热处理对ZK60镁合金显微组织及疲劳断裂行为的影响

发布时间：2018-09-12 15:05

【摘要】：镁合金具有密度低、强度高、良好的阻尼性和减震性能,而且它的热容量低、凝固速度快、铸造成型性能好。镁合金的循环利用性好,回收再利用率高,回收熔化时所消耗的能量较少,并且使用回收的镁合金制作构件时不会降低其力学性能。被誉为“21世纪最具发展的绿色工程材料”。ZK60镁合金作为目前商用变形镁合金中强度最高者,提高构件的塑性,对扩大镁合金的应用至关重要。但是商业镁合金板材总会存在一定的缺陷,进而影响性能,使用热处理的方法可以有效地减少缺陷。因此,论文通过研究热处理条件下ZK60镁合金微观组织的变化,探讨其对疲劳断裂行为的影响,这对进一步推动ZK60镁合金在实际生产生活中的应用具有一定的意义。论文采用万能试验机对ZK60镁合金板材以及热处理后试样的拉伸性能进行分析,利用疲劳试验机分析镁合金的疲劳裂纹扩展特征。利用SEM、XRD等测试手段对热处理后镁合金进行物相分析。ZK60镁合金板材晶粒尺寸26μm,固溶处理以后的试件晶粒平均尺寸大约是31μm。之后进行时效热处理从5h~20h时,晶粒尺寸先减少后变得粗大。时效10h时,晶粒的尺寸最小。ZK60镁合金板材和固溶处理以后的拉伸强度分别是294MPa和301MPa。时效处理后试样的强度为325MPa,相对ZK60镁合金板材来说有较大的提高,延伸率以及屈服强度都有所提高,通过XRD分析可知,ZK60镁合金板材主要有由α-Mg,MgZn,MgZn2相组成,固溶处理之后试样中的MgZn相全部融入基体中,Mg Zn2大量溶入到基体中只有少量残余,但是时效处理后有MgZn和MgZn2重新析出。试样受到循环载荷次数为107时,ZK60镁合金板材的疲劳强度为91.62MPa,试件在经过固溶处理(T4)及固溶时效处理(T6)之后疲劳强度分别为102.46MPa和117.76MPa,经过时效处理之后试件的疲劳强度相对于ZK60镁合金板材提高28.31%。ZK60镁合金板材的疲劳断口为脆性断裂断口,而热处理之后在裂纹萌生区发现许多凹坑,裂纹扩展区域存在准解理断裂特征。裂纹扩展速率试验表明,时效5h的试件裂纹的扩展速率稍慢于固溶处理的试件。ZK60镁合金时效10h,抵御裂纹扩展能力得到提高。时效15h时,裂纹扩展速率没有继续下降,反而有所提高。当时效20h时裂纹扩展速率加快。所以固溶处理之后,时效10h时,ZK60镁合金的抵抗裂纹扩展的性能最优。通过分析裂纹扩展速率的断口,在裂纹开始扩展阶段,固溶处理的试样断口含有小坑,属于脆性断裂断口,时效处理的断口相对平滑。在时效处理试件的稳定扩展区可以发现二次裂纹。在时效处理的瞬断区内有许多韧窝,它可以提高抵御裂纹扩展的能力。通过塑性半径公式计算出ZK60镁合金板材,固溶处理的试样和时效处理后的试样裂纹尖端的塑性半径分别为10.5μm和10.1μm,平均晶粒尺寸大于塑性半径,发现时效处理的ZK60镁合金相对于固溶处理的ZK60镁合金具有更小的塑性半径,即有更小的塑性区,所以使得试件抵御裂纹扩展能力加强。
[Abstract]:Magnesium alloys have the advantages of low density, high strength, good damping and damping properties, low heat capacity, fast solidification speed and good casting formability. ZK60 magnesium alloy is regarded as the most developed green engineering material in the 21st century. As the highest strength of commercial wrought magnesium alloy, improving the plasticity of components is very important for expanding the application of magnesium alloy. Therefore, this paper studies the microstructure changes of ZK60 magnesium alloy under heat treatment, and discusses its influence on fatigue fracture behavior, which has certain significance for further promoting the application of ZK60 magnesium alloy in practical production and life. The tensile properties of ZK60 magnesium alloy were analyzed by fatigue testing machine and the fatigue crack propagation characteristics were analyzed by SEM and XRD. The grain size of ZK60 magnesium alloy sheet was 26 micron. The average grain size of ZK60 magnesium alloy after solution treatment was about 31 micron. The tensile strength of ZK60 magnesium alloy sheet is 294 MPa and 301 MPa respectively after solution treatment. The strength of the sample after aging treatment is 325 MPa, which is higher than that of ZK60 magnesium alloy sheet. The elongation and yield strength are both improved by XRD analysis. It is known that ZK60 magnesium alloy sheet is mainly composed of a-Mg, MgZn and MgZn2 phases. After solution treatment, all MgZn phases in the sample are incorporated into the matrix, while Mg-Zn2 is dissolved into the matrix with a small amount of residual, but MgZn and MgZn2 are re-precipitated after aging treatment. The fatigue strength of ZK60 magnesium alloy sheet is 9 when the number of cyclic loading is 107. The fatigue strength of the specimens after solution treatment (T4) and solution aging treatment (T6) were 102.46 MPa and 117.76 MPa, respectively. The fatigue strength of the specimens after aging treatment was 28.31% higher than that of ZK60 magnesium alloy sheet. The fatigue fracture of ZK60 magnesium alloy sheet was brittle fracture, and the crack initiation zone was found after heat treatment. The crack growth rate test showed that the crack growth rate of the specimens aged for 5 h was slightly slower than that of the specimens treated by solid solution. The crack growth rate of ZK60 magnesium alloy increases at h, so the crack growth resistance of ZK60 magnesium alloy is the best after solution treatment and aging for 10 h. By analyzing the fracture surface of crack growth rate, at the beginning of crack growth stage, the fracture surface of the sample treated by solution treatment contains pits, which belongs to brittle fracture surface, and the fracture surface of aging treatment is relatively smooth. Secondary cracks can be found in the stable propagation zone of the specimens.There are many dimples in the transient fracture zone after aging treatment,which can improve the resistance to crack propagation.The plastic radius of ZK60 magnesium alloy sheet is calculated by the plastic radius formula.The plastic radius of the crack tip of the solution treated specimens and the aging treated specimens are 10.5 and 10.1 microns,respectively. The grain size is larger than the plastic radius. It is found that the aging treatment ZK60 magnesium alloy has a smaller plastic radius than the solution treatment ZK60 magnesium alloy, that is, there is a smaller plastic zone, which makes the specimen more resistant to crack propagation.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.22;TG166.4

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