝化膜的速率较快,但易对工件表面产生过度磨损,工件表面的粗糙度值随着磨料粒径的减小而逐渐减小。另一方面,当使用粒径较小的磨料进行研磨时,去除钝化膜的效率较低,且钝化膜往往无法得到完全去除,加工完后仍有部分钝化膜存留在工件表面,致使加工后的表面粗糙度值较大。

  • 主轴转速对表面粗糙度的影响如图8d所示, 工件表面的粗糙度值随主轴转速的增加而先减小后增大。在主轴转速较低时,随着主轴转速的增加,磁性磨粒对工件表面钝化膜的刮擦速率逐渐增加,钝化膜去除效率逐渐提高,但当主轴转速过高时,磁性磨料容易被甩出加工区域,从而使参与研磨的有效磨料减少,且在高速的情况下,磁性磨料易磨损,降低钝化膜的去除速率,最终使表面粗糙度值升高。

  • 3.3 加工参数优化及结果

  • 3.3.1 响应面分析法优化加工参数

  • 以任意两个加工参数为自变量,以表面粗糙度为因变量,构建6个响应曲面,所构建的工件表面粗糙度响应曲面如图9所示,通过观察该响应曲面,发现在部分响应曲面中存在极小值,因此可以根据式(8)所构建的回归模型进行优化分析来寻找一组使工件表面粗糙度值最小的加工参数,利用软件MINITAB中的响应面优化器对加工参数进行优化,优化后的加工参数如下:电解温度28.282 8℃,电解电压12.27V, 磨料粒径183.33 μm,主轴转速1 115.15r/min。

  • 图9 工件表面粗糙度响应曲面

  • Fig.9 Response surface of workpiece surface roughness

  • 3.3.2 优化后的试验结果

  • 由于无法准确地在试验设备上设定优化后的加工参数,所以将优化后的加工参数的近似值(即电解温度28℃, 电解电压12V, 磨料粒径180 μm,主轴转速1 100r/min) 作为试验条件, 并运用改良后的最佳加工参数对材料为Ti6Al4V的合金模具型腔内表面进行加工,分别采用MAF和EMAF两种加工方法进行对比试验,以此来判断两种加工方法的优劣,优化后的加工参数如表5所示。

  • 表5 优化后的MAF和EMAF加工参数

  • Table5 Processing parameters of MAF and EMAF after optimization

  • 通过观察加工后工件表面形貌和测定工件表面的粗糙度值来评价加工后的工件表面质量,工件表面形貌和粗糙度值如图10所示,其中,图10a为工件表面的原始形貌,从图中可以清晰地看到工件表面有很多凹坑和凸起,并且在表面附着一层黑色的氧化皮,其表面粗糙度Ra 10.7 μm,使用MAF加工60min后,工件表面的粗糙度降为 Ra 0.52 μm,凹坑等缺陷得到较为明显的去除,表面粗糙度降低了95.14%,使用EMAF先电解5min后进行MAF加工60min后,工件表面的缺陷得到进一步去除,表面粗糙度进一步降低, 最终表面粗糙度值达到 Ra 0.354 μm,表面粗糙度降低了96.69%,工件表面质量得到极大提升。产生这种现象的原因是因为单纯的MAF所加工的对象是工件的基体,而工件的基体材料为硬度较高的钛合金,随着加工时间的增加,磨料易于磨损,磨料寿命降低,从而导致加工效果不理想,而EMAF加工则不受工件材料硬度的影响,其所加工的对象为电解后在工件表面形成的钝化膜, 钝化膜的硬度要低于工件基体,在使用磁粒刷研磨加工的过程中,磨料磨损相对较慢,使用寿命变长, 从而可以获得较好的表面质量,且残存在工件表面上的钝化膜可以在一定程度上起到抗氧化和耐腐蚀的作用。

  • MAF和EMAF两种加工方式随时间对工件表面粗糙度的影响如图11所示,从图中可以看出, 在0~30min,使用EMAF加工的工件表面粗糙度下降较快,并且在30min以后表面粗糙度的变化不大,基本趋于稳定,而使用MAF加工的工件表面粗糙度一直呈下降趋势,在50min左右才趋于稳定,由此可以得出,相对于单纯的MAF加工, EMAF的加工效率更高,这是因为使用EMAF加工过程中,相对于硬度较高的工件基体材料,磁性磨粒可以比较容易地去除由电解产生的钝化膜,从而可以获得较高的材料去除效率,综上,将电解加工与MAF加工融合,可以在工件表面性能、磨料使用寿命和加工效率等方面有效提升MAF加工的技术性能。

  • 图10 工件表面形貌和粗糙度值

  • Fig.10 Surface topography and roughness of workpiece

  • 图11 MAF和EMAF对工件表面粗糙度的影响

  • Fig.11 Influence of MAF and EMAF on surface roughness of workpiece

  • 4 结论

  • (1)使用电解辅助磁粒研磨的加工方法可以显著改善SLM成型零件型腔内表面的表面质量。

  • (2)进行了Box-Behnken试验设计,构建了预测表面粗糙度的二次回归模型,并通过方差分析的方法验证了该模型的有效性和合理性。

  • (3)用响应面分析法优化了加工参数,并且联系实际确定了最佳的加工参数:电解温度28℃,电解电压12V,磨料粒径180 μm,主轴转速1 100r/min。

  • (4)在最佳加工工艺参数下,分别采用单纯磁粒研磨和电解辅助磁粒研磨两种加工方法加工工件,使用单纯磁粒研磨加工后工件表面粗糙度降低95.14%,而使用电解辅助磁粒研磨加工后工件表面的粗糙度降低了96.69%,电解辅助磁粒研磨的加工效果和加工效率要优于单纯的磁粒研磨。

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