热处理对Mg-6Zn-1Cu-xCe合金组织和性能的影响

发布时间：2019-01-20 13:23

【摘要】：镁合金在高温下容易燃烧,形成氧化夹杂降低产品性能,半固态成形工艺可在较低成形温度下制备镁合金产品,既能减少夹杂的产生又能延长模具使用寿命。然而,目前用于半固态成形的镁合金局限在AZ91等牌号合金上,这些合金热处理强化作用较差,不利于发挥半固态压铸成形后可进行热处理的优势,因此,研究适用于半固态成形的新型镁合金势在必行。Mg-Zn-Cu系列合金热处理强化作用较好,但该合金系的半固态组织变化及优化研究尚不深入。本文在ZC61合金的基础上添加Ce元素,并借助XRD、OM、SEM、TEM等手段及拉伸试验来进行以下研究。对ZC61+xCe合金组织和性能进行研究后表明:ZC61合金的铸态组织主要由基体α-Mg、CuMgZn及MgZn_2共晶相组成。Ce元素的添加量增加时,有少量的Mg_12Ce相形成,合金晶粒逐步细化,抗拉强度和延伸率均呈先上升后下降的趋势。合金断裂方式由解理断裂向准解理断裂转变,再向解理断裂转变。在Ce含量为0.5%时,合金晶粒最为细小,且抗拉强度和延伸率均达到最大值。采用等温热处理法研究Ce含量,保温温度和保温时间对合金半固态组织的影响。结果表明:随Ce含量增加,非枝晶颗粒明显细化,且当Ce含量达到0.7%时,固相率会急剧下降。Mg-6Zn-1Cu-0.5Ce合金在600℃保温25min时,可获得理想半固态组织,其颗粒尺寸、形状因子和固相率分别为57μm、1.16和68%。另外,半固态组织中的固相主要由初生α_1(Mg)相和二次凝固的α_2(Mg)相组成。半固态组织演变过程为:共晶组织溶解与α-Mg的粗化→分离→球化→合并与长大。颗粒分离机制包括亚晶界浸润机制和根部重熔机制。当固、液两相达到平衡阶段时,非枝晶颗粒发生Ostwald熟化。研究了热处理对Mg-6Zn-1Cu-0.5Ce合金铸态组织和性能的影响,结果表明:在405℃固溶24h后,共晶相会发生大量溶解,沿晶界分布的残余共晶相以细小、弥散的颗粒状存在。经时效后,CuMgZn和MgZn_2等沉淀相分别在晶界和晶内均有析出。提高时效温度后,沉淀相的数量减少,抗拉强度和显微硬度降低。在160℃时效20 h时,抗拉强度和显微硬度均达到最大值,分别为256MPa和84VHN。分析峰值时效下的TEM照片可知,垂直于滑移面的棒状β_1'相强化作用明显,而平行于盘状滑移面的β_2'相的强化效果则弱了很多。对比断口形貌可知:在铸态拉伸时,断裂方式为沿晶及解理断裂;在固溶态拉伸时,断裂方式为穿晶及准解理断裂;在时效态拉伸时,断裂方式为穿晶及混合断裂。研究了热处理对Mg-6Zn-1Cu-0.5Ce合金半固态组织的影响后表明,半固态组织的固溶转变过程如下:共晶相溶解→α_2-Mg生长并形成细小颗粒→细小颗粒粗化及合并→初生颗粒合并→初生颗粒的粗化及长大。半固态组织在时效后,沉淀相主要在二次凝固区析出。时效温度提高后,析出相数量减少,且初生固相颗粒长大。
[Abstract]:Magnesium alloy can easily burn at high temperature and form oxidized inclusions to reduce product properties. Semi-solid forming process can produce magnesium alloy products at lower forming temperature, which can not only reduce the inclusion generation but also prolong the service life of die. However, the magnesium alloys used in semi-solid forming at present are limited to AZ91 and other grade alloys, which have poor heat treatment strengthening effect, which is not conducive to the advantage of heat treatment after semi-solid die-casting forming. It is imperative to study the new magnesium alloy suitable for semi-solid forming. The heat treatment strengthening effect of Mg-Zn-Cu series alloy is good, but the research on the microstructure change and optimization of semi-solid alloy system is not deep. In this paper, Ce element is added to ZC61 alloy, and the following research is carried out by means of XRD,OM,SEM,TEM and tensile test. The microstructure and properties of ZC61 xCe alloy were studied. The as-cast structure of ZC61 alloy was mainly composed of matrix 伪-Mg,CuMgZn and MgZn_2 eutectic phase. When the addition of Ce element increased, a small amount of Mg_12Ce phase was formed, and the grain of ZC61 alloy was gradually refined. The tensile strength and elongation showed a trend of first rising and then decreasing. The fracture mode of alloy changed from cleavage fracture to quasi-cleavage fracture and then to cleavage fracture. When the content of Ce is 0.5, the grain size of the alloy is the smallest, and the tensile strength and elongation of the alloy reach the maximum value. The effects of Ce content, holding temperature and holding time on the semi-solid microstructure of the alloy were studied by isothermal heat treatment. The results show that with the increase of Ce content, the non-dendritic particles are refined obviously, and when the content of Ce reaches 0.7, the solid phase ratio will decrease sharply. The ideal semi-solid structure can be obtained by holding 25min at 600 鈩，

本文编号：2412075