摘要
坩埚移动式喷射沉积技术作为一种先进的材料制备新技术,在制备大尺寸合金及金属基材料方面具有显著的优越性。然而喷射沉积坯料通常存在一定数量的孔隙,颗粒表面存在一定数量的氧化膜,颗粒之间未能完成良好的冶金结合状态。因些需要致密化和塑性变形才能获得理想的组织和性能。楔形压制是一种新型的压制工艺,即通过局部变形、多道次小变形累积实现大变形的致密化加工方法。本文在恒温楔压的基础上引入温度梯度,很好的解决了恒温楔压时高向变形不完全的问题,大大减化了工艺流程,本文主要对比研究了恒温楔压与梯温楔压两种加工工艺对SiC/7090铝基复合材料致密化的影响,并系统的研究了梯温楔压的致密化规律,通过对比实验发现梯温楔压很好地解决了恒温楔压时一次加工后致密化效果不佳的问题,这对大尺寸喷射沉积多孔材料的后续致密化和塑性变形的研究具有重要的指导意义,本文主要得出如下结论:
(1)通过模拟坯料的加热过程,得知对坯料加热时,当加热时间为90min时,坯料的温度梯度最为理想,呈现出136℃的温度梯度。通过实验中对坯料的加热过程得知,当加热为时间为90min时,坯料的正面及侧面分别呈现出180℃及250℃的温度梯度。
(2)梯温楔压时,坯料首先呈现出“双曲线”的形状,在中期与后期,坯料与模壁完全接触。恒温楔压时,坯料始终呈现出“倒梯形”的形状。梯温楔压初期上层与下层的金属流动优于中层的金属,中期与后期,金属的流动趋于均衡,最终整体基本达到致密化效果。恒温楔压时,坯料的上层金属首先发生流动,上层的流动速率大于中下层的金属,在整个压制过程中,中下层金属始终未与模壁接触。
(3)梯温楔压与恒温楔压时,最大高向压下量均为25%,在同一位置的取样点,梯温楔压的相对密度均高于恒温楔压的相对密度。梯温楔压与恒温楔压时都是上层的硬度值最大,最大值分别为117.3HB与118.3HB,梯温楔压时硬度值分布比较均匀,大部分区域的硬度值都大于100HB,而恒温楔压的硬度值分布不均匀,底层的最大硬度值仅为68.5HB。
(4)梯温楔压过程中,高向变形量为5%,15%以及25%时,在长度方向上,沿压头运动方向,坯料的相对密度逐渐增大。在宽度方向上,坯料中间的相对密度最小,坯料边缘的相对密度较大。
(5)梯温楔压过程中,当高向变形量为5%时,相对密度呈现出上层最大,下层次之,中间层最小的分布规律。当高向变形量为15%与25%时,相对密度呈现出上层最大,中间层次之,下层最小的分布规律。
(6)梯温楔压过程中,当高向变形量为25%时,试样的抗拉强度为304.52MPa,伸长率为2.4%,这些力学性能指标均优于此高向变形量时的恒温楔压坯料。温楔压后的断口上并无明显的大孔洞。
As an advanced materials preparation technology, spray deposition has shown remarkable superiority in producing large-sized alloys and metal matrix composites. However, the spray deposited preforms usually contain a certain quantity of porosity. Besides, the oxide film on the surface of the particles leads to weak metallurgical bonding between the particles. In order to obtain an ideal structure and performance, densification and plastically deformation are needed further. Wedge pressing is a new technology of pressing, in which the large deformation can be obtained through the sum-up of local deformation and multi-step small deformation. In this thesis, temperature gradient is introduced to constant temperature wedge pressing, which solves the problem of abundant deforming in short transverse through constant temperature wedge pressing. The process is simplified through the technology of temperature gradient wedge pressing. Effect of gradient temperature wedge pressing and constant temperature wedge pressing on densification of Spray deposited SiCp/7090 composite is compared. The regularity of densifying for gradient temperat- ure wedge pressing is also studied. It is found that the problem that the degree of densifying is inadequate by once pressing in constant temperature wedge pressing is solved through gradient temperature wedge pressing. The research is vital not only to the further densification but also to the further research on plastically deforming. The conclusion of this thesis can be concluded as following:
(1) By simulating the process of heating the billet, it shows that when the billet is heated for 90 minutes, the billet’s temperature gradient is 136℃,which is perfect. It can be seen in the experiment that when the billet is heated for 90 minutes, the billet’s temperature gradient is 180℃in the front face and is 250℃in the side face.
(2) In the process of gradient temperature wedge pressing, the shape of the billet is“hyperbolic”, and then the billet contacts the die wall absolutely. In the early stage of the process of gradient temperature wedge pressing, the flowing of the metal in the upper bed and the low bed of the billet is prior to the metal in the middle bed. In the intermediate stage and the anaphase the flowing of the metal balances, the whole of the billet is densified. In the process of constant temperature wedge pressing, the metal in the upper bed begins flowing firstly; the flow rate of the metal in the intermediate is greater than that in the anaphase. In the whole pressing process, the metal in the intermediate and the anaphase does not contact the die wall.
(3) The maximum reduction in height is 25% in the two pressing process. The relative density of the sample plot in the process of gradient temperature wedge pressing is higher than that of the process of the constant temperature wedge pressing. It is found that the hardness is 117.3HB on the upper bed in the process of gradient temperature wedge pressing, while it is 118.3HB in the process of constant temperature wedge pressing. The hardness of the billet in the process of gradient temperature wedge pressing distributes uniformly, most of the hardness is more than 100HB. The hardness of the billet in the process of constant temperature wedge pressing distributes non-uniformly. The greatest hardness in the anaphase is 68.5HB.
(4) In the process of gradient temperature wedge pressing, when the reduction of the billet in height is 5%, 15% and 25% respectively, the relative density becomes greater and greater along the pressure head moves in length. The relative density in the middle of the billet is minimum, the relative density in the edge is greater in width.
(5) In the process of gradient temperature wedge pressing, when the reduction of the billet in height is 5%, the regularity of relative density distribution is found that the relative density in upper layer is the greatest, and the relative density is the least in the intermediate layer.
(6) In the process of gradient temperature wedge pressing, when the reduction of the billet in height is 25%, the tensile strength is 304.52MPa and the elongation is 2.4%, the mechanical property is prior to that of constant gradient temperature wedge pressing. No evident pore can be seen in the fracture of the sample.
引文
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