基于复合细化的7075铝合金流变成形微观组织调控技术

Microstructure cintrol technology of rheocasting 7075 aluminum alloy based on hybrid refiner

彭锦康

11930427

硕士

材料工程

卢宏兴

2021-05-20

2021-06-07

南方科技大学

深圳

7075铝合金具有密度小、比强度高等优点，是汽车轻量化的重要材料之一，在汽车工业中具有广阔的运用前景。7075铝合金属于锻造铝合金，通常通过锻造工艺成型，制造成本较高，且不适合复杂结构成型。该合金具有较高的铸造热裂倾向，且熔体流动性较差，不适宜采用常规铸造工艺成型。在半固态流变成形中，金属熔体被转变为含有一定比例近球形初生固相颗粒的半固态金属，熔体表观黏度升高，在充型过程流动平稳，可以有效降低气孔缺陷生成，且凝固收缩率降低，能够有效降低缩孔、热裂等铸造缺陷，可制造高品质的近净成形件，达到以铸代锻的效果。但是，7075铝合金的半固态加工工艺窗口较窄，在实践中，初生固相颗粒尺寸和形貌不均匀，是普遍存在的问题。并且，随着半固态流变成形技术应用的深入，对初生固相颗粒尺寸的要求越来越高，更小的初生固相颗粒尺寸才能满足未来薄壁件的制造要求。最新相关研究进展显示，在金属型浇铸和增材制造工艺条件下，TiN/Ti复合细化剂对7075铝合金都能起到较好的晶粒细化效果和强化作用，为7075流变成形微观组织调控提供了新的解决思路。在半固态流变成形工艺条件下，金属熔体凝固分为三个阶段，依次为快冷形核阶段、缓冷准平衡凝固阶段以及快冷凝固成形阶段。在这一特定冷却条件下，TiN/Ti复合细化剂是否能对7075铝合金起到良好的细化和强化效果，有待系统性研究。本文主要研究了TiN/Ti复合细化剂对7075铝合金凝固行为的影响、对7075铝合金流变成形件微观组织及性能的影响，以及经复合细化剂处理的流变成形件的微观组织及性能在热处理过程中的演变规律。主要研究成果如下：(1)随着TiN/Ti复合细化剂添加量从0增加到0.8wt.%，α-Al相的起始析出温度从637.8ºC提高到61.8ºC、析出温度范围从41.7ºC增加到56.6ºC，材料的半固态加工温度窗口从6.3ºC增大到10.3ºC。温度场监测结果表明，添加TiN/Ti细化剂有利于提高半固态浆料准稳定阶段的降温速率的均匀性。(2)随着TiN/Ti复合细化剂添加量从0增加到0.8wt.%，流变成形件的α-Al晶粒尺寸先减小后增大，当添加量为0.4wt.%时，晶粒尺寸达到最小值，为65.5 μm。经过固溶热处理后，未经过细化处理的试样晶粒显著长大，而经过细化处理的试样晶粒未发生显著长大。(3)随着TiN/Ti复合细化剂添加量从0增加到0.8wt.%，铸态试样的抗拉强度和屈服强度先增加后减小，当添加量为0.4wt.%时，抗拉强度达到最大值，为251.9 MPa，当添加量为0.6wt.%时，屈服强度达到最大值，为146 MPa。经过T6热处理后，随着TiN/Ti复合细化剂添加量从0增加到0.8wt.%，试样的抗拉强度和屈服强度先增加后减小，当添加量为0.2wt.%，抗拉强度达到最大值，为496 MPa，当添加量为0.6wt.%时，屈服强度达到最大值，为446 MPa。

7075 aluminum alloy is considered to be one of the important lightweight materials with the advantages of low density and high specific strength, so it has a broad application prospect in the automotive industry. As a forged aluminum alloy, 7075 aluminum alloy is usually formed by a forging process, so it has a high manufacturing cost and is not suitable for complex structure forming. At the same times, the alloy has a high tendency of casting hot cracking, and the melt fluidity is poor, so it is not suitable for forming by conventional casting process. In semi-solid rheological forming, molten metal is transformed into semi-solid metal, which contains a certain percentage of near-spherical primary solid particles and the metal slurry shows an increase in viscosity. Thus, the slurry flows smoothly and the solidification shrinkage rate decreases in the filling process, effectively reducing the casting defects such as shrinkage cavity and cracking. As a result, near-net-shape parts with high performance could be manufactured through semi-solid forming, achieving the effect of casting instead of forging. However, the microstructure control of 7075 aluminum alloy by rheo-forming is still a problem due to the narrow semi-solid processing window of 7075 aluminum alloy. It is common fact that the size and morphology of primary solid particles are not uniform in practice. Moreover, with the further application of semi-solid rheo-forming technology, the requirements for the size of primary solid particles are smaller and smaller. Only the smaller primary solid particle size can satisfy the requirements of future thin-walled parts manufacturing. The latest research progress shows that TiN/Ti composite refiners have good composite refining effects on 7075 aluminum alloy under the conditions of metal mold casting and additive manufacturing. In the semi-solid rheological forming process, the solidification of the metal melt can be divided into three stages: the rapid cooling in the nucleation stage, the slow cooling in the quasi-equilibrium solidification stage, and the rapid cooling in the solidification stage. Under this special cooling condition, whether TiN/Ti composite refiner can have the same refining and strengthening effect on 7075 aluminum alloy remains to be systematically studied. This paper mainly studies the effect of TiN/Ti composite refiner on the solidification behavior of 7075 aluminum alloy, the effect of this refiner on the microstructure and properties of the rheological formed parts of 7075 aluminum alloy, and the evolution law of the microstructure and properties of the rheological formed parts treated by the composite refiner during the heat treatment process. The main research results are as follows:(1) With the addition of TiN/Ti composite refiner from 0 to 0.8wt.%, the initial precipitation temperature of α-Al phase increased from 637.8ºC to 61.8ºC, and the precipitation temperature range increased from 41.7ºC to 56.6ºC, and the semi-solid processing temperature window of the material has increased from 6.3ºC to 10.3ºC. The temperature field monitoring results show that the addition of TiN/Ti refiner is beneficial to improve the uniformity of the cooling rate of the semi-solid slurry in the quasi-stable stage.(2) With the addition of TiN/Ti composite refiner increasing from 0 to 0.8wt.%, the α-Al grain size of the rheological shaped part first decreases and then increases. When the addition amount is 0.4wt.%, the crystal grain size reaches the minimum value of 65.5 μm. After solution heat treatment, the grains of the sample without the refining treatment grew significantly, while the grains of the sample after the refining treatment did not grow significantly.(3) As the addition of TiN/Ti composite refiner increases from 0 to 0.8wt.%, the tensile strength and yield strength of the as-cast sample increase first and then decrease. When the addition is 0.4wt.%, The tensile strength reaches the maximum value of 251.9 MPa, and when the addition amount is 0.6 wt.%, the yield strength reaches the maximum value of 146 MPa. After T6 heat treatment, as the addition amount of TiN/Ti composite refiner increases from 0 to 0.8wt.%, the tensile strength and yield strength of the sample first increase and then decrease. When the addition amount is 0.2wt.%, the resistance of tensile strength reaches the maximum value of 496 MPa, and when the addition amount is 0.6 wt.%, the yield strength reaches the maximum value of 446 MPa.

微观组织 力学性能 TiN/Ti复合细化剂 7075铝合金 流变成形

microstructure mechanical property TiN/Ti composite refiner 7075 aluminum alloy rheology

中文

独立培养

南方科技大学

彭锦康. 基于复合细化的7075铝合金流变成形微观组织调控技术[D]. 深圳. 南方科技大学,2021.