磨削深度对 γ-TiAl合金表面完整性和疲劳性能的影响
Influence of Grinding Depth on the Surface Integrity and Fatigue Property of γ-TiAl Alloy
通讯作者: 刘仁慈,rcliu@imr.ac.cn,主要从事TiAl合金及其部件研制的研究
责任编辑: 肖素红
收稿日期: 2022-02-28 修回日期: 2022-04-10
基金资助: |
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Corresponding authors: LIU Renci, professor, Tel:
Received: 2022-02-28 Revised: 2022-04-10
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作者简介 About authors
倪明杰,男,1996年生,硕士
γ-TiAl合金部件需要磨削加工以保证其装配精度,而低塑性金属间化合物γ-TiAl合金的力学性能对磨削表面完整性极为敏感。本工作研究了磨削深度对γ-TiAl合金样品表面形貌、表层显微组织、显微硬度等表面完整性参数及最终疲劳性能的影响。结果表明,0.5 mm及以上深度磨削样品表面易产生裂纹,而0.2 mm及以下深度磨削样品表面完好,这与磨削变形热温升导致的冷却收缩拉应力过大有关。随着磨削深度增大,粗糙度参数的轮廓算术平均偏差(Ra)和微观不平度的十点平均高度(Rz)增大,偏度(Rsk)减小。表层γ + α2片层沿磨削方向弯曲变形,相应变形层厚度随着磨削量增大而增大,而显微硬度由表及里先减小后增大。γ-TiAl合金650℃加载440 MPa旋转弯曲疲劳寿命随着磨削深度增大而减小,磨削深度0.05 mm的样品疲劳寿命大于106 cyc,磨削深度0.2 mm的样品疲劳寿命约为104 cyc,其断口裂纹源可观察到磨削痕迹,这与其表面沟槽应力集中有关。γ-TiAl合金疲劳寿命随着Rz的增大而减小,2者呈非线性关系;当Rz小于4 μm时,疲劳寿命稳定在106 cyc以上。
关键词:
γ-TiAl is a family of promising structural materials with low density, high stiffness, and good oxidation and creep resistances at elevated temperatures. They can replace heavier nickel-based alloys at 600-800oC; thus, they are used in the construction of low-pressure turbine blades for aeroengines, i.e., General Electric next-generation and leading edge aviation propulsion. Grinding is an important processing step in blade production to ensure the accuracy of assembling. However, the limited ductility and fracture toughness at room temperature and low thermal conductivity of γ-TiAl alloys narrow the parameter windows of the grinding process. Cracks often form on the surface when the processing parameters are not well controlled. Additionally, grinding greatly influences the surface integrity (i.e., roughness, microstructure, and hardness), which influences the mechanical properties, especially those of brittle γ-TiAl alloys that are sensitive to notch. Grinding depth is a major parameter in blade production because it influences quality and efficiency. Investigating the effect of grinding depth on the surface integrity and fatigue properties of γ-TiAl samples is necessary to optimize the grinding process and identify the major factors of surface integrity that guarantee optimal mechanical properties. In this work, cast γ-TiAl alloy (Ti-45Al-2Nb-2Mn-1B, atomic fraction, %) samples were ground with different depths. The surface integrity (surface roughness, microstructure, and microhardness) and fatigue properties of the samples were compared. Cracks were detected in samples ground to 0.5 and 1 mm depths, while no cracks were detected in samples ground to 0.2 mm or less depths, this is related to the tensile stress induced by temperature increase caused by deformation heat. With the increased grinding depth, the number and depth of grooves increased and for the surface roughness parameters, arithmetic mean deviation and 10-point mean roughness (Rz) increased, while skewness decreased. The γ + α2 lamellae bended in the surface layer and layer thickness increased with the increased grinding depth. The microhardness initially decreased and then increased from the surface to the interior. The rotating bending fatigue life at 650oC under a load of 440 MPa decreased with the increased grinding depth: it was > 106 cyc at 0.05 mm grinding depth but dropped to ~104 cyc at 0.2 mm grinding depth. Fracture surface analysis showed that the cracks mainly nucleated at the surface grooves caused by grinding, which resulted in stress concentration and reduced the fatigue life of samples ground to 0.2 mm depth. The fatigue life decreased with increasing Rz, but remained above 106 cyc when Rz was less than 4 μm. A nonlinear relationship between fatigue life and Rz was shown.
Keywords:
本文引用格式
倪明杰, 刘仁慈, 周浩浩, 杨超, 葛术宇, 刘冬, 史凤岭, 崔玉友, 杨锐.
NI Mingjie, LIU Renci, ZHOU Haohao, YANG Chao, GE Shuyu, LIU Dong, SHI Fengling, CUI Yuyou, YANG Rui.
γ-TiAl合金高温强度高、导热系数低、与刀具亲和力强以及材料加工硬化速率高,导致接触表面局部产生大量变形热,这使得γ-TiAl合金机加工难度大[9~11]。而加工过程的高切削应力和区域温升的综合作用使其表面或亚表面极易损伤,甚至产生裂纹而导致零件报废[12~14],这极大地限制了γ-TiAl合金的应用。此外,机加工工艺对零件的表面加工质量、表面形貌、表面显微组织和表面应力状态等有着重要影响,从而影响机加工部件的最终力学性能[15,16]。γ-TiAl合金作为金属间化合物结构材料,其室温塑性和断裂韧性较低,缺口敏感性大[2~4],这使得其力学性能对表面更为敏感,因此γ-TiAl合金的机加工表面完整性一直是研究热点[17~22]。已有研究[23~25]表明,加工表面对γ-TiAl合金的抗拉强度、疲劳极限等力学性能有着重要影响。
1 实验方法
采用水冷铜坩埚感应悬浮熔炼Ti-45Al-2Mn-2Nb-1B (原子分数,%) 合金二次自耗熔炼铸锭,浇铸入Y2O3面层陶瓷模壳制备出铸板。铸板经过1260℃、150 MPa热等静压4 h后,在1010℃保温8 h退火去应力。采用线切割加工成尺寸为50 mm × 25 mm × 10 mm的试片,而后采用80号SiC砂轮磨削加工试片,如图1a所示。通过装卡样品的工装夹具相对砂轮移动距离来控制磨削深度(ap),分别为0.05、0.1、0.2、0.5和1 mm。
图1
图1
γ-TiAl试片与旋转弯曲疲劳样品磨削加工示意图
Fig.1
Schematics of grinding setup of γ-TiAl plate (a) and fatigue test sample (b) (vs—linear speed of grinding wheel, vw—workpiece linear speed, ap—grinding depth; T represent the transversal section of sample, and L represent the longitudinal section of sample)
采用荧光渗透法检测磨削试片表面裂纹,采用VHX-1000系列显微观察系统观察磨削表面低倍形貌,采用Micro XAM 3D白光干涉仪检测磨削表面三维轮廓,根据ISO 4287标准沿垂直磨削方向截取直线,获取粗糙度高度特性参数(轮廓算术平均偏差(Ra))、微观不平度(十点平均高度(Rz)),形状特性参数(偏度(Rsk)和陡度(Rku))。从磨削试片中取样,利用Axiovert 200 MAT金相显微镜(OM)观察平行磨削方向截面(L截面)和垂直于磨削方向截面(T截面)的表层显微组织。采用Apero扫描电镜(SEM)配备的电子背散射衍射(EBSD)探头分析磨削深度1 mm样品L截面磨削表层的显微组织,结合基体γ相片层厚度和检测效率将扫描步长设定为0.15 μm,使用HKL Channel 5软件获得表层显微组织的Kikuchi带衬度分布图(band contrast map,BC map)、物相分布图(phase map)、取向成像图(inverse pole figure map,IPF map)和取向波动图(kernel average misorientation map,KAM map)。采用FM-700e显微Vickers硬度计测定L截面磨削表层显微硬度随深度的变化,压痕间距10~20 μm,为了避免晶粒差异导致的硬度波动,显微硬度变化曲线在同一晶粒内获得。
根据不同深度磨削试片检测分析结果,选择磨削表面质量较好的深度(0.05、0.1和0.2 mm)外圆磨削加工旋转弯曲疲劳样品,根据GB/T 4337—2015《金属疲劳试验旋转弯曲方法》加工旋转弯曲疲劳试样,磨削深度通过移动样品沿砂轮径向距离控制,如图1b所示,磨削加工工艺参数与试片磨削相同,其中样品旋转线速度等于试片移动速率(vw),为50 mm/min。采用Micro XAM 3D白光干涉仪检测旋转弯曲疲劳试样圆弧工作段的表面三维轮廓和表面粗糙度。旋转弯曲疲劳实验利用QBWP-10000旋转弯曲疲劳试验机按照GB/T 2013—4335标准完成,实验温度650℃,频率为83.3 Hz,应力比R = -1,试样工作段最小直径位置加载最大应力为440 MPa,检测不同深度磨削样品的旋转弯曲疲劳寿命,使用MERLIN Compact SEM观察断口形貌。
2 实验结果
2.1 磨削深度对样品表面形貌的影响
图2
图2
不同深度磨削γ-TiAl试片表面荧光渗透检测结果
Fig.2
Fluorescent penetration inspect (FPI) results of γ-TiAl plates ground with 0.05 mm (a, b), 0.1 mm (c, d), 0.2 mm (e, f), 0.5 mm (g), and 1 mm (h) depths
图3
图3
不同深度磨削γ-TiAl试片的表面形貌
Fig.3
Surface morphologies of γ-TiAl plates ground with 0.2 mm (a, b) and 0.5 mm (c, d) depths under low (a, c) and high (b, d) magnifications (Inset in Fig.3c shows the morphology of the whole grinding surface of plate, the rectangle shows the region observed in Fig.3c)
不同深度磨削γ-TiAl试片的三维轮廓形貌如图4a~e所示。可以看出,样品表面磨削方向较为平直,而垂直磨削方向存在较明显的周期性起伏,但起伏高度随样品磨削深度改变而略有差别。图4f给出了不同深度磨削γ-TiA试片垂直于磨削方向的二维线性轮廓。可以看出,所有样品都存在波长约300 μm的周期性起伏,起伏高度差随着磨削深度的增大而增大,其中磨削深度为0.05 mm的样品最大高度差不足5 μm,而磨削深度为1 mm的样品最大高度差超过6.7 μm。此外,周期性起伏内还有更小的周期性起伏,其波长约为10 μm,高度差不超过1 μm,如图4f中插图所示。较大的周期性起伏主要与砂轮平整度有关,而较小周期性起伏与砂轮磨粒切削工件有关。表1给出了不同深度磨削γ-TiAl试片垂直磨削方向的表面线粗糙度参数检测结果。可以看出,随着磨削深度增大,Ra和Rz增大;而Rsk和Rku略有差别,其中Rsk随着磨削深度增大而减小,磨削深度较大时甚至小于0。而Rku都小于3,其与3的差值随着磨削深度增大而增大。
表1 不同深度磨削γ-TiAl试片垂直磨削方向表面粗糙度
Table 1
Grinding depth / mm | Ra / μm | Rz / μm | Rsk | Rku |
---|---|---|---|---|
0.05 | 0.66 | 4.30 | 0.281 | 2.97 |
0.1 | 0.84 | 4.43 | 0.176 | 2.98 |
0.2 | 0.93 | 4.87 | -0.137 | 2.37 |
0.5 | 1.01 | 4.98 | -0.079 | 1.84 |
1 | 1.21 | 5.01 | 0.011 | 2.33 |
图4
图4
不同深度磨削γ-TiAl试片表面三维轮廓和垂直磨削方向二维线性轮廓曲线
Fig.4
Surface topographies of γ-TiAl plate ground with 0.05 mm (a), 0.1 mm (b), 0.2 mm (c), 0.5 mm (d), and 1 mm (e) depths and surface profile perpendicular to grinding direction (f) (Inset in Fig.4f shows the magnification of 400-500 μm profiles of 0.05 and 0.1 mm depths)
图5给出了不同深度磨削旋转弯曲疲劳γ-TiAl试样圆弧工作段的典型三维轮廓形貌。可以看出,疲劳样品表面未观察到图4中波长300 μm的周期性起伏,仅观察到砂轮磨粒切削形成的较小周期性起伏,但该类样品局部可观察到明显的沟槽,如箭头所示,其深度和宽度随着磨削深度的增大而增大。表2给出了上述旋转弯曲疲劳样品工作段圆弧位置轴向粗糙度检测结果。可以看出,样品粗糙度Ra和Rz随着磨削深度的增大而增大,同样深度磨削样品的粗糙度略有波动,其中0.2 mm深度磨削样品的粗糙度,特别是Rz数值波动较大,这可能与砂轮磨削细小圆弧工作段受力集中,更容易磨损有关。Rsk多数小于0,除磨削深度0.05 mm的5#样品外,0.05和0.1 mm样品的Rsk均大于-0.6,而磨削深度0.2 mm样品的Rsk最小值达到-1.010,其轮廓线的中线更加偏向峰顶,这表明大磨削深度容易导致表面尖锐沟槽的深度或比例增大。Rku数值分布没有明显规律,其数值在3上下波动,这表明磨削深度对Rku影响不大。
图5
图5
不同深度磨削旋转弯曲疲劳γ-TiAl样品圆弧工作段表面三维轮廓
Fig.5
Surface topographies of fatigue test γ-TiAl samples ground with 0.05 mm (a), 0.1 mm (b), and 0.2 mm (c, d) depths
表2 不同深度磨削旋转弯曲疲劳γ-TiAl样品圆弧工作段轴向表面粗糙度及其650℃加载440 MPa疲劳寿命
Table 2
Grinding depth / mm | No. | Ra / μm | Rz / μm | Rsk | Rku | Nf / cyc |
---|---|---|---|---|---|---|
0.05 | 1 | 0.525 | 2.819 | -0.134 | 2.52 | 1978066 |
2 | 0.669 | 4.222 | -0.574 | 3.73 | 188264 | |
3 | 0.590 | 3.364 | -0.472 | 2.83 | 2963463 | |
4 | 0.491 | 2.885 | 0.390 | 2.85 | 2950000 | |
5 | 0.469 | 2.956 | -0.882 | 4.20 | 7530000 | |
0.1 | 1 | 0.563 | 3.233 | -0.586 | 3.19 | 1860544 |
2 | 0.747 | 4.232 | -0.452 | 2.92 | 2900787 | |
3 | 0.631 | 3.582 | -0.083 | 2.79 | 9709 | |
4 | 0.773 | 4.286 | 0.232 | 2.64 | 16000 | |
5 | 0.452 | 2.717 | -0.452 | 3.29 | 1960000 | |
0.2 | 1 | 1.351 | 8.325 | -0.485 | 3.62 | 35200 |
2 | 2.831 | 13.573 | -0.236 | 2.55 | 12200 | |
3 | 1.695 | 9.565 | -1.010 | 4.29 | 15500 | |
4 | 1.389 | 7.466 | -0.406 | 2.64 | 13284 | |
5 | 1.649 | 9.941 | -0.921 | 3.77 | 5149 |
2.2 磨削深度对样品表层显微组织及其显微硬度的影响
图6a给出了0.05 mm深度磨削γ-TiAl试片L截面表层显微组织的OM像。可以看出,其表面轮廓较为平直,当表层γ + α2片层界面与表面成较大角度时,白色α2片沿磨削方向弯曲变形,如箭头所示;而片层界面与表面接近平行的白色α2片未观察到明显变形,如圆圈所示。其他深度磨削试片L截面也观察到类似现象,如图6b~e所示。对比不同深度磨削试片L截面表层显微组织可以发现,片层弯曲程度和深度都随磨削深度增大而增大。不同深度磨削γ-TiAl试片L截面表层片层弯曲深度统计后发现,0.05 mm深度磨削试片的变形层弯曲深度约为11 μm,而1 mm深度磨削试片的弯曲深度达到23 μm。在表层显微组织中还可观察到红色硼化物也发生沿磨削方向弯曲变形,个别硼化物破碎断裂,如图6b中箭头所示。而磨削深度为0.5和1 mm的样品在片层弯曲位置观察到微裂纹,分别如图6d和e中箭头所示,这与图2荧光渗透检测结果相吻合。
图6
图6
不同深度磨削γ-TiAl样品L截面和T截面表层显微组织的OM像
Fig.6
OM images near the surface of γ-TiAl plates ground with 0.05 mm (a, f), 0.1 mm (b), 0.2 mm (c), 0.5 mm (d), and 1 mm (e) depths in the L sections (a-e) and T section (f)
为进一步研究表层的变形情况,使用EBSD分析1 mm深度磨削试片L截面表层显微组织的取向信息和应变分布,如图7所示。图7a给出了表层显微组织的Kikuchi带衬度分布图。可以看出,样品表面到片层弯曲深度位置之间区域的衬度较暗,Kikuchi带质量较差,解析率非常低,这表明该区域发生较为严重的变形。图7b给出了表层显微组织的物相分布图,图7c给出了取向成像图,结合2者可看出,表面与弯曲深度位置间的γ片发生明显的晶粒破碎和取向变化,如图7c中方框所示。同时表面硼化物和晶界α2片也发生破碎和晶体取向变化,如图7c中箭头所示。图7d给出了表层显微组织取向波动图。可以看出,变形主要集中于表层20 μm深度范围和晶界,其中距离表面越近,相应γ片晶粒内部的取向波动越大,这表明其变形程度越大。由以上分析可知,显微组织变化显示磨削表层发生较大的塑性变形,但变形程度从表面到内部逐渐减小,从图7d可看出,1 mm磨削试片的表层变形深度不小于40 μm。
图7
图7
磨削深度为1 mm的γ-TiAl试片L截面表层EBSD分析
Fig.7
Band contrast map (a), phase map (b), inverse pole figure (IPF) map (c), and kernel average misorientation (KAM) map (d) near the surface of plate ground with 1 mm depth in the L section
图8给出了不同深度磨削γ-TiAl试片L截面表层组织显微硬度由表及里的深度变化。可以看出,随距表面距离增加,不同深度磨削样品表层显微硬度先降低后增大,显微硬度变化拐点在距离表面30~60 μm之间,其中磨削深度越大,相应表面显微硬度越大,显微硬度变化拐点深度越大。
图8
图8
不同深度磨削γ-TiAl试片L截面表层显微硬度随深度的变化
Fig.8
Variations of microhardness with distance to the surface of plates at different grinding depths
2.3 磨削深度对旋转弯曲疲劳性能的影响
表2还列出了不同深度磨削加工旋转弯曲疲劳γ-TiAl样品在650℃加载440 MPa的疲劳寿命。可以看出,相同深度磨削样品的疲劳寿命略有波动,其中磨削深度为0.05 mm的样品疲劳寿命大多超过106 cyc,最小也超过了105 cyc;而磨削深度为0.2 mm的样品疲劳寿命大多未超过2 × 104 cyc,个别样品寿命未达到104 cyc;磨削深度为0.1 mm的样品疲劳寿命介于前述2者之间,但数值波动较大。但总体而言,磨削深度越小,相应样品的疲劳寿命越大。
图9给出了不同寿命旋转弯曲疲劳γ-TiAl试样的典型断口形貌。从断口宏观形貌(图9a、c、e和g)可以看出,样品裂纹呈放射线扩展,其裂纹源均靠近表面,但在更高倍数(图9b、d、f和h)下可观察到裂纹萌生位置略有差别,其中疲劳寿命长的0.05 mm深度磨削样品(图9b)裂纹萌生位置可观察到光滑、平直的小平面,其近平行于样品轴向,如图9b中箭头所示,该类小平面为α2 + γ两相片层界面,距离表面约100 μm。疲劳寿命较长的0.1 mm深度磨削样品(图9d)裂纹源也观察到类似情况,其距离样品表面约50 μm,如图9d中箭头所示。而疲劳寿命异常的0.1 mm深度磨削样品(图9f)裂纹源可观察到明显的划痕,其深度约13 μm,如图9f中箭头所示。疲劳寿命较短的0.2 mm深度磨削样品(图9h)裂纹源也观察到划痕,其深度为6~8 μm,如图9h中箭头所示。以上断口形貌观察表明,裂纹萌生位置与磨削表面质量密切相关。
图9
图9
不同深度磨削旋转弯曲疲劳γ-TiAl样品断口的二次电子形貌
Fig.9
Low (a, c, e, g) and high (b, d, f, h) magnified SE-SEM images showing fracture surfaces of fatigue γ-TiAl samples ground with 0.05 mm (a, b), 0.1 mm (c-f), and 0.2 mm (g, h) depths (a, b) Nf = 7.53 × 106 cyc (c, d) Nf = 1.86 × 106 cyc (e, f) Nf = 9709 cyc (g, h) Nf = 1.22 × 104 cyc
3 分析讨论
3.1 磨削裂纹产生的原因
式中,C为工件的热容,f为工件进给速率,vs为砂轮线速度。
本工作仅研究磨削深度的影响,vw、f和vs等磨削工艺参数一致。γ-TiAl合金的C随温度变化很小[11],因此,可认为Ta∝
图10
图10
γ-TiAl合金磨削表层温度变化与应力示意图
Fig.10
Schematic of stress and temperature in the grinding surface layer of γ-TiAl plate (Fp—grinding force, α—angle between radius and ch-ord or angle between chord and grinding direction, Fa—axial component force of Fp, Ft—tangential component force of Fp, Fn—normal component force of Fp, R—grinding wheel radius, Ta—average temperature in the surface of the workpiece contacted with wheel, fn—tensile stress along depth caused by thermal contraction, fr—tensile stress parallel to grinding direction caused by thermal contraction)
从图6和7可知,γ-TiAl合金磨削加工的组织变形主要集中于表层20 μm深度范围内,而γ-TiAl合金的导热系数小[11],导致磨削表层的变形热不能迅速传导至基体,因此样品表层深度方向也存在温度差。冷却液不断冷却工件磨削表面,这就导致了工件表面和基体内部温度低,而2者之间的表面变形层芯部温度高,如图10所示,其在后续冷却过程中产生沿深度方向的拉应力(fn)。磨削深度越大,变形热温升越大,相应表面变形层冷却收缩变形越大,产生的fn越大。TiAl合金塑性较低[2,4,9],当其受力超过临界值就会萌生裂纹并迅速扩展。因此,磨削深度越大,相应冷却收缩形成的拉应力越容易导致样品裂纹,这解释了图6d和e中磨削深度为0.5和1 mm的样品表层显微组织微裂纹的形成原因。
3.2 磨削深度对样品表面粗糙度与显微硬度的影响
磨削样品L截面与T截面的表层显微组织具有明显的差异,前者片层均沿磨削方向偏转,变形层厚度大,后者片层向磨粒耕梨形成的V型槽两侧偏转,变形层厚度小,这与磨削过程中砂轮磨粒对工件的作用力,即磨削力(Fp)有关。已有研究[13]表明,磨削力与ap具有如下关系:
式中,K为与材料特性相关的常数。因此,ap越大,相应Fp越小。而砂轮对工件施加的Fp可分解为砂轮法向力(Fn)、砂轮切向力(Ft)和砂轮轴向力(Fa),如图10所示,其中Fn最大,而Fa最小[13]。在Fn和Ft作用下,L截面片层晶粒沿磨削方向弯曲变形;而在Fn和Fa作用下,T截面片层晶粒沿砂轮轴向弯曲变形。根据前述磨削力分析可知,Ft大于Fa,相应L截面片层受力大,变形程度大,这解释了L和T截面显微组织变形层厚度的差别。对于平面磨,Fa方向取决于磨粒与工件接触面的方向,而磨粒形状不规则导致作用力方向变化,因此片层沿磨粒两侧偏转,而Ft沿磨削方向,片层沿磨削方向偏转弯曲[8],这解释了L截面与T截面的片层变形和表面线性轮廓差异。L截面的片层沿磨削方向的弯曲变形主要与Ft有关,结合图10几何分析与作用力分解可知,ap、砂轮半径(R)、接触弦长(l)、砂轮法向夹角(α)有如下几何关系:
因此,Ft与Fp和ap具有如下关系:
根据上述关系可知,ap越大,相应Ft越大,其对片层沿磨削方向的作用力越大,相应片层弯曲深度越大,这解释了片层弯曲深度随着磨削深度增大而增大的实验现象。
试片与棒状旋转弯曲疲劳试样表面粗糙度参数Ra和Rz均随着磨削深度的增大而增大,如表1和2所示,但随磨削深度的增大趋势不一样,前者较为平缓,而后者在磨削深度为0.2 mm时迅速增大。磨削通过砂轮上磨粒对工件的滑擦、耕梨和切削作用而形成一定纹理的表面,相应工件表面粗糙度参数与砂轮表面形貌、磨粒粒度及其分布、工件与砂轮相互作用等密切相关[13]。作用力是相互的,砂轮磨削工件的同时,其自身也受工件的摩擦磨损作用,特别是磨削工件温度较高的情况下磨损更为严重。由
不同深度磨削样品的表层显微硬度随着距表面距离增加先减小后增大,拐点位置在距表面30~60 μm处,这与磨削变形引起的表层残余应力有关。磨削易在样品表层形成残余压应力[15]。结合图7d可以看出,随着距表面距离的增大,表层显微组织的应变减小,相应由变形引起的残余压应力随之下降。结合图10和前述分析可知,磨削变形热温升热胀冷缩导致表层一定深度位置形成拉应力,最终拉应力减小直到趋于初始平衡状态。而拉应力导致材料显微硬度减小[15,26]。因此,表层组织显微硬度随着残余压应力的减小而减小,并在残余拉应力区域达到最小值,随着拉应力减小到平衡状态,其硬度也缓慢上升,这解释了显微硬度先下降后增大的变化趋势。结合图6可知,磨削深度越大,变形层厚度越大,变形残余压应力层越厚,相应显微硬度变化拐点深度越大[26],这解释了不同深度磨削样品显微硬度变化拐点差异。
3.3 表面粗糙度与疲劳性能的关系
图11
图11
十点平均高度(Rz)与650℃加载440 MPa疲劳寿命的关系
Fig.11
Relationship between Rz and Nf tested at 650oC under a load of 440 MPa
γ-TiAl合金疲劳断口表现出明显的脆性断裂特征,裂纹都萌生于样品表面,如图9所示。但更高倍数下可观察到,长寿命样品(Nf大于106 cyc)的裂纹源与样品表面有一定距离,如图9b和d所示,这类样品裂纹萌生与大角度片层界面两相变形不协调,两相片层界面易应力集中开裂有关[27,28]。而短寿命样品(Nf小于105 cyc)的裂纹源基本位于样品表面,甚至可观察到明显的磨削痕迹,如图9f和h所示。结合表面三维形貌与线性轮廓检测结果可知,短寿命样品裂纹源萌生于磨削表面沟槽,这与沟槽位置的应力集中有关。已有研究[15,29]表明,样品表面形貌对材料疲劳性能有着重要影响,样品表面粗糙度参数越大,缺口效应越明显,应力集中系数相应增大,疲劳性能越差。Neuber[30]使用标准粗糙度参数计算应力集中系数(Kt):
式中,n为应力状态相关常数(n = 1时为剪切力,n = 2时为拉应力),b为轮廓单元宽度,t为轮廓单元高度,ρ为缺口处曲率半径。
4 结论
(1) 磨削深度对γ-TiAl合金磨削表面质量有着重要影响,磨削深度不超过0.2 mm的样品未产生裂纹,而磨削深度为0.5和1 mm的样品表面产生裂纹,这与磨削变形热导致的局部温度升高在后续冷却过程收缩受限有关。
(2) 随着磨削深度增大,γ-TiAl合金表面垂直磨削方向的起伏增大,沟槽增多,相应粗糙度高度参数Ra和Rz增大,而粗糙度形状轮廓参数Rsk减小。
(3) γ-TiAl合金磨削表层显微组织中片层弯曲变形,弯曲深度随着磨削量增大而增大。表层显微硬度由表及里先下降后上升,拐点位置随着磨削深度增大而增大。表层显微组织及其显微硬度分析结果表明,磨削表层变形层厚度为40~60 μm。
(4) γ-TiAl合金650℃旋转弯曲疲劳寿命随着磨削深度的增大而下降,磨削深度为0.05 mm的样品疲劳寿命稳定在106 cyc以上,磨削深度为0.2 mm的样品疲劳寿命约为104 cyc,后者疲劳寿命显著降低与磨削表面沟槽应力集中易萌生裂纹有关。
(5) γ-TiAl合金650℃加载440 MPa旋转弯曲疲劳寿命随着Rz的增大而减小,2者呈非线性关系,当Rz小于4 μm时,疲劳寿命稳定在106 cyc以上。
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