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USRP处理45钢微观梯度力学性能演变及残余应力分布∗

  • 许全军

    机 构：

    天津大学材料科学与工程学院 天津 300350

    ×
  • ， 刘秀国

    机 构：

    天津大学材料科学与工程学院 天津 300350

    ×
  • ， 龚宝明

    机 构：

    天津大学材料科学与工程学院 天津 300350

    ×
  • ， 邓彩艳

    机 构：

    天津大学材料科学与工程学院 天津 300350

    ×
  • ， 王东坡

    机 构：

    天津大学材料科学与工程学院 天津 300350

    ×
  • ， 刘永

    机 构：

    天津大学材料科学与工程学院 天津 300350

    ×

天津大学材料科学与工程学院 天津 300350；

Evolution of Micro-gradient Mechanical Properties and Residual Stress Distribution of 45 Steel by USRP

  • XU Quanjun

    Affiliation：

    School of Materials Science and Engineering, Tianjin University, Tianjin 300350 , China

    ×
  • ， LIU Xiuguo

    Affiliation：

    School of Materials Science and Engineering, Tianjin University, Tianjin 300350 , China

    ×
  • ， GONG Baoming

    Affiliation：

    School of Materials Science and Engineering, Tianjin University, Tianjin 300350 , China

    ×
  • ， DENG Caiyan

    Affiliation：

    School of Materials Science and Engineering, Tianjin University, Tianjin 300350 , China

    ×
  • ， WANG Dongpo

    Affiliation：

    School of Materials Science and Engineering, Tianjin University, Tianjin 300350 , China

    ×
  • ， LIU Yong

    Affiliation：

    School of Materials Science and Engineering, Tianjin University, Tianjin 300350 , China

    ×

School of Materials Science and Engineering, Tianjin University, Tianjin 300350 , China；

作者简介:

许全军,男,1993年出生,硕士研究生。主要研究方向为表面强化和结构疲劳。E-mail:qj_xu@tju.edu.cn

刘秀国(通信作者),男,1976年出生,博士,副研究员,硕士研究生导师。主要研究方向为超声表面处理、结构疲劳。E-mail:liuxiuguo@tju.edu.cn

中图分类号:TG159

文献标识码:A

DOI:10.11933/j.issn.1007-9289.20210825002

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目录contents

    摘要

    针对材料表层梯度结构力学性能及残余应力分布研究不够深入的问题,对正火态 45 钢进行超声表面滚压加工 (USRP) 处理,在材料表层制备出微观梯度结构,并进行金相、SEM 和 EBSD 分析。 结果表明,根据形变程度将表层微结构分为 3 个区域:强变形区、微变形区和未影响区,厚度最高可达 680 μm。 接着采用应变梯度理论(MSG)修正的仪器化压痕法对试样表层力学性能进行分析,发现试样表层的显微硬度、弹性和塑性变形性能均呈现明显的梯度变化。 最后,采用轮廓法和 X 射线衍射法相结合的方式测量试样表面及内部残余应力分布。 测试结果表明,处理后试样表面形成一定深度的残余压应力场,在距离表面约 700 μm 处压应力峰值仍可达 809. 6 MPa,并且两种测试方法具有较好的一致性。 系统分析了试样表层在微观组织、屈服强度、残余应力分布和塑性流变性能方面出现的明显梯度变化。

    Abstract

    To address the lack of in-depth study on the mechanical properties and residual stress distribution of the gradient structure of the material surface, optical microscope (OM), scanning electron microscope (SEM) and electron back scattering diffraction (EBSD) analyses are carried out on the surface gradient structure of normalized 45 steel obtained by ultrasonic surface rolling process (USRP). The structure is analyzed by optical microscope (OM), scanning electron microscope( SEM) and electron back scattering diffraction (EBSD). Several distinct zones can be identified by the plastic deformation, i. e. strongly plastic distorted, plastic deformed and undeformed areas. The thickness of the plastically deformed area increases with the increase of treatment time, and the maximum thickness of the plastic deformation layer can reach 680 μm. Moreover, the instrumental indentation method based on the strain gradient theory (MSG) is used to determine the gradient mechanical properties of the surface. It is found that the microhardness, elasticity and plastic deformation properties show obvious gradient changes along the surface depth. Finally, the residual distribution along the depth is determined by the contour method and X-ray diffraction, respectively, and very good agreement can be obtained by the two methods. The results show that residual compressive stress field is formed on the surface of the sample after treatment, and the compressive stress peak value can reach 809. 6 MPa at a depth of about 700 μm from the surface. Systematic analysis of the obvious gradient changes in the surface layer of the sample in terms of microstructure, yield strength, residual stress distribution and plastic rheological properties.

  • 0 前言

  • 结构的抗疲劳性能和耐蚀性与材料的表面状态密切相关。因此,通过表面处理的方法可以有效地改善构件的服役性能,如近些年开发的超声表面滚压加工 (USRP) 技术[1-2]。该处理方法将传统滚压技术和超声冲击技术复合而成,依靠强烈的机械塑性变形来诱发金属材料表面细晶或纳米化,在一定深度范围内形成梯度组织结构,有效改善了材料表面粗糙度和力学性能,并对表面的损伤程度远小于传统的滚压、喷丸等工艺,有效的保证了结构表面完整性[3-5]。

  • 王东坡等[6-7] 研究了USRP对金属材料表面纳米化程度的影响,结果表明经USRP后40Cr钢的表层形成了等轴纳米晶组织,表面纳米层的厚度达到了150 μm,材料表面性能有了明显改善。 USRP技术可以显著提高材料表层组织的显微硬度和耐磨性,且经USRP处理后材料表层的晶粒和晶界数量明显增加[8-9]。 USRP技术能够引入高深度的残余压应力场,经USRP处理后材料表层残余应力通常呈现梯度分布,且残余压应力场深度随着加工次数的增加而变大,能够很好地改善材料的表面性能,提高机械零部件的抗疲劳性能和耐磨损性能[10-11]。曹小建等[12]研究了超声冲击技术对TC4钛合金塑性变形层、拉压疲劳性能和断裂特征的影响,结果表明TC4钛合金的残余压应力和拉压疲劳性能有明显提高。近些年,USRP技术也被用于金属3D打印构件的后处理,通过锻打的方式细化材料组织的晶粒尺寸,能大幅提高试件的强度、韧性及构建的疲劳寿命和可靠性[13-15]。

  • 本文对正火态45钢进行超声表面滚压加工 (USRP) 处理,在材料表层制备出微观梯度结构,对不同加工遍数的表面处理结果进行了金相、SEM和EBSD分析;采用应变梯度理论(MSG) 修正的仪器化压痕法对试样表层力学性能进行分析;采用轮廓法和X射线衍射法相结合的方式测量试样表面及内部残余应力分布。系统分析了试样表层在微观组织、屈服强度、残余应力分布和塑性流变性能方面出现的明显梯度变化。

  • 1 试验方法

  • 1.1 试验材料与USRP工艺

  • 试验材料为 ϕ60mm的正火态45钢,其化学成分(质量分数,%) 为C 0.45、Si 0.15、Mn 0.65、Cr 0.23、Ni 0.28、Cu 0.20、Fe余量。采用自主研发的超声表面滚压装置对正火态45钢圆棒进行USRP处理,从左往右滚压1次记为加工1遍。主要参数为:加工头为 ϕ15mm的硬质合金球,主轴转速为560r/min,进给量为0.076mm/r,静压力为500N, 振幅为10 μm。

  • 1.2 微观组织表征

  • 制备金相试样时,将试样打磨至7000#砂纸, 抛光,用4%硝酸酒精溶液腐蚀,酒精冲洗吹干,最后采用OLYMPUS GX51光学显微镜和JSM-7800F热场发射扫描电镜( SEM) 进行组织观察。 EBSD样品制备过程是将试样磨至7000#,用体积分数为95%的无水乙醇和5%的高氯酸溶液进行电解抛光,电解抛光参数为电压30V,时间24s。 EBSD测试时, 工作电压为20kV, 扫描步长为0.3 μm。采用JSM-7800F热场发射扫描电镜 ( SEM)观察试样显微组织,结合EDAX探头获得电子背散射衍射原始数据,从而进行晶粒微观的晶体结构分析。

  • 1.3 仪器化压痕测试与残余应力测试

  • 仪器化压痕测试能够有效地对材料局部的力学性能进行评估,相较于其他测试方法(如原位拉伸、微柱压缩试验等)更加经济便捷。采用Nano indenter G200进行纳米压痕试验,压头为金刚石玻氏压头,位移分辨率为0.01nm,载荷分辨率为50nN,试验采用最大载荷控制模式进行,最大加载载荷从30 μN依次增加至150 μN。在试验中,加载速率和卸载速率均为1mN/s,在最大载荷处保载2s。

  • 残余应力的轮廓法测试步骤如下:首先将试样与刚性平台固定,防止其转动和平动。然后采用MAKINO DUO 43慢走丝切割机将试样从中间切开,平均切割速度为1mm/min,切割丝( 铜丝) 的直径为0.25mm,对试样进行一次成型的连续切割;接着使用接触式的HEXAGON三坐标测量仪测量切割面的表面轮廓。最后根据试样尺寸和三坐标轮廓数据建立有限元模型进行计算,得到试样切割面应力分布全貌。轮廓法测试原理如图1所示。为验证轮廓法测试结果的准确性和有效性,同时采用PROTO X射线对试样表面及亚表面深度的残余应力分布进行测试,测试过程中采用电解抛光去除表层材料。

  • 以经过6遍USRP处理为例,轮廓法数据分析过程如下:先对经过二维平直化处理的轮廓数据进行处理,删除Z方向上较大的误差点,将切割面两个轮廓上的数据进行平均,以消除切割过程中切应力变化和轮廓测试带来的误差,得到平均后的表面轮廓如图2所示。接着采用样条曲线将平均值进行拟合,减小由个别点的测量误差导致的最终结果误差,以及切面表面粗糙度引起的轮廓数据局部小幅波动对应力重构的干扰,得到切割面光滑拟合后的曲面轮廓点云图。最后建立有限元模型,将三坐标轮廓数据作为边界条件加载到模型界面上,对模型进行全约束,防止在计算过程中出现刚性移动,模型变形如图3所示,进行弹性分析,可得到垂直于两个切割面的应力分布全貌,如图4所示为残余应力在切面上的二维分布云图。

  • 图1 轮廓法测试原理和步骤

  • Fig.1 Principle and procedure of the contour method

  • 2 试验结果与分析

  • 2.1 微观组织分析

  • 图5 为经过6遍和30遍USRP处理的试样显微形貌对比图,从图中可以看出,经过USRP处理后材料表层因机械挤压发生强烈塑性变形,从而导致表层材料发生显著的晶粒细化,形成梯度结构。经USRP处理后,试样的微结构大致可分为3个区: 强变形区Ⅰ、微变形区Ⅱ和未影响区Ⅲ。经过6遍USRP处理后,强变形区Ⅰ厚度大约为160 μm,微变形区Ⅱ厚度大约为110 μm。经过30遍USRP处理后,强变形区Ⅰ厚度大约为500 μm,微变形区Ⅱ厚度大约为180 μm,塑性变形层厚度为680 μm。图6为未经处理的试样,可以看出经过30遍USRP处理后,45钢传统组织中渗碳体的层状结构发生断裂, 成为了较小尺度颗粒。

  • 图2 切割面平均后的表面轮廓数据

  • Fig.2 Surface profile data after cutting surface average

  • 图3 加载边界条件后变形的有限元模型(放大500倍)

  • Fig.3 Deformed finite element model after loading boundary conditions (500times)

  • 图4 基于轮廓法测量试样表面残余应力分布云图

  • Fig.4 Measurement of residual stress distribution cloud diagram on sample surface based on profile method

  • 图5 经过不同遍数USRP处理的试样金相图

  • Fig.5 OM images of samples with different USRP times

  • 图6 未处理与经30遍处理试样(距表层20 μm)SEM图

  • Fig.6 SEM images of samples with base material and 30times (20 μm from the surface) USRP processing

  • 综合分析可知,塑性变形层和晶粒细化层的厚度均随着USRP处理遍数的增加而增加,此时随着合金球的继续挤压,挤压的能量由表层向材料内部传递,致使内部材料发生塑性变形,最终形成了梯度结构。随着USRP遍数的增加,材料塑性变形将加剧,塑性变形层的厚度增加,所以晶粒细化层的厚度将同时增加。

  • 2.2 晶粒取向图和晶界角度分布

  • 图7 为未处理试样与经过15遍USRP处理试样的的晶粒取向图和晶界分布图。对比图7a、7c和7e可得,未经USRP处理时,试样同一晶粒内颜色相对统一,经过15遍USRP处理后,晶粒被挤压细化,同一晶粒内部颜色不再统一,是由变形过程中晶格旋转产生的变形带所致。

  • 对于图7b、7d和7f,图中蓝色的线条代表大角度晶粒边界 (15°~180°),红色的线条代表小角度晶粒边界 (2°~15°)。由图可得,经USRP处理后, 材料表层组织的大小晶界数量明显增多,这是由于位错的运动以及滑移带聚集和交织不断切割粗大晶粒,形成新的晶界,所以晶界的数量明显增加,表层晶粒得到细化。

  • 2.3 纳米压痕法反推梯度力学性能技术

  • 考虑尺度效应的影响,根据ZHANG等[16-17] 建立的计算方法来确定试样表层组织的梯度力学性能。根据JI等[18-19]的研究,假设被测组织的弹塑性行为满足如下关系[20] :

  • 图7 未处理试样与经过15遍USRP处理试样 (距表层20 μm)的IPF图和EBSD晶界分布图

  • Fig.7 IPF images and EBSD grain boundary maps with untreated samples and 15times USRP (20 μm from the surface) processing

  • σ=Eε σ⩽σyσ=σy1+EσyεPn σ>σy
    (1)

  • 式中,E 为弹性模量,ε 为总应变,σy 为屈服强度,n 为加工硬化指数,εp 为塑性应变,塑性应变定义为总应变 ε 减去屈服应变 εy,如下式:

  • εp=ε-εy
    (2)

  • 图8 为母材和不同USRP处理遍数下材料表层微观组织(深度为20 μm)的载荷-深度曲线。从图中可以看出,纳米压痕试验曲线在加载段呈抛物线状,在载荷保持阶段为水平直线段,卸载后随着材料的弹性恢复,压痕深度略有减小,USRP处理遍数越多,最大压入深度和剩余压痕深度越小。

  • NIX和GAO [21]用几何必须位错的概念准确预测了晶体材料的压痕尺寸效应,考虑晶粒尺寸对位错移动的限制作用,根据NIX和GAO的应变梯度理论( MSG),屈服强度 σy 可通过以下计算方法确定:

  • 图8 不同USRP处理遍数的仪器化压痕试验载荷-深度 (P-h) 曲线

  • Fig.8 Instrumented indentation test load-depth (P-h) curve with different USRP processing times

  • HH0=1+h*h
    (3)

  • 式中,H 是压痕深度 h 下的名义硬度值,h ∗ 是一个与压头形状和材料有关的特征长度,H 0 是与尺寸无关的硬度值。由式(3)可知仪器化压痕名义硬度的平方 H 2 与压入深度的导数1/h 成正比,当1/h=0时,可以得到与尺寸无关的硬度值H 0 的值。

  • 材料的屈服强度 σy 可由下式计算:

  • H0=4.15σy
    (4)

  • 根据DAO等[20] 提出的方法,可以计算出塑性应变 εp 为0.033时的应力 σ0.033,计算如下:

  • Cσ0.033=-1.131ln⁡Erσ0.0333+13.635ln⁡Erσ0.0332-30.594ln⁡Erσ0.033+29.267
    (5)

  • 式中,C 是载荷-深度曲线的加载曲率,简约弹性模量E r 可通过下式进行计算:

  • Er=πS2Ac
    (6)

  • 式中,压头的接触面积函数Ac=24.56hc2-5246hc+290362hc12-2384536hc14+5261354hc18-3156731hc116, S 是用卸载曲线上半段测得的接触刚度,有效压入深度h c 可通过下式计算:

  • hc=hmax-0.75PmaxS
    (7)

  • 式中,P max 和h max 分别是最大压入载荷和最大压入深度。各组织的弹性模量 E 可通过下式得到:

  • 1Er=1-v2E+1-vi2Ei
    (8)

  • 式中,E i=1 140GPa是金刚石压头的弹性模量,νi=0.07是金刚石压头的泊松比,ν=0.3是各组织的泊松比。根据式(5)可以得到比应力 σ0.033,因此各组织的加工硬化指数 n 可由下式计算:

  • σ0.033=σy1+Eσy×0.033n
    (9)

  • 至此,通过计算得出USRP处理1、6和15遍所得表面组织的力学性能参数见图9。由图6a和6b得,经USRP处理后,材料表层组织的弹性模量和显微硬度均有明显提高,材料组织中的微孔洞对材料的弹性模量影响很大,微孔洞越多弹性模量呈线性下降[22],而USRP处理能降低45钢表层组织的孔隙率[23],使材料表层的微结构更加致密,最终形成一定厚度的细晶结构,从而提高了材料表面组织的弹性模量和显微硬度。在相同USRP处理遍数下, 显微硬度和弹性模量均沿着样品厚度方向,由材料表面向材料内部逐渐减小,且随着加工遍数的增加,显微硬度和弹性模量变化范围越来越大。经过15遍USRP处理后试样的显微硬度最高为9.7GPa, 比心部未影响区4.3GPa提高了1.3倍,弹性模量最高为655GPa,比心部未影响区245GPa提高了1.7倍,当深度超过120 μm时,表面显微硬度和弹性模量的变化趋势减缓并趋于稳定。 6遍USRP处理的表面显微硬度和弹性模量最高为7.4GPa和472GPa,1遍USRP处理的表面显微硬度和弹性模量最高为5.8GPa和345GPa。由此可得经过USRP处理后,材料表面组织的刚度和硬化程度均有明显的提高,且加工遍数的增加显然影响着表层组织的显微硬度和弹性模量的变化范围。

  • 图9 不同USRP处理遍数距表层不同深度力学性能变化

  • Fig.9 Changes in mechanical properties of different depths from the surface with different USRP processing times

  • 由图9c和9d可以看出,经过USRP处理后,材料表层组织的屈服强度得到了明显的提高,加工硬化指数明显降低。在相同USRP处理遍数下,材料表层组织的屈服强度沿着样品厚度方向,由材料表面向材料内部逐渐减小,材料表层组织的加工硬化指数沿着样品厚度方向,由材料表面向材料内部逐渐增大,且随着加工遍数的增加,屈服强度和加工硬化指数均有明显的变化。经15遍USRP处理的屈服强度和加工硬化指数分别为827.2MPa和0.026 3,屈服强度比心部未影响区570.1MPa提高了45.1%,加工硬化指数比心部未影响区0.083 5降低了68.5%。 6遍USRP处理的表层屈服强度和加工硬化指数分别为731.5MPa和0.041 2,1遍USRP处理的表层屈服强度和加工硬化指数分别为669.6MPa和0.055 6。由此可得经过USRP处理后,材料表层组织的塑性性能有了明显的提高,且加工遍数的增加显然影响着表层组织的塑性性能。

  • 由图9分析可得,正火态45钢经USRP处理后,通过位错运动将原始粗大晶粒分割成尺寸较小的晶粒,并发展成小角度的亚晶界,随着挤压的继续作用,更多的位错在亚晶界处产生,晶界两侧的取向差不断增大,晶粒取向趋于随机分布,同时组织中的渗碳体发生细化甚至溶解[24],在试样表层形成了一定厚度取向各异的细小晶粒组织,大大提高了材料表面的力学性能。

  • 2.4 残余应力分析

  • 对于传统的残余应力测试方法,如X射线衍射法,只能测量材料近表层的残余应力,需要结合逐层剥离的方法才能获得残余应力随深度的变化趋势。而轮廓法可以通过一次切割及数值反演分析,获得整个截面上的应力分布,被广泛应用于残余应力测试中[25-27]。采用轮廓法测量USRP处理试样表面和内部残余应力分布,并用X射线衍射法测量试样表层残余应力分布,进而验证轮廓法测试结果的准确性和有效性。

  • 图10 为经过6遍和15遍USRP处理后试样内部残余应力分布。由图10a和10b可知,经过6遍USRP处理后,试样表面残余应力随着试样表层深度的增加,由残余压应力转变为拉应力,最后又转变为压应力,在距试样表面约10mm范围内,残余应力场梯度变化较大。残余压应力场深度大约为2mm,且残余压应力场梯度变化极大,残余压应力先增大后减小,试样最表层残余压应力为416.3MPa,试样表面最大残余压应力出现在距试样表层约500 μm处,值为489.6MPa。在距试样表层2.5~26mm范围内,试样内部残余应力均为拉应力,试样最大残余拉应力出现在距试样表层约5mm处,值为137.6MPa。由图10c和10d可知,经过15遍USRP处理后,试样表面残余应力随着试样表层深度的增加,由残余压应力转变为拉应力,接着又转变为压应力,最后变化趋势减缓。在距试样表面约12mm范围内,残余应力场梯度变化较大。残余压应力场深度大约为2.1mm,在距试样表层约700 μm处压应力峰值为809.6MPa;最大残余拉应力出现在距试样表层约4.5mm处,值为209.5MPa。再者,从图10b和图11b分析可得,残余压应力场深度和最大残余压应力值随着USRP处理遍数的增加而增大,并且基于轮廓法测试残余应力结果和X射线衍射法测试结果基本吻合。

  • 图10 6遍USRP处理后试样内部残余应力分布

  • Fig.10 Residual stress distribution inside the sample after 6USRP processing

  • 图11 15遍USRP处理后试样内部残余应力分布

  • Fig.11 Residual stress distribution inside the sample after 15USRP processing

  • 3 结论

  • (1) 试样表层微结构分为3个区域:强变形区、微变形区和未影响区,表面细晶层的厚度随处理遍数的增加而增加,表层组织的大小角度晶界数量明显增加。

  • (2) 试样表面残余应力场呈梯度分布,残余压应力场深度和最大残余压应力值随USRP处理遍数的增加而增大,试样表面的力学性能明显改善。

  • (3) 对比轮廓法和X射线衍射法测试所得残余应力结果,验证了轮廓法在USRP处理试样残余应力测试中的准确性和有效性,为研究USRP技术对材料表面性能和疲劳性能的影响提供参考。

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  • 基本信息

    中图分类号: TG159
    文献标识码: A
    DOI: 10.11933/j.issn.1007-9289.20210825002

    基金信息

    国家自然科学基金资助项目(51771135)；
    Supported by National Natural Science Foundation of China(51771135).；

    引用信息

    稿件历史

    收稿日期: 2021-08-25

  • 参考文献

    • [1] NOSE T,SHIMANUKI H.3538 Analyze the influence of ultrasonic peening on fatigue life in pad welded joint by FLARP [J].Proceedings of the JSME Annual Meeting,2006,1:691-692.

    • [2] HE T T,DING Z M,SHEN C B,et al.Mechanisms and characteristics of ultrasonic impact treatment on steel surface[J].Advanced Materials Research,2014,835-836:649-653.

    • [3] LU L X,SUN J,LI L,et al.Study on surface characteristics of 7050-T7451 aluminum alloy by ultrasonic surface rolling process [J].International Journal of Advanced Manufacturing Technology,2016,87(9-12):2533-2539.

    • [4] 徐滨士.纳米表面工程[M].北京:化学工业出版社,2004.XU Binshi.Nano surface engineering [ M].Beijing:Chemical Industry Press,2014.(in Chinese)

    • [5] LU K,LU J.Surface nanocrystallization(SNC)of metallic materials-presentation of the concept behind a new approach[J].Journal of Materials Science & Technology,1999,15(3):193-197.

    • [6] 王东坡,宋宁霞,王婷.纳米化处理超声金属表面[J].天津大学学报,2007,2:228-233.WANG Dongpo,SONG Ningxia,WANG Ting,et al.Metal surface nanocrystallization by uitrasonic processing [J].Journal of Tianjin University,2007,2:228-233.(in Chinese)

    • [7] 龚宝明.金属超声表面滚压加工纳米化研究[D].天津:天津大学,2008.GONG Baoming.The research of metallic surface nanocrystallization by ultrasonic rolling extrusion processing[D].Tianjin:Tianjin University,2008.(in Chinese)

    • [8] WU B,WANG P P,PYOUN Y S,et al.Effect of ultrasonic nanocrystal surface modification on the fatigue behaviors of plasma-nitrided S45C steel [J].Surface and Coatings Technology,2012,213:271-277.

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