【作者】 刘文才；

【导师】 翟春泉； 董杰； 张平；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2011， 博士

【摘要】 镁合金运动类零部件减重效应大,在交变载荷作用下容易以疲劳方式失效,研究清楚镁合金的疲劳失效行为和提升其疲劳性能意义重大。本文选取商业牌号高强变形镁合金ZK60和新型高强镁稀土合金GW103K为研究对象,首先研究不同状态、不同类型试样的高周疲劳性能和疲劳断裂行为;其次开发适合镁合金喷丸强化的表面处理关键设备;然后表征喷丸处理后合金表面形变层组织、结构和性能特征;再研究喷丸处理合金的高周疲劳性能和疲劳断裂行为;最后尝试解释喷丸强化镁合金机理。ZK60和GW103K镁合金不同状态、不同类型试样的高周疲劳性能研究结果表明,挤压态高于铸态,时效态高于挤压态,而固溶态低于挤压态。挤压态和挤压+时效态ZK60合金光滑试样的107次疲劳强度分别为140和150MPa;挤压态ZK60合金Kt=2.3和2.7缺口试样疲劳强度Kt.?-1分别为150和160MPa;挤压+时效态ZK60合金Kt=2.3和2.7缺口试样疲劳强度Kt.?-1分别为155和165MPa;而铸态(85MPa)、铸态+T6态(105MPa)、挤压+T4态(110MPa)、挤压+T6态(110MPa)、挤压态(150MPa)、挤压+过时效态(160MPa)、挤压+欠时效态(160MPa)、挤压+峰时效态(165MPa)GW103K合金光滑试样依次具有更高的疲劳强度。同等状态下,GW103K合金的疲劳强度高于ZK60合金。裂纹复型和疲劳断裂行为研究结果表明,挤压态和挤压+时效态ZK60镁合金光滑试样的疲劳裂纹萌生于试样表面,且只有一个疲劳裂纹源,裂纹从萌生源向试样横截面的对面呈扇形扩展,裂纹萌生寿命远大于扩展寿命;而缺口试样的疲劳裂纹萌生于缺口根部,且有多个裂纹源,裂纹由四周向中心扩展,裂纹萌生寿命远小于扩展寿命。不同状态GW103K镁合金具有不同的疲劳裂纹萌生、扩展和断裂行为。铸态、铸态+T6态、挤压+T4态和挤压+T6态合金的疲劳裂纹主要萌生于亚表面的气孔或氧化物等缺陷处,疲劳强度由非扩展裂纹决定;挤压态和时效态合金的疲劳裂纹主要萌生于表面挤压条带组织中的粗大晶粒区域,疲劳强度由疲劳裂纹萌生决定;铸态和挤压+T4态合金的疲劳断裂方式以穿晶扩展为主;铸态+T6态和挤压+T6态合金以沿晶扩展为主;挤压态和时效态合金为沿晶和穿晶混合扩展模式。喷丸处理后ZK60和GW103K镁合金表面变形层微观组织的研究结果表明,随喷丸强度增加,各状态合金的表面变形层深度、微观组织细化程度、显微硬度、残余压应力和表面粗糙度均增加。挤压织构显著弱化。喷丸强度为0.10mmN时,挤压态ZK60和GW103K合金部分晶粒尺寸达到150nm,表面粗糙度Ra约为1?m,变形层显微硬度增幅分别为40和45HV0.05,最大残余压应力分别约为75和120MPa。GW103K合金变形层显微硬度增幅以及最大残余压应力均高于ZK60合金,且疲劳过程中残余压应力松弛幅度更小。喷丸处理ZK60和GW103K镁合金的疲劳寿命表现出很大的喷丸强度敏感性和过喷效应,高周疲劳寿命随喷丸强度增加先增大,达到一个峰值,然后急剧降低。挤压态和挤压+时效态ZK60合金光滑试样的合适喷丸强度范围为0.02~0.15mmN,缺口试样的合适喷丸强度范围为0.20~0.50mmN;挤压态和挤压+时效态GW103K合金的合适喷丸强度范围为0.05~0.30mmN。挤压态和挤压+时效态ZK60镁合金光滑试样在最佳喷丸强度0.05mmN条件下,疲劳强度分别提高到180和195MPa,缺口试样分别在最佳喷丸强度0.30和0.40mmN条件下,疲劳强度分别提高到220和240MPa;铸态(0.40mmN)、铸态+T6态(0.50mmN)、挤压态(0.10mmN)、挤压+欠时效态(0.10mmN)、挤压+峰时效态(0.10mmN)、挤压+过时效态(0.10mmN)、挤压+T4态(0.30mmN)和挤压+T6态(0.40mmN)GW103K镁合金光滑试样在各自最佳喷丸强度条件下,疲劳强度分别提高到115、125、215、230、240、205、140和140MPa。时效状态合金喷丸强化效果优于非时效状态合金,缺口试样优于光滑试样,GW103K合金优于ZK60合金。喷丸后ZK60和GW103K镁合金疲劳断裂行为分析表明,挤压态和挤压+时效态ZK60合金光滑试样的疲劳裂纹萌生源先由表面转移到亚表面,而后又回到表面,裂纹源数量由未喷丸和较小喷丸强度时的单个变为高喷丸强度时的多个;而缺口试样的疲劳裂纹仍萌生于缺口根部,裂纹源数量仍为多个。铸态、铸态+T6态、挤压+T4态和挤压+T6态GW103K合金疲劳裂纹的萌生位置以及扩展行为与喷丸前无明显变化,而挤压态和挤压+时效态合金的裂纹萌生位置则由表面转移到亚表面,挤压态GW103K合金疲劳裂纹主要沿解理面扩展,挤压+时效态合金以剪切韧窝合并方式扩展。所有状态GW103K合金的疲劳裂纹萌生源数量仅为一个,不随喷丸强度变化。喷丸处理ZK60和GW103K镁合金疲劳性能分析结果表明,喷丸引起合金变形层组织细化、残余压应力和显微硬度的增加有利于提高其高周疲劳性能,而表面粗糙度的增加不利于提高高周疲劳性能。喷丸强度过大时,表面粗糙和微裂纹会降低组织细化、残余压应力和显微硬度的强化效果。ZK60和GW103K镁合金的拉压疲劳微观变形行为和喷丸强化机理分析表明,拉压疲劳循环过程中,挤压态和挤压+时效态ZK60合金在压缩过程中以孪生变形为主,拉伸过程以去孪或孪生回复为主,存在显著的拉压不对称性和孪生–去孪现象;而挤压态和挤压+时效态GW103K合金在压缩和拉伸过程中均以位错滑移为主,无明显拉压不对称性和孪生–去孪现象。喷丸处理过程中,挤压态和挤压+时效态ZK60合金以位错滑移和孪生为主,而挤压态和挤压+时效态GW103K合金以位错滑移为主。喷丸处理形成的孪晶在随后疲劳循环过程中的去孪会引起残余压应力松弛,同时较高密度的孪晶增加了疲劳裂纹的潜在萌生源,降低喷丸强化效果。更多还原

【Abstract】 At present, it is a hot topic of research on magnesium alloys due to the possible weight saving. However, limited fatigue durability restricts the range of their applications. Shot peening is a powerful method of enhancing the fatigue performance of structural metallic materials. The present study was aimed firstly at investigating the high cycle fatigue properties and fatigue fracture behavior of the ZK60 and GW103K magnesium alloys in different conditions. The key equipments such as shot peening and rotation bending fatigue machine for magnesium alloys were machined. Further study was put on the high cycle fatigue properties and fatigue fracture behavior of peened ZK60 and GW103K alloys. Make a final attempt to explain the shot peening strengthening mechanism of magnesium alloy. ZK60 and GW103K alloys in different conditions as well as different types of amples possessed different high cycle fatigue properties. The fatigue strengths at 107 cycles of smooth and notched specimens (Kt=2.3 and 2.7) of the as-extruded (or extruded-aged) ZK60 alloy were 140, 150 and 160 MPa (or 150, 155 and 165 MPa), respectively. The fatigue strengths of smooth specimens for the as-cast, cast-T6, extruded-F, extruded-UA, extruded-PA, extruded-OA, extruded-T4 and extruded-T6 GW103K alloys were 85, 105, 150, 160, 165, 160, 110 and 110MPa, respectively.The study on crack initiation, propagation and fracture behaviors by using the replica method and SEM show that the fatigue cracks of smooth specimens for the as-extruded and extruded-aged ZK60 alloys initiated at sample surface with only one initiation site. Its fatigue crack nucleation life was great longer than the crack propagation life. Comparatively, the fatigue cracks of the notched specimens for the as-extruded and extruded-aged ZK60 alloys initiated at the notch root surface with multiple initiation sites, and its fatigue crack nucleation life was great shorter than the crack propagation life.The fatigue cracks of the as-cast, cast-T6, extruded-T4 and extruded-T6 GW103K alloys initiated at porosities or inclusions that lie subsurface, and their fatigue strengths were determined by non-propagation cracks. Comparatively, the fatigue cracks of the as-extruded and extruded-aged alloys initiated at the regions with coarse grains that lie surface, and their fatigue strengths were determined by crack initiation.The research on the surface characteristics of the peened ZK60 and GW103K alloys show that the deformation depth, grains refinement degree, microhardness, compressive residual stress and surface roughness increased with the increasing of Almen intensity. Comparatively, at the same Almen intensity, the microhardness improvement and maximum compressive residual stress in the GW103K alloy were higher than those in the ZK60 alloy with similar condition. Furthermore, the relaxation degree of compressive residual stress of the GW103K alloy during fatigue cycling is significantly less than that of the ZK60 alloy.A pronounced overpeening effect was observed in the ZK60 and GW103K alloys. The fatigue life depends on the Almen intensity, with the fatigue life first dramatically increased with the Almen intensity, and then similarly dramatically decreased. The optimum shot peening process window of the as-extruded and extruded-aged ZK60 alloys is 0.02~0.15mmN for smooth specimens and 0.20~0.50mmN for notched specimens. For the as-extruded and extruded-aged GW103K alloys, the optimum shot peening process window is 0.05~0.30mmN.The optimum Almen intensities of the as-extruded and extruded-aged ZK60 alloys are 0.05 mmN for the smooth specimens, and 0.30 and 0.40mmN for the notched specimens, respectively. Compared to the unpeened smooth specimens, the increase of fatigue strengths for 107 cycles was 40 MPa for the as-extruded ZK60 and 45 MPa for the extruded-aged ZK60; while compared to the unpeened notched specimens, the increase of fatigue strength ( ? a ?Kt, Kt=2.3) was 70 MPa for the as-extruded ZK60 and 85 MPa for the extruded-aged ZK60. The optimum Almen intensities of the as-cast, cast-T6, extruded-F, extruded-UA, extruded-PA, extruded-OA, extruded-T4 and extruded-T6 alloys were 0.40, 0.50, 0.10, 0.10, 0.10, 0.10, 0.30 and 0.40mmN, respectively. The fatigue strengths of the peened GW103K alloys in eight conditions were 30, 20, 65, 70, 75, 40, 30 and 30MPa higher than those of the unpeened specimens, respectively.The fractography observation by SEM shows that the fatigue crack nucleation site of smooth specimen of ZK60 alloy shifted from surface to the subsurface, and then shifted to the surface again, with the increase in Almen intensity. Meanwhile, a significantly higher number of fatigue crack initiation sites resulted from overpeening can be seen. Compared to smooth specimen, shot peening exhibits no significant effect on the fatigue crack initiation of notched specimen. The fatigue cracks of the peened GW103K alloys in different conditions initiated in subsurface at all Almen intensities, and they still have one crack initiation site with the increase in Almen intensity. The small fatigue crack in the peened as-extruded GW103K alloy grew alont the cleavage planes. In contrast, the small crack in the extruded-aged GW103K alloy propagated by the coalescence of the sheared dimples. The measurement results on the surface and subsurface characteristics in the peened magnesium alloys show that the improvements of grain refinement, compressive residual stress and microhardness were efficient to improve high cycle fatigue properties, while surface roughening accelerates the nucleation and early propagation of cracks.For the as-extruded and extruded-aged ZK60 alloys, during the compression and tension fatigue cycling, mechanical twinning dominates deformation in compression, while dislocation slip dominates deformation in tension, i.e., the as-extruded and extruded-aged ZK60 alloys exhibit twinning and detwinning phenomenon. Comparatively, dislocation slip dominates deformation in both compression and tension in the as-extruded and extruded-aged GW103K alloys. Similarly, the twinning deformation for the as-extruded and extruded-aged ZK60 alloys and dislocation slip for the as-extruded and extruded-aged GW103K alloys are the grains refinement mechanism by shot peening. The twinning induced by shot peening and detwinning during fatigue cycling leads to the residual stress relaxation. Meanwhile, the possibility of these twins acting as crack initiation sites decreases the positive effect of shot peening on fatigue resistance.更多还原