摘要
镁合金板材轧制工艺的不成熟，如道次变形量小，轧制道次多，中间多次加热、材料利用低等问题，严重制约着镁合金板材的广泛应用，因此，探索出高效、高性能、高材料利用率的镁合金板材轧制工艺，有利于促进变形镁合金的发展与应用。本文为此以ZK61镁合金铸坯和挤压坯为研究对象，研究了多道次降温热轧工艺及轧制过程中的组织性能演变，以求获得一条既有利于提高效率又可改善板材综合性能的轧制工艺路线，并重点研究了轧制温度和道次变形量对2.0mm以下薄板组织、织构及性能的影响，以寻求制备ZK61镁合金薄板的热轧工艺。根据镁合金冷变形特点，开拓出小变形量多道次累积冷轧工艺，可以使累积冷变形量达到20-35%，成功制备出了0.4-0.7mm厚性能优异、表面质量良好的ZK61镁合金薄板板材。本文还研究了热轧和冷轧过程的沉淀析出问题，并深入分析了晶粒尺寸、织构对板材力学性能的影响。
ZK61镁合金24mm厚铸坯经30%道次变形量6道次连续降热轧制获得了晶粒尺寸7.2μm、延伸率24%、屈服和抗拉强度分别约为208MPa和306MPa的2.0mm镁合金板材，并完全消除了中间加热工序，体现了的多道次降温轧制的高效性。对比20-40%不同道次变形量下的ZK61镁合金挤压坯多道次降温热轧过程中的组织性能演变，发现30%道次变形量对挤压坯热轧板材的组织性能改善仍具有很好的适用性。系统研究了道次变形量20-50%、轧制温度160-300℃对2.0mm板材单道次热轧轧后板材组织性能的影响，得出有利于ZK61镁合金薄板组织细化和性能优化的30-40%道次最佳变形量区间及230-250℃最佳温度区间，轧后板材组织晶粒细化至约3.2μm且尺寸分布均匀、延伸率稳定高于22%、屈服和抗拉强度分别高达222MPa和324MPa。采用40%逐次降温的热轧工艺，成功制备出延伸率高达30%的ZK61镁合金0.7mm厚薄板。随后探索了小变形多道次累积的冷轧工艺方案，成功制出板厚0.42-0.50mm、表面平整的ZK61细晶镁合金薄板，板材延伸率稳定高于25%、屈服强度高于220MPa、抗拉强度约为280MPa。进一步的180℃2h退火处理消除ZK61镁合金冷轧薄板的板平面强度各向异性，使得镁合金薄板板材延伸率进一步升高到了30%以上，对镁合金冲压性能的改善有重要意义。
对轧制过程中板材组织内Mg-Zn相析出研究表明，300℃以下热轧易于促进应变诱导沉淀析出，沉淀析出相形态随热轧进行由长杆型（I-type）和短棒型（II-type）为主，逐渐转变为以短棒型（II-type）和球型（III-type）为主。随后时效处理显示，175℃仅需0.5h便达到峰值时效状态，而通常则需时效10h以上。因此，多道次降温轧制伴随沉淀析出，有利于缩短板材生产的整体周期，实现了时效强化和细晶强化同时发生的效果。
板材的力学性能与组织密切相关，晶粒尺寸以Hall-Petch关系式的方式影响着板材的强度。本文研究表明，随织构状态不同，镁合金中存在着滑移主导的Hall-Petch关系和孪生主导的Hall-Petch关系。织构的改变引起滑移主导的Hall-Petch关系的斜率k在120-246MPa·μm~(1/2)之间变化，并与Schmid因子m成反比。孪生主导的Hall-Petch关系表现出恒定的k值约为300MPa·μm~(1/2)。板材组织状态对板材塑性的影响显著，晶粒细化对板材拉伸过程中的均匀延伸率影响甚微，主要表现通过提高应变速率敏感指数改善板材拉伸时的非均匀延伸率。织构状态直接决定着板材的均匀延伸率，表现为随着垂直于拉伸方向基面织构的减弱均匀延伸率不断提高。基于硬化行为的研究表明，这种均匀延伸率对织构的依赖性源于室温拉伸硬化行为中存在的类似的织构依赖关系，并拟合得到一指数型关系， δ_u=δ_0+c1exp (c_2K_(III), K_(III)<0)。变形过程中的孪生一定程度可通过延缓应变硬化方式提高均匀延伸率。
Wide application of magnesium alloy sheets is limited by the immaturemagnesium alloy plate manufacturing technology at present. Therefore, exploring arolling processing technology with high efficiency and high quality for magnesiumalloy sheets is of great significance to promote the development of magnesium alloy.In this study, as-cast and as-extruded billets of ZK61magnesium alloy were used asthe starting materials to study multi-pass rolling process with decreasingtemperature and microstructure evolution, for the purpose of reducing productioncost and improving mechanical properties at room temperature. Further study wasfocused on the effect of rolling temperature and thickness reduction on theperformance of hot-rolled sheets with2.0mm thickness to design a warm rollingprocess for fabricating thick ZK61magnesium alloy sheets. In addition,precipitation during rolling process as well as relationships between microstructureand mechanical properties was also studied.
As-cast ZK61magnesium alloy billet was subjected to6-pass rollingdeformation with30%thickness reduction per pass. The final ZK61magnesiumalloy sheets with grain size7.2μm attained an improved elongation of24%. It wasfound that30%thickness reduction applied on the rolling of as-extruded billet stillhad a good applicability. The effects of rolling parameters including rollingtemperature and reduction, on the microstructure and mechanical properties ofZK61magnesium alloy sheets rolled at160-300℃with thickness reductionranging from20-40%were investigated. Microstructure evolution of the as-rolledsheets was controlled by the rolling reduction and rolling temperature. Grain size ofZK61magnesium alloy was refined effectively at rolling reduction of30%-40%and rolling temperature of230-250℃. Homogeneous dynamic recrystallizedmicrostructure with an average grain size of3.2μm was fabricated, and theelongation was enhanced to about24%by grain refinement. Then, through coldrolling process, fine-grained ZK61magnesium alloy sheet with a thickness of0.4-0.7mm, yield strength of more than220MPa, tensile strength of about280MPa,and enhanced elongation of25%was successfully fabricated. Further annealingtreatment of180℃for2h can be used to eliminate the strength anisotropy and toimprove the elongation to above30%. This is of great significance for theapplication of magnesium alloy sheets in the field of stamping.
Precipitation during rolling process was promoted by hot rolling under300℃ due to the effect of strain-induced precipitation. Particles of needle-like I type androd-like II-type were primarily distributed in ZK61magnesium alloy, and graduallytransformed into with rod-like II-type and spherical III-type. Subsequent agingtreatments suggested that a peak aging state occurred at about175℃for0.5h.Thus, multi-pass hot rolling process is beneficial to shortening the production cycleas a whole.
Mechanical properties of sheets are closely related to microstructure. Grainsize effect on strength can be described by Hall-Petch equation. Tensile testssignificantly reveal two different Hall-Petch relations with the variation of texture,corresponding to the mechanical response. One is multislip-yield Hall-Petch fittingwith a texture dependent slope ranging from120to246MPa·μm~(1/2)and a slightdecreasing friction stress of about142MPa with basal texture weakening, which isstrongly dependent on combination of activated slip systems rather than thedominated; the other is twinning-yield Hall-Petch fitting, showing a uniqueempirical equation ofσ25+3001/2y≈d, indicating an insensitivity of twinning-yield strength to texture. This could be ascribed to the lower values in {1012}twinning and basal slip, and an unchanged but lower friction stress in term of theprofuse twinning with the onset of yielding. Tensile tests along transverse directionat room temperature revealed that post-uniform strain may be controlled by theenhancement of increasing strain-rate sensitivity and the deterioration of grainboundary sliding in grain refining, resulting to a maximal fracture strain at the grainsize of about6μm. Although the uniform strain is not associated with grain size, itcan be related exclusively to the texture patterns and increased with basal intensityweakening. The texture dependence of uniform strain was characterized by anexponential relationship on estimated Taylor factor deduced from the Kocks-Mecking hardening model. Additional results and discussions presented on theeffect of twinning deformation suggest that uniform strains can also be enhanced bytwinning deformation via retarding strain hardening.
引文
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