【作者】 刘强；

【导师】 潘复生； 宋江凤；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2023， 博士

【摘要】 镁合金由于具有密度低、比强度高、比刚度高、阻尼性能好和环境友好等优点,是最有潜力的轻量化材料之一,被誉为21世纪最具发展前景的“绿色工程材料”。随着3C产品、轨道交通工具的快速发展和轻量化要求,镁合金板材具有巨大的应用潜力,其大规模应用对节能减排和实现双碳目标具有非常重要的实际战略意义。目前,镁合金薄板带材已在交通汽车、精密仪表部件、3C产品、蚀刻板、音响振动膜等得到越来越多的应用。但是,由于镁合金具有密排六方结构,室温塑性成形能力差,容易在传统轧制工艺中出现边裂现象,边裂的出现使得成品板材需切边处理,大大浪费板材,使得轧制镁薄板成本非常高,成为制约镁合金薄板发展和应用的瓶颈,限制了镁合金薄板的大规模应用。因此,如何控制或避免轧制薄板中的边裂对板材的成本控制和大规模应用至关重要。而为了更好的控制边裂,则必须开展镁合金轧制薄板边裂的基础研究,掌握边裂控制工艺原型。前期研究表明在线加热轧制是一种可以有效改善镁合金薄板边裂的方法。此外,课题组前期研究发现低温下挤压的Mg-2.0Zn-1.5Mn(ZM21)合金和Mg-1.0Mn-0.5Al(MA10)合金具有十分优异的综合力学性能,发展潜力巨大。然而在较低温度下轧制镁合金板材,通常极易产生边裂缺陷。因此,本论文以ZM21、MA10合金和商用AZ31合金为研究对象,采用在线加热轧制技术在相对较低的温度下进行轧制,开展在线加热轧制薄板边裂形成机理的基础研究,以期获得薄板边裂控制技术原型。本课题主要研究了轧制变形量、轧制温度、板材初始状态等对在线加热轧制三种镁合金薄板边裂的影响,并对比了相同温度下高温原位拉伸的裂纹萌生和扩展以及三种镁合金板材边裂行为差异。主要研究结果如下:(1)AZ31、ZM21、MA10合金挤压板材在150°C下进行在线加热单道次轧制,随着道次变形量从30%增加到75%,边裂程度呈现增加趋势。单道次变形量为30%时,三种镁合金板材边部均未出现裂纹。单道次变形量为45%时,AZ31、ZM21合金板材纵截面开始出现微小裂纹,边部板面无明显边裂;而MA10合金板材板面和纵截面出现少量边裂。当单道次变形量继续增加到60%和75%时,三种镁合金板材边裂更加严重。对比发现,三种镁合金板材在150℃下单道次在线加热轧制时,AZ31合金板材边裂行为最轻微,其次为ZM21合金,MA10合金边裂行为最严重。(2)AZ31、ZM21、MA10合金挤压板材在150℃下单道次在线加热轧制时,边裂纹主要萌生于纵截面,后续扩展至轧制板面形成一定深度的边裂。随着单道次压下量增加,轧制后边部均残留少部分粗大晶粒。由于初始挤压板材中存在粗大晶粒,使其在大应变下与周围细小晶粒组织应变不协调,导致微裂纹主要萌生于细小晶粒和具有硬取向的粗大变形晶粒的交界处。此外,在轧制后的ZM21、AZ31合金板材边部也发现微裂纹萌生于再结晶细晶粒区域。宏观上,三种镁合金挤压板材单道次在线加热轧制较小压下量后,板材边部纵截面观察到凹凸不平,形成微小沟槽。且在凹陷底部出现撕裂损伤而形成局部裂纹源。随着单道次应变量的增加,局部损伤加重并扩展至板面。(3)对比铸态、挤压态、轧制态AZ31板材进行在线加热轧制的边裂行为,在250℃下单道次轧制50%,初始挤压态板材未见明显边裂,而初始轧制态的轧板比初始铸态的轧板边裂更严重。由于初始挤压态板材存在基面双峰织构,并且部分晶粒c轴平行于板材横向(TD),使得轧制过程中变形更均匀,因此未产生剪切带和边裂;而初始轧态的板材为强基面织构,在线加热轧制过程中变形极不均匀而产生大量的剪切带,同时产生了严重的边裂。并且随着轧制压下量的增加,剪切带与轧制方向的角度减小,边裂更严重。此外,初始铸态的轧板边部微裂纹主要萌生于局部再结晶区域和发生再结晶的孪晶内部区域,初始轧态的轧板边部微裂纹主要萌生于剪切细晶带。(4)研究了挤压态ZM21合金纵截面取样和板面预制裂纹取样150°C下的原位拉伸行为,分析了裂纹萌生和扩展机制。在纵截面取样的原位拉伸中,在大小晶粒共存的非均匀区域,微裂纹易萌生于大小晶粒之间和细晶粒区具有低基面SF(硬取向)晶粒的晶界或晶界三角连接处。而在晶粒相对细小的均匀区域,微裂纹也易萌生于具有低基面SF(硬取向)晶粒的晶界或晶界三角连接处。主要由于在这些晶界或晶界三角连接处不能很好地协调变形,容易累积位错导致应力集中。且在晶界三角连接处萌生的微裂纹很可能沿着具有最大基面施密特因子差异的晶界扩展。而在板面预制裂纹原位拉伸试验中,随着位移的增加,应变主要集中在裂纹尖端。在裂纹尖端区域存在明显的应变梯度分布,随着与裂纹尖端边界距离的增加,应变逐渐减小。此外,在裂纹尖端区域,由于基面施密特因子差异较大的相邻晶粒变形不兼容,容易在晶粒间的晶界优先萌生微裂纹,并促进预制裂纹的快速扩展。(5)随着在线加热轧制温度的升高,MA10合金轧制板材边裂行为显著改善。轧制温度的升高,板材边部微观组织更均匀、动态再结晶(DRX)程度更高、应变分布更均匀以及激活了更多非基面滑移,最终使得板材成形性增加、边裂减小。此外,发现MA10合金板材进行多道次在线加热轧制后,轧制板材边部的基面最大织构强度随着轧制温度的升高而增加,归因于轧制后基面取向晶粒数量的增加。更多还原

【Abstract】 Magnesium alloy is one of the most potential lightweight materials because of its high specific stiffness,high specific strength,low density,good damping performance and environmental friendly advantages,and is known as the most promising "green engineering materials" in the 21 st century.With the rapid development of 3C products,rail transportation and lightweight requirements,magnesium alloy sheet has huge application potential.Its large-scale application has very important strategic significance for energy saving and emission reduction and the realization of the double carbon target.At present,magnesium alloy sheet and strip have get more and more applications in the automotive field,3C products,and precision instrument parts,audio vibration film,etching plate and so on.However,because magnesium alloy has a hexagonal closepacked crystal structure,its room temperature plastic deformation capacity is poor,traditional rolling process is prone to edge cracking phenomenon.Edge cracking needs the finished sheet to cut edge processing,leading to a great waste of rolled sheet and thus high cost.This becomes a bottleneck to limit the development and large-scale application of magnesium alloy sheet.Therefore,how to control or avoid the edge cracking in rolled sheet is crucial to control the cost and large-scale application of the sheet.And in order to better control the edge crack,it is necessary to carry out the basic research of edge crack in magnesium alloy rolled sheet and master the prototype of controlling edge crack.The preliminary research shows that on-line heating rolling is an effective method to improve the edge cracking of magnesium alloy sheet.In addition,our group found that the Mg-2.0Zn-1.5Mn(ZM21)alloy and Mg-1.0Mn-0.5Al(MA10)alloy extruded at low temperature have very excellent comprehensive mechanical properties and great potential for development.However,the rolling of Mg alloy sheets at lower temperatures is usually very prone to edge cracking.Therefore,in this thesis,ZM21,MA10 alloy and commercial AZ31 alloy sheets are studied and rolled at relatively low temperatures using on-line heating rolling technology to carry out basic research about the formation mechanism of edge cracks and obtain a prototype of edge crack control technology for Mg thin sheets.The effect of rolling deformation,rolling temperature and initial state on the edge cracking of three kinds of magnesium alloy sheets rolled by on-line heating rolling was mainly studied,and the difference of crack initiation and expansion.In addition,edge cracking behavior of magnesium alloy sheets were also compared with tensile cracking in in-situ tensile experiment at the same temperature.The main findings are as follows:(1)AZ31,ZM21 and MA10 alloy extruded sheets were rolled in a single pass by online heating rolling at 150°C.The degree of edge cracking shows an increasing trend as the pass reduction increases from 30% to 75%.When the single-pass reduction was 30%,no cracks appears at the edges of all three magnesium alloy sheets.When the reduction of single-pass is 45%,AZ31 and ZM21 alloy sheets start to show small cracks in the longitudinal section,and there is no obvious edge crack on the rolling plane of the sheet;while a small amount of edge crack appears on the rolling plane and longitudinal section of MA10 alloy sheet.When the reduction of single-pass increases to 60% and 75%,the edge cracking behavior of all three magnesium alloy sheets becomes more severe In comparison,it was found that the edge cracking behavior of AZ31 alloy sheet is the least,followed by ZM21 alloy,and the edge cracking behavior of MA10 alloy rolled sheet exhibits most severe when the three magnesium alloy sheets were rolled in a single-pass on-line heating rolling at 150°C.(2)When AZ31,ZM21 and MA10 alloy extruded sheets were rolled by on-line heating rolling at 150°C,the edge cracks initiated in the longitudinal section subsequently extend to the rolling plane to form certain depth.With the increase of single-pass reduction,a few coarse grains were remained at the edge after rolling.The presence of coarse grains in the initial extruded sheets cause uncoordinated strain with surrounding fine grains under large deformation,resulting in the initiation of microcracks mainly at the junction of fine grains and coarse deformed grains with hard orientation.In addition,microcracks were also found within the recrystallized fine grain region at the edge of the ZM21 and AZ31 alloys rolled sheets.Macroscopically,unevenness is observed in the longitudinal section of the sheet after single-pass on-line heating rolling of the three magnesium alloy extruded sheets with relatively smaller reduction,and tiny grooves are also formed.Moreover,its bottom appears tear damage and forms local microcrack source.With the increase of the single-pass reduction,the local damage increases and expands into rolling plane.(3)Comparing the edge cracking behavior of as-cast,as-extruded,and as-rolled AZ31 sheets,no significant edge cracking is observed in the initial extruded sheet after on-line heating rolling at 250°C with a single-pass reduction of 50%.While the edge cracking is more severe in the initial rolled sheet than that in the rolled sheet with as-cast state.Due to the existence of the basal bimodal texture of the initial extruded sheet,and part of the grain c-axis parallel to the transverse direction(TD)of the sheet,making the rolling deformation process more uniform,and therefore do not produce shear bands and edge cracks.While the initial sheet with rolled state has a strong basal texture at the edge,leading to the extremely uneven deformation during online heating rolling process and generation of a large number of shear bands,as well as serious edge cracks.And with the increase of rolling reduction,the angle between the shear band and the rolling direction decreases,resulting in the more serious edge crack.In addition,microcracks at the edge of rolled sheet with as-cast state mainly initiate in the fine-graind region with local recrystallization and inside internal area with twin recrystallization.And microcracks at the edge of sheets with initial rolled state mainly initiate inside the shear fine-grianed bands.(4)The in-situ tensile cracking behavior of longitudinal cross-sectional sample and prefabricated crack sample on plate surface of ZM21 alloy in the extruded sheet at 150°C was investigated to analyze the crack initiation and propagation mechanisms.It is found that for the in-situ tension of longitudinal cross-sectional samples,microcracks tend to initiate both between large and small grains and the grain boundaries or triple joints of grain boundaries inside the fine grain region with low basal SF(hard orientation)grains in the non-uniform region where large and small grains coexist.In the homogeneous region with relatively small grains,microcracks also tend to occur at the grain boundaries or triple junctions of grain boundaries with low-basal SF(hard orientation)grains.The main reason is that these grain boundaries or triple joints of grain boundaries can not well coordinate with deformation and are prone to accumulate dislocations,leading to stress concentration.The microcracks initiated at triple joints of grain boundaries are likely to propagate along the grain boundaries with the largest basal Schmid factor difference.In contrast,in the in-situ tensile test of prefabricated cracks on the plate surface,the strain was mainly concentrated at the crack tip with increasing displacements.There is a clear strain gradient distribution in the crack tip region.It can be seen that the strain distribution gradually decreases with the increase of distance from the crack tip boundary.In addition,around the crack tip region,the incompatibility of intergranular deformation due to the large difference in basal Schmid factor tends to preferentially initiate the microcracks at the grain boundaries and promotes the rapid expansion of prefabricated cracks.(5)The edge cracking behavior of MA10 alloy rolled sheets is significantly improved with the increase of the on-line heating rolling temperature.The increase in rolling temperature results in more homogeneous microstructure at the edge,higher degree of dynamic recrystallization(DRX),more uniform strain distribution,and activation of more non-basal slip,which ultimately leads to an increased sheet formability and reduce the edge cracking.In addition,it is found that the maximum basal texture intensity of the rolled sheet at the edge of MA10 alloy sheets increases with increasing rolling temperature after on-line heating rolling with multiple passes,which is attributed to the increase of the number of basal oriented grains after rolling.更多还原