张翔1,3, 王晓溪2, 张德坤1, 曹秉宇2, 周怡2
(1. 中国矿业大学 机电工程学院,江苏 徐州 221116;2. 徐州工程学院 机电工程学院,江苏 徐州 221018;3. 高端工程机械智能制造国家重点实验室,江苏 徐州 221004)
摘要:
基于传统等通道转角挤压(Equal Channel Angular Extrusion, ECAE)的变形原理,开发设计出一种具有球形空腔结构的新型复合大塑性变形工艺——等通道球形转角挤压(Equal Channel Angular Extrusion with Spherical Cavity, ECAE-SC)。以ECAE工艺为对比参照,开展了室温条件下工业纯铝单道次ECAE-SC挤压实验研究,进行了金相显微分析、显微硬度测试和球-面接触往复式摩擦磨损试验,获得了变形材料的显微组织、显微硬度、摩擦系数、磨损率和三维磨损形貌等特征参数.研究表明:ECAE-SC球形转角的圆滑过渡可大幅提高模具外角处金属的流动性,有效避免产生传统ECAE单道次变形“死区”,挤出试样头部翘曲现象得到显著改善；室温单道次ECAE-SC变形后,工业纯铝晶粒细化明显,显微硬度值由初始36.6HV增加至58.7HV,比同条件下ECAE变形试样(52.8HV)提高了11.2%；ECAE-SC变形材料耐磨性能最好,并表现出良好的摩擦磨损稳定性,磨损机制从磨粒磨损和氧化剥层磨损为主导的混合磨损机制转变为以磨粒磨损为主导的磨损机制。
关键词: 剧烈塑性变形 等通道球形转角挤压 工业纯铝 显微硬度 耐磨性能
DOI：10.11951/j.issn.1005-0299.20190008
分类号:TG376；TG113.2
文献标识码:A
基金项目:国家自然科学基金资助项目(51905462)；江苏省“六大人才高峰”高层次人才选拔培养资助项目(GDZB-127)；江苏省科协青年科技人才“托举工程”资助培养项目(苏科协发〔2018〕202号).
Influence of spherical cavity on microstructure and properties of commercially pure aluminum during equal channel angular extrusion process
ZHANG Xiang1,3, WANG Xiaoxi2, ZHANG Dekun1, CAO Bingyu2, ZHOU Yi2
(1. School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou 221116, China; 2. School of Mechanical and Electrical Engineering, Xuzhou University of Technology, Xuzhou 221018, China; 3. State Key Laboratory of Intelligent Manufacturing of Advanced Construction Machinery, Xuzhou 221004, China)
Abstract:
A new continuous severe plastic deformation (SPD) technique named equal channel angular extrusion with spherical cavity (ECAE-SC) was proposed based on the modification of the conventional equal channel angular extrusion (ECAE) die. ECAE process was used as a comparison. Commercially pure aluminum (Al-1060) was subjected to ECAE-SC for a single pass at room temperature. The influence of spherical cavity on microstructure and properties were evaluated through microstructure analysis, Vickers microhardness (HV) testing, and reciprocating friction-wear test. Vickers microhardness values, friction coefficients, wear rates, and three-dimensional wear morphologies were obtained as well. Results showed that the spherical cavity of ECAE-SC die could greatly improve the metal flow at the outer corner of the die, thus reducing “dead zone” and head bending of 1P- ECAE processed billet. After one pass of ECAE-SC, the grains were refined, and microhardness was increased from 36.6HV in the as-cast state to 58.7HV, which was 11.2% higher than that of ECAE process (52.8HV). Compared with ECAE, ECAE-SC processed aluminum exhibited better wear resistance and wear stability. The wear mechanism of commercially pure aluminum after ECAE process was dominated by a mixed wear mechanism containing abrasive wear and oxidation wear, while that after ECAE-SC process was a wear mechanism dominated by abrasive wear.
Key words: severe plastic deformation (SPD) ECAE-SC commercially pure aluminum microhardness wear resistance
张翔, 王晓溪, 张德坤, 曹秉宇, 周怡. 球形分流对等通道转角挤压工业纯铝组织性能的影响[J]. 材料科学与工艺, 2020, 28(6): 43-48. DOI: 10.11951/j.issn.1005-0299.20190008.
ZHANG Xiang, WANG Xiaoxi, ZHANG Dekun, CAO Bingyu, ZHOU Yi. Influence of spherical cavity on microstructure and properties of commercially pure aluminum during equal channel angular extrusion process[J]. Materials Science and Technology, 2020, 28(6): 43-48. DOI: 10.11951/j.issn.1005-0299.20190008.
基金项目 国家自然科学基金资助项目(51905462);江苏省"六大人才高峰"高层次人才选拔培养资助项目(GDZB-127);江苏省科协青年科技人才"托举工程"资助培养项目(苏科协发[2018]202号) 通信作者 王晓溪，E-mail：wxx19851109@sina.com 作者简介 张翔(1986—)，男，高级工程师，博士;
张德坤(1971—)，男，教授，博士生导师 文章历史 收稿日期: 2019-01-10 网络出版日期: 2019-09-03
Contents Abstract Full text Figures/Tables PDF
球形分流对等通道转角挤压工业纯铝组织性能的影响
张翔1,3, 王晓溪2
, 张德坤1, 曹秉宇2, 周怡2 1. 中国矿业大学 机电工程学院，江苏 徐州 221116;
2. 徐州工程学院 机电工程学院，江苏 徐州 221018;
3. 高端工程机械智能制造国家重点实验室，江苏 徐州 221004
收稿日期: 2019-01-10; 网络出版日期: 2019-09-03
基金项目: 国家自然科学基金资助项目(51905462);江苏省"六大人才高峰"高层次人才选拔培养资助项目(GDZB-127);江苏省科协青年科技人才"托举工程"资助培养项目(苏科协发[2018]202号)
作者简介: 张翔(1986—)，男，高级工程师，博士;
张德坤(1971—)，男，教授，博士生导师.
通信作者: 王晓溪，E-mail：wxx19851109@sina.com.
摘要: 基于传统等通道转角挤压(Equal Channel Angular Extrusion, ECAE)的变形原理，开发设计出一种具有球形空腔结构的新型复合大塑性变形工艺——等通道球形转角挤压(Equal Channel Angular Extrusion with Spherical Cavity, ECAE-SC)。以ECAE工艺为对比参照，开展了室温条件下工业纯铝单道次ECAE-SC挤压实验研究，进行了金相显微分析、显微硬度测试和球-面接触往复式摩擦磨损试验，获得了变形材料的显微组织、显微硬度、摩擦系数、磨损率和三维磨损形貌等特征参数.研究表明：ECAE-SC球形转角的圆滑过渡可大幅提高模具外角处金属的流动性，有效避免产生传统ECAE单道次变形“死区”，挤出试样头部翘曲现象得到显著改善；室温单道次ECAE-SC变形后，工业纯铝晶粒细化明显，显微硬度值由初始36.6HV增加至58.7HV，比同条件下ECAE变形试样(52.8HV)提高了11.2%；ECAE-SC变形材料耐磨性能最好，并表现出良好的摩擦磨损稳定性，磨损机制从磨粒磨损和氧化剥层磨损为主导的混合磨损机制转变为以磨粒磨损为主导的磨损机制。
关键词: 剧烈塑性变形 等通道球形转角挤压 工业纯铝 显微硬度 耐磨性能
Influence of spherical cavity on microstructure and properties of commercially pure aluminum during equal channel angular extrusion process
ZHANG Xiang1,3, WANG Xiaoxi2
, ZHANG Dekun1, CAO Bingyu2, ZHOU Yi2 1. School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou 221116, China;
2. School of Mechanical and Electrical Engineering, Xuzhou University of Technology, Xuzhou 221018, China;
3. State Key Laboratory of Intelligent Manufacturing of Advanced Construction Machinery, Xuzhou 221004, China
Abstract: A new continuous severe plastic deformation (SPD) technique named equal channel angular extrusion with spherical cavity (ECAE-SC) was proposed based on the modification of the conventional equal channel angular extrusion (ECAE) die. ECAE process was used as a comparison. Commercially pure aluminum (Al-1060) was subjected to ECAE-SC for a single pass at room temperature. The influence of spherical cavity on microstructure and properties were evaluated through microstructure analysis, Vickers microhardness (HV) testing, and reciprocating friction-wear test. Vickers microhardness values, friction coefficients, wear rates, and three-dimensional wear morphologies were obtained as well. Results showed that the spherical cavity of ECAE-SC die could greatly improve the metal flow at the outer corner of the die, thus reducing "dead zone" and head bending of 1P- ECAE processed billet. After one pass of ECAE-SC, the grains were refined, and microhardness was increased from 36.6HV in the as-cast state to 58.7HV, which was 11.2% higher than that of ECAE process (52.8HV). Compared with ECAE, ECAE-SC processed aluminum exhibited better wear resistance and wear stability. The wear mechanism of commercially pure aluminum after ECAE process was dominated by a mixed wear mechanism containing abrasive wear and oxidation wear, while that after ECAE-SC process was a wear mechanism dominated by abrasive wear.
Keywords: severe plastic deformation (SPD) ECAE-SC commercially pure aluminum microhardness wear resistance
近年来，超细晶材料(Ultrafine Grained，UFG)因其具有比传统粗晶材料更加优异的组织与性能而备受关注[1-2]。剧烈塑性变形(Severe Plastic Deformation，SPD)[3-6]作为一种新兴的塑性变形方法，具有强烈的晶粒细化能力，被国际材料学界公认为是制备块体超细晶材料最有前途的先进技术，应用潜力巨大。目前，人们利用SPD法已成功制备出超细晶结构铝、铜、镁、钛及其合金[7-9]。常见的代表性SPD工艺有等通道转角挤压(ECAE)[10]、高压扭转(High Pressing Torsion, HPT)[11]、挤扭(Twist Extrusion, TE)[12]以及多向锻造(Multiple Forging, MF)[13]等，这些工艺在材料组织性能调控，粉末冶金固结等方面发挥了重要作用。为进一步提高SPD过程中块体UFG材料的制备效率，防止变形坯料过早出现损伤缺陷，以传统ECAE工艺为基础的复合SPD技术相继被开发设计出来，如挤压-等通道转角挤压(Extrusion-Equal Channel Angular Pressing, Ex-ECAP)[14]、等径角挤扭(Equal Channel Angular Pressing and Torsion, ECAPT)[15]、扭转-等通道挤压(Twist Channel Angular Pressing, TCAP)[16]与背压-等通道挤压(Back Pressure-Equal Channel Angular Pressing, BP-ECAP)[17]等。然而，前期SPD工艺研究的重点主要集中于变形材料的微观组织与力学性能方面，而对于其摩擦性能关注较少。铝及其合金由于化学活性高、粘性强及耐磨、抗擦伤性能较差，限制了其在易磨损零部件中的应用。因此，研究新型铝材强化工艺并提高其耐磨性能对扩大铝及其合金的适用范围、提高UFG材料表面质量具有十分重要的意义。
本文基于传统ECAE技术原理，提出一种带有球形空腔结构的新型等通道球形转角挤压(ECAE-SC)工艺，在自行设计的模具上对工业纯铝进行了室温单道次ECAE-SC变形，并与相同条件下的ECAE变形进行对比分析，在组织观察与硬度测试基础上，采用球-面接触往复式摩擦磨损试验，结合磨痕三维形貌表征与磨损表面SEM观察，重点研究了球形分流对ECAE变形材料摩擦磨损行为的影响规律，探讨分析了变形材料摩擦磨损机理。
1 实验1.1 工艺原理新型ECAE-SC工艺变形原理如图 1所示。ECAE-SC模具型腔由两个相交成一定角度的圆形等截面通道组成，模具转角处增设一直径大于通道尺寸的球形空腔。工件从模具竖直入口通道放入，在冲头挤压力作用下经模具转角依次发生镦粗与近似纯剪切变形，待材料完全充满球形空腔后，被“减径”挤出至水平出口通道，截面尺寸恢复至初始未变形状态，实现了“一次挤压，多种工艺，连续成形”的复合变形效果。由于工件在变形前后截面尺寸未发生变化，因此可进行多道次重复挤压变形以累积更大的塑性应变量，进而达到改善材料微观组织与提高综合性能的目的。
图 Figure1(Fig.Figure1)
图 1 ECAE-SC工艺示意图Fig.1 Schematic diagram of ECAE-SC process1.2 实验材料与方法实验材料为均匀化退火后的工业纯铝(1060)棒料，工件尺寸为Φ15 mm×80 mm，在YD32G-100型液压机上完成室温单道次ECAE-SC挤压变形。ECAE-SC模具通道为圆截面，通道直径为15 mm，两通道间夹角为90°，转角处球形空腔直径为23 mm，挤压凹模实物如图 2所示。工件与模具通道内壁间采用石墨-机油进行润滑，以减小挤压变形过程中摩擦带来的不利影响，挤压速度约为1 mm/s。此外，为验证新型ECAE-SC工艺变形效果，进行了相同条件下的工业纯铝单道次ECAE室温挤压变形，对两个实验结果进行对比分析。
图 Figure2(Fig.Figure2)
图 2 实验用ECAE-SC挤压凹模Fig.2 The experimental ECAE-SC die挤压后，在垂直于挤出方向(图 1)的横截面上取样进行组织与性能检测。在Leica DMI5000M倒置金相显微镜上观察金相显微组织(Optical Microstructure, OM)，在德国KB30S全自动维氏硬度试验机上进行显微硬度测试，加载载荷为100 g，保压时间15 s。球-面接触往复式摩擦磨损试验在美国UMT-2摩擦磨损试验机上进行。试验前将样品表面用800#砂纸打磨至Ra≈0.16 μm，随后在无水乙醇中超声波清洗10 min，吹干备用。试验条件为：室温(约25 ℃)，干摩擦，相对湿度45%~55%，对摩副为GCr15钢球(Φ10 mm，洛氏硬度60~62HRC)，施加载荷为20 N，滑动速度2 mm/s，滑动时间20 min。试验过程中，摩擦系数曲线及实时数据由试验机自动记录。试验结束后采用称量法测量材料磨损量，取3次试验平均值作为平均磨损量，最终换算成平均磨损率Wr[18]。采用布鲁克公司ContourGT-K型非接触式三维光学形貌仪测定磨痕三维轮廓及深度。利用Inspect S50扫描电镜对磨损表面形貌进行观察分析。
2 结果与讨论2.1 变形特征图 3给出了不同变形条件下挤压工件的变形过程实物图，可以看出，新型ECAE-SC工艺下工件整体变形连续，材料在模具通道与球形空腔内部填充良好，挤出工件外形完整，宏观无裂纹，表面质量良好。对比传统ECAE挤压试样可以发现，ECAE-SC工艺下材料变形更加平稳，截面过渡更圆滑，转角处金属球状形貌和尺寸与球形空腔结构完全吻合，有效避免了传统ECAE单道次变形常出现的变形“死区”，挤压后工件头部翘曲现象得到了显著改善(图 3局部放大图)。
图 Figure3(Fig.Figure3)
图 3 挤压工件实物图Fig.3 Macrograph of the processed billets这是由于模具转角处大尺寸球形空腔的设计对材料变形起到了分流和缓冲作用，减小了金属上下层流动速度的差异，材料在转角处实现了“镦粗-剪切-挤压”的连续、复合和高效变形，内部获得了较高的塑性应变累积。此外，出口通道减径结构为球形空腔内的变形材料提供了合理的反向背压，增大了工件内部的静水压力，其塑性变形能力和变形均匀性得到进一步提高。
2.2 显微组织图 4为工业纯铝试样在不同变形条件下的金相显微组织，可以看出：初始未变形材料(图 4(a))晶粒十分粗大，形状和尺寸不均匀；单道次ECAE-SC挤压后(图 4(c))，在剧烈塑性应变诱导作用下，大量晶粒被拉长呈条带状，发生了破碎和细化，组织变得细小和均匀，且变形效果明显优于传统ECAE(图 4 (b))。这是由于在ECAE-SC模具球形转角处，材料不仅受到近似纯剪切作用，还同时发生镦粗和挤压两种变形，在多种塑性变形形式的耦合作用下，材料实际受到剧烈的简单剪切变形作用。由于塑性应变的不可逆性，导致材料内部累积的变形量有增无减，晶粒细化效果十分明显。
图 Figure4(Fig.Figure4)
图 4 不同变形条件下试样显微组织Fig.4 Microstructure of specimens under different deformation conditions:(a)as received; (b)ECAE; (c)ECAE-SC2.3 显微硬度图 5给出了工业纯铝试样在不同变形条件下横截面的平均显微硬度，可以看出，新型ECAE-SC工艺对工业纯铝力学性能的提升效果明显，试样硬度值由初始未变形的36.6HV升高至58.7HV，比传统ECAE挤压态硬度(52.8HV)增加了11.2%。这是由于工业纯铝经复合剧烈ECAE-SC变形后，材料内部累积了较高的塑性应变，晶粒不断细化，晶内位错密度增大，晶界数目增多。而大量晶界使得位错进一步运动受阻，塞积在晶界附近产生局部应力。同时ECAE-SC减径挤压所提供的反向背压有效提高了材料的塑性变形能力，随着变形程度的增大，材料变形抗力不断增大，宏观上表现为材料硬度值的上升。
图 Figure5(Fig.Figure5)
图 5 不同变形条件下试样的平均硬度Fig.5 Average microhardess of specimens under different deformation conditions2.4 磨损性能图 6和图 7分别为工业纯铝试样在不同条件下的摩擦系数对比曲线及平均摩擦系数、平均磨损率的变化规律，可以发现，3种状态下(初始未变形、1P-ECAE和1P-ECAE-SC)摩擦系数曲线变化趋势基本相同，均呈现出“急剧上升-逐渐下降-趋于稳定”的变化规律(图 6)。
图 Figure6(Fig.Figure6)
图 6 不同变形条件下试样摩擦系数曲线Fig.6 Friction coefficient curves of specimens under different deformation conditions图 Figure7(Fig.Figure7)
图 7 不同变形条件下试样的平均摩擦系数与磨损率Fig.7 Average friction coefficient and wear rate of specimens under different deformation conditions仔细观察还可以发现，随着材料变形程度的增加，试样摩擦曲线异常波动的频率与幅度逐渐减小，平均摩擦系数和磨损率不断降低，耐磨性能提升明显。由于ECAE-SC工艺集镦粗、剪切和挤压等多种不同形式的变形于一体，材料内部累积塑性应变量最高，耐磨性能最好。如图 7所示，与传统ECAE相比，试样平均摩擦系数从初始的0.591减小到0.525，平均磨损率从初始的0.167×10-5 mm3/m降至0.137×10-5 mm3/m，降幅分别为12.6%和21.9%，表现出较高的耐磨性和良好的摩擦磨损稳定性。
2.5 磨损形貌图 8为不同变形条件下工业纯铝试样表面磨痕的三维形貌图，图中横、纵坐标表示磨痕整体的长度与宽度，右侧颜色标尺表示磨痕深度(z方向)。由于本文采用球-面接触往复式摩擦磨损试验，磨痕长度基本固定(约5 mm)，而磨痕宽度取决于对磨钢球的压入深度，压入越深，球面接触宽度越宽即磨痕越宽。
图 Figure8(Fig.Figure8)
图 8 不同变形条件下试样表面磨痕轮廓图Fig.8 Surface profile of wear scar under different deformation conditions:(a)as received; (b)ECAE; (c)ECAE-SC从图 8可以看出，初始未变形试样(图 8 (a))表面耐磨性较差，磨损表面的磨痕宽度和z向深度最大，即磨损体积最大。随着材料变形程度的增加，试样磨损宽度与深度均不断减小，磨痕面积进一步缩小。其中，新型复合ECAE-SC工艺下工业纯铝试样磨损体积最小(图 8 (c))，耐磨性能最好，有力地验证了前述试样磨损率的变化规律(图 7)。这表明复合SPD工艺下变形材料剧烈的塑性应变积累，不仅能够细化材料晶粒，提高力学性能，还有利于改善其耐磨性能，从而为传统材料表面强化和改性研究提供了有益的启发和全新的思路。
不同变形条件下工业纯铝试样磨损表面SEM形貌如图 9所示。对比图 9可以发现，ECAE-SC变形材料表面最为光滑，犁沟分布最为均匀且犁沟边缘附着的磨料最细小，ECAE变形材料次之，未变形材料最差。观察图 9 (a)可以看出，初始材料表面磨损最严重，磨痕处除磨屑和犁沟外，沿着竖直滑动方向还产生了鳞片状表层与剥落坑。可以认为，其磨损过程中铝材表面及生成的氧化膜更易产生剥落形成磨屑，即存在一定程度的剥层磨损。这种磨屑作为一种区别于铝材与对磨钢球的硬质颗粒可视为第3种材料，整个磨损过程类似于三体磨损。随着材料变形程度的增大，磨损表面的鳞片状表层逐渐减少(图 9 (b))，磨损表面逐渐变得光滑平整，犁沟相互平行且趋于均匀分布，深度和宽度也进一步减小，如图 9(c)所示。这表明随着材料内部累积塑性应变量的不断增加(由初始未变形→ECAE→ECAE-SC)，工业纯铝试样磨损机制从以氧化剥层磨损为主导逐渐过渡至以磨粒磨损和氧化剥层磨损为主导的混合磨损机制，进而发展为以磨粒磨损为主导的磨损机制。由于磨损是材料内部抵抗摩擦力变形的一种外在表现，与材料内在性能尤其是硬度密切相关。因此，材料硬度越高，在外力产生的应力作用下，发生剥落的程度越轻，这一研究结论与本文图 5显微硬度结果相吻合。
图 Figure9(Fig.Figure9)
图 9 不同变形条件下试样磨损表面形貌Fig.9 Wear surface morphologies of specimens under different deformation conditions: (a) as received; (b) ECAE; (c) ECAE-SC3 结论1) ECAE-SC工艺“球形分流”的结构设计实现了材料复合、连续、高效变形，避免了传统ECAE单道次变形模具外角处出现变形“死区”。挤出工件外形完整，宏观无裂纹，表面质量良好，头部翘曲得到显著改善。
2) ECAE-SC工艺集镦粗、剪切和挤压变形于一体，有效提高了材料塑性应变的累积效率。1道次变形后，工业纯铝晶粒细化明显，平均显微硬度从初始未变形态的36.6HV增加至58.7HV，相比于传统ECAE挤压态增加了11.2%。
3) ECAE-SC工艺能够有效提高工业纯铝的耐磨性能，试样平均摩擦系数和平均磨损率分别为0.525和0.137×10-5 mm3/m，比传统ECAE工艺降低了12.6%和21.9%，表现出较高的耐磨性和良好的摩擦磨损稳定性。
4) 与传统ECAE工艺相比，ECAE-SC变形材料磨损表面鳞片状表层减少，犁沟状形貌宽度和深度减小，磨痕最窄最浅。工业纯铝磨损机制从以氧化剥层磨损为主导过渡至以磨粒磨损和氧化剥层磨损为主导的混合机制，最后发展为以磨粒磨损为主导的磨损机制。
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