【作者】 冯凯；

【导师】 郝远；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2013， 博士

【摘要】 半固态成形技术被认为是一种最具先进性的金属成形加工技术,其特点是易于近终成形、成形温度低、节约能源等。镁合金在一般成形过程中容易氧化燃烧,而采用半固态成形可有效的解决此问题。目前,采用半固态成形的镁合金主要有AZ91、AM60和AM50等牌号合金,首先是这些合金对半固态成形可控性的要求不能够完全满足的,其次是合金本身的热处理强化效果不太明显,从而不能有效地发挥半固态成形材料可通过热处理强化的优势。因此,有必要开发一种适合半固态成形的高强镁合金。开发合金的思路主要是,先获得强度较高且热处理强化效果显著的镁合金,要求该合金满足半固态成形,并具有较强的可控性,然后利用半固态成形以及热处理提高其力学性能。Mg-Zn-Al合金具有较高的室温及高温力学性能,成本低廉,其研究和应用较为广泛,并且热处理强化效果较好,所以可利用半固态成形对其进一步强化。本课题系统地研究了Mg-Zn—Al合金铸态及热处理组织和力学性能,并且对其进行半固态组织非枝晶化研究,最终选择了热处理强化效果最好并满足半固态成形要求的ZA74合金进行触变压铸研究以及后期热处理强化。主要研究结果如下：1.对Mg-(5～20%)Zn-(0～6%)Al合金组织和性能进行系统地研究。表明,合金主要由α-Mg、MgZn、Mg32(Al,Zn)49及Mg17Al12相组成。随合金元素Zn和A1增加,合金由粗大的树枝晶向细小的等轴晶转变,晶粒尺寸逐渐减小,而晶界处的共晶相数量逐渐增多,尺寸逐渐增大。Mg-Zn-Al合金具有较高的室温抗拉强度,在180-240MPa之间,其中ZA54、ZA56以及ZA202合金的抗拉强度分别可达到227、228和237MPa；随着Zn含量的增加,合金的延伸率逐渐降低。2.Mg-Zn-Al合金经过热处理后,抗拉强度得到显著提高,其中ZA74和ZA72合金热处理后抗拉强度最高,分别为338MPa和337MPa,较铸态分别提高了68%和66%,其次抗拉强度较高的是ZA54和ZA56合金,分别可达到320MPa和305MPa,而随着合金元素的增多,热处理后晶界化合物相严重影响合金的力学性能。对ZA74合金热处理工艺研究,其最佳热处理工艺参数为：固溶工艺350℃+28h水淬,时效工艺165℃+24h。热处理强化主要是固溶强化和时效强化,时效过程中在晶界和基体内部析出细小弥散的强化相,对合金力学性能贡献较大。3.利用等温热处理法对Mg-Zn-Al合金半固态组织演变及机理进行研究。表明,颗粒尺寸与Zn含量有直接关系,随Zn含量增加,得到的固相颗粒尺寸逐渐减小,并且当Zn含量大于7%以后,固相颗粒尺寸在50μm以下,其中,ZA202合金的固相颗粒尺寸可达到28.9μm,Al元素可减小初生固相颗粒的粗化速率；Mg-Zn-Al合金半固态组织演变主要经历组织的粗化、分离、球化以及最终的合并、长大阶段。当液相较少,固相颗粒分离的机制主要是液相沿着晶界、亚晶界浸润和扩展；而有一定的液相存在时,根部重熔机制在固相颗粒分离中起主导作用。4.对ZA74合金进行半固态触变成形及热处理强化研究。表明,半固态成形可显著提高合金力学性能,ZA74合金触变压铸后的抗拉强度可达到256MPa,较金属型(201MPa)提高27%；经热处理抗拉强度可达到352MPa,较铸态提高了38%,与AZ91D半固态触变压铸热处理后的抗拉强度(230MPa)相比,提高幅度为53%。更多还原

【Abstract】 The semi-solid forming technology has been considered to be one of the most advanced metal forming technology, which is characterized by easy to near-net shape, low molding temperature, and energy conservation. Magnesium alloy is easily oxidized and burned in the usual forming process, but the semi-solid forming technology is effective to solve this problem. At present, the semi-solid forming of magnesium alloys is mainly for the existing alloys of AZ91D, AM60and AM50. However, on the one hand, these alloys can not fully meet the requirements of semi-solid forming controllability, on the other hand, they can not give full play to the strengths advantage of semi-solid forming material by heat treatment because of their bad strengthening effect of heat treatment. Therefore, it is necessary to develop a high-strength magnesium alloy for semi-solid forming. First, to obtain the high strength magnesium alloys which have remarkable effect of heat treatment strengthening, then using semi-solid forming to improve its mechanical properties, and finally through the heat treatment further strengthened. Mg-Zn-Al alloy has higher mechanical properties at room temperature and high temperature, and it has low cost, therefore it widely researched and used, further strengthen it by semi-solid forming is feasible. The microstructure and mechanical properties of Mg-Zn-Al alloy at cast and heat-treated condition were investigated in this study. The main results are found as follows:1. The microstructure and mechanical properties of Mg-(5-20%)Zn-(0-6%)A1alloys were studied. The results indicated that the alloys are mainly composed of a-Mg、MgZn、Mg32(Al,Zn)49and Mg17Al12. With increasing of Zn and Al, the coarse dendrites transite to fine equiaxed grains and the grain size is gradually reduced; While the number of eutectic phase and its size both gradually increased. Mg-Zn-Al alloys have higher tensile strength at room temperature, which is between180-240MPa, the tensile strength of ZA54、ZA56and ZA202reaches to227,228and237MPa, respectively. The elongation of alloys decreased gradually with increasing of Zn content.2. The tensile strength of Mg-Zn-Al alloys has significantly improved by heat treatment, while the tensile strength of ZA74and ZA72alloy is338and337MPa, respectively. Compared to as-cast alloys, it has been improved by68and66%, respectively. The tensile strength of ZA54and ZA56alloy could get up to320MPa and305MPa. With the increasing of alloying elements, the compound phases distributed on the grain boundaries which seriously affect the mechanical properties of alloys. The heat treatment process parameters of ZA74alloy are:solution350℃+28h, aging165℃+24h. The main reasons of heat treatment strengthen are solution and aging strengthening. The precipitates at the grain boundaries and within the matrix during aging treatment, which benefit to the improved mechanical properties.3. The semisolid microstructure evolution process and mechanism analysis of ZA74magnesium alloy are investigated by isothermal heat-treatment. The results indicate that the particle size has a direct relationship with the Zn content. With the increasing of the Zn content, it could obtain fine particles, and when the Zn content is greater than7%, the size of particles is less than50μm, and the particle size of ZA202alloy is28.9μm. The coarsening rate of solid particles is reduced by Al element. During partial remelting, the main mechanism of Mg-Zn-Al alloy is eutectic microstructure solution, solute diffusion, coarsening, dendrite separation, spheroidization and final coarsening. Furthermore, the factor of solute diffusion, energy fluctuation and constituent fluctuation, which result in the appearance of subboundary and root remelting, which are the separation mechanism of particle during partial remelting. When the liquid phase is less, solid particle separation mechanism is liquid phase infiltrate liquid and extend along the grain boundary and sub-boundary; and when there is a certain liquid, root remelting mechanism plays a leading role in the separation of solid particles.4. Choosing the ZA74alloy to thixotropic forming and heat treatment, it show that the semisolid forming can significantly improve the mechanical properties of ZA74alloy. The tensile strength of ZA74alloy is256MPa used thixotropic die-casting, compared to the metal forming increased by27%. The tensile strength of semisolid thixoforming ZA74alloy is352MPa after heat treatment, which is increased by38%compared to the cast alloy, and compared with AZ91D alloy (230MPa) increased by53%.更多还原