【作者】 张琪；

【导师】 马宗义； 肖伯律；

【作者基本信息】 中国科学技术大学 ， 材料加工工程， 2013， 博士

【摘要】 搅拌摩擦加工(Friction stir processing, FSP)是近年来发展起来的一种新型材料塑性加工制备技术,目前已在超细晶材料的制备、材料的微观组织改性以及金属基复合材料的合成制备等方面得到了应用。本文选择Al-Ti和Al-TiO2两个铝基复合材料常用的反应体系为研究对象,通过真空热压和FSP制备原位复合材料,系统研究了FSP激活原位反应的本质原因和FSP过程中增强相的形成机制及制备工艺参数对原位复合材料组织和性能的影响。主要开展了以下方面的研究工作：研究了真空热压和FSP参数对Al-Ti系原位复合材料微观组织和力学性能的影响。在真空热压过程中,Ti与A1之间发生反应扩散生成Al3Ti0随着热压温度提高和热压保温时间的延长,生成的Al3Ti数量增多。随后的FSP不仅能够激活Al-Ti反应,而且细化了热压过程中形成的Al3Ti颗粒,导致Al3Ti在基体中的均匀分布。这些微观组织的变化导致了原位复合材料的拉伸强度显著提高。但拉伸强度随FSP转速的变化趋势却依赖于热压过程中的Al-Ti反应程度：对于热压中Al-Ti反应未发生或部分发生的样品,拉伸强度随着FSP转速的提高而提高；而对于热压中Al-Ti反应完全发生的样品,拉伸强度随着FSP转速的提高而降低。定量研究了退火和FSP过程中Al-Ti的反应动力学。退火和FSP过程中的Al-Ti反应均可用经验公式VAl3Ti=ktn来描述。Al-Ti反应在525℃和550℃为扩散控制,在575-650℃则为界面反应控制。在转速为1000和2000rpm的FSP过程中的Al-Ti反应均为界面反应控制。相对于退火过程,FSP过程中Al-Ti反应速率常数显著增加,这主要是由于FSP过程中剧烈塑性变形降低了反应激活能所致。有效温度可以用来表征FSP对机械活化的贡献,工具转速为1000和2000rpm的FSP过程中反应体系的有效温度分别为710和716℃。研究了Cu或Mg添加对Al-Ti体系制备复合材料的微观组织和力学性能的影响。在Al-Ti体系中添加少量的Cu导致FSP过程中出现了少量的共晶液相,因此强化了Al-Ti反应,生成了更多的Al3Ti颗粒。同时Cu与基体A1生成Al2Cu, Al2Cu在FSP过程中经历了溶解-再析出过程,而部分Cu则以固溶原子的形式保留在基体中。在Al-Ti体系中添加少量的Mg可以导致基体中生成具有细小孪晶片层的Ti2Mg3Al18颗粒,因而导致FSP样品中增强相体积分数的增加。Cu和Mg的添加引入了更多的强化方式和增强相含量,因此导致原位复合材料室温强度的大幅度提高,但却严重降低了材料的塑性。采用搭接率为50%的FSP搭接加工制备的大面积原位复合材料组织相对均匀,各道次之间并无明显的过渡区,搭接加工过程中Al-Ti原位反应程度与原位4道次基本相同。FSP搭接Al-Ti样品加工区横截面硬度分布均匀,拉伸性能也与原位4道次样品接近。FSP搭接加工Al-Ti-Cu样品的硬度随着与第一道次距离的增加而逐渐增加,拉伸性能也低于原位4道次的样品,这主要由于距离第1道次加工区较近的材料经历了较多次数的热循环,导致基体中Al2Cu数量增加,尺寸增大造成的。经过固溶处理后,搭接加工Al-Ti-Cu样品的硬度分布变得均匀,拉伸强度大幅度提高。研究了FSP过程中Al-TiO2系原位复合材料的反应机制和增强相形成机制以及所制备原位复合材料的强化机制。FSP激活Al-TiO2反应可归因于FSP过程中剧烈的塑性变形导致的元素扩散加速以及机械活化作用。FSP过程中Al2O3和Al3Ti的形成机制分别为变形协助的界面反应和变形协助的溶解-析出机制。FSP复合材料的强化机制包括：载荷传递、晶粒细化强化和Orowan强化,其中Orowan强化对FSP复合材料的屈服强度贡献最大。系统研究了热压和FSP参数对Al-TiO2系原位复合材料组织和性能的影响。在温度为500℃、保温时间为5min的热压过程中,TiO2和Al未发生反应。在热压温度为550℃、保温时间为50min的热压过程中,TiO2和Al发生部分反应。在热压温度为630℃、保温时间为240min的热压过程中,TiO2和Al完全反应,生成粗大的、多边形的Al3Ti和150nm左右的Al2O3。对于热压过程中未反应和部分反应的样品,随后的FSP激活了Al-TiO2反应,生成了尺寸约为80nm的Al3Ti和A1203颗粒,拉伸性能相对于FSP纯铝也有较大的提高。对于热压中完全反应的样品,FSP将多边形的Al3Ti变为椭圆形,并使A1203和Al3Ti均匀分布于基体中。由于基体中的Al3Ti相对粗大,导致拉伸性能低于热压过程中未反应和部分反应的FSP样品。对于热压过程中未反应的样品,增强相含量随FSP行进速度的降低而增加,但晶粒尺寸受行进速度影响较小,FSP样品的强度也随行进速度的降低而增加。当行进速度为25mm/min时,工具转速由1000rpm增加至2000rpm对FSP样品的微观组织影响和力学性能影响较小。水下两道次FSP对样品中增强相尺寸和分布影响较小,但细化了基体晶粒,因而导致样品强度的增加和延伸率的降低。更多还原

【Abstract】 Friction stir processing (FSP) is a new solid state processing technique for plastic processing and fabrication of metallic materials, and has found several applications in fabrication of ultra-fine grained materials, microstructural modification and in situ synthesis of intermetallic compounds and metal matrix composites. In the present study, in situ aluminum matrix composites (AMCs) were fabricated via vacuum hot pressing (VHP) and subsequent FSP in Al-Ti and Al-TiO2systems. The reasons why in situ reaction took place during FSP and the formation mechanism of in situ reinforcing particles during FSP were studied. The effects of processing parameters on the microstructures and properties of in situ AMCs were also investigated.For in situ composites based on the Al-Ti system, the formation mechanism of Al3Ti and the effects of VHP and FSP parameters on the resultant microstructure and mechanical properties were investigated. Al3Ti formed due to reactive diffusion between Al and Ti during VHP, and the number of Al3Ti increased with increasing the temperature and holding time of the VHP. FSP not only induced the Al-Ti reaction, but also resulted in a significant refinement of Al3Ti, thereby creating a homogeneous distribution of Al3Ti particles in the Al matrix. These microstructural changes led to a significant improvement in the tensile properties of the in situ Al3Ti/Al composite. However, the change trends of the tensile properties of the FSP samples were dependent on the extent of the Al-Ti reaction during VHP. For the samples in which the Al-Ti reaction did not take place or take place partly during VHP, the tensile strength increased with the increase of tool rotation rate, while for the sample in which the Al-Ti reaction took place completely, the tensile strength decreased with the increase of tool rotation rate.The kinetics of the Al-Ti reaction during isothermal annealing and FSP was studied. The Al-Ti reaction during isothermal annealing and FSP can be described by an empirical relationship:VAl3Ti=ktn. The reaction was diffusion-controlled at525and550℃, and interfacial-reaction-controlled at575-650℃. The Al-Ti reaction during FSP at rotation rates of1000and2000rpm was interfacial-reaction-controlled. The reaction rate constant k during FSP was substantially enhanced compared to that during annealing, which can be attributed to the decrease in activation energy caused by severe deformation during FSP. The effective temperature was proposed to show the contribution of the mechanically activated effect of FSP, and was calculated to be 710and716℃for FSP with rotation rates of1000and2000rpm.The effects of Cu or Mg addition on the microstructure and mechanical properties of in situ composites based on the Al-Ti system were studied. Cu addition in the Al-Ti system enhanced the Al-Ti reaction and resulted in the formation of more Al3Ti particles due to the presence of a small amount of liquid phase during FSP. Al2Cu experienced dissolution and precipitation during FSP, and part of Cu was kept in the Al matrix as solute after FSP. Mg addition in the Al-Ti system led to the formations of some Ti2Mg3Al18particles with fine twin lamellas during FSP, resulting in an increase in the total volume fraction of reinforcing particles. Cu and Mg addition increased the strength of the in situ composites substantially due to introduction of more strengthening modes or more reinforcing particles, however the elongation decreased dramatically.Large-area bulk in situ AMCs based on the Al-Ti and Al-Ti-Cu systems were fabricated by overlapped FSP with an overlapping ratio of50%. The extent of Al-Ti reaction during overlapped FSP was similar with that in in situ4pass FSP. The hardness distribution in the cross section of overlapped FSP Al-Ti sample was relatively homogeneous, and the tensile properties were also similar with those of the Al-Ti sample subjected to in situ4pass FSP. The hardness of overlapped FSP Al-Ti-Cu samples increased with the increase of the distance from the first pass, and the tensile strength was lower than that of in situ4pass FSP Al-Ti-Cu sample. This was caused by the increase and coarsening of Al2CU in the matrix close to the first pass due to more thermal cycles experienced. For overlapped FSP Al-Ti-Cu sample, the hardness distribution became homogeneous and tensile properties increased after solid solution treatment.The reactive mechanism of Al-TiO2reaction and the strengthening mechanism of in situ composites based on the Al-TiO2system were investigated. The occurrence of the Al-TiO2reaction during FSP can be attributed to the enhanced solid diffusion and mechanical activation effect caused by the severe deformation of FSP. The formation mechanisms of Al2O3and Al3Ti during FSP are determined to be a deformation-assisted interfacial reaction and deformation-assisted solution-precipitation. The strengthening mechanisms of the FSP in situ composites included load transferring, grain refinement and Orowan strengthening, among which Orowan strengthening contributed the most to the yield strength of the composites.The effects of VHP and FSP parameters on the microstructure and mechanical properties of in situ composites based on the Al-TiO2system were studied. The Al-TiO2reaction hardly took place during VHP at500℃with a holding time of5min. And the Al-TiO2reaction took place partly during VHP at550℃with a holding time of50min. All the TiO2reacted with Al to form coarse and polygonal Al3Ti particles and Al2O3particles with a size of150nm during VHP at630℃with a holding time of240min. For samples in which the Al-TiO2reaction took place hardly or partly, subsequent FSP induced the Al-TiO2reaction, resulting in the formation of Al3Ti and Al2O3with a size of about80nm. The tensile properties of the FSP in situ composites increased substantially compared with those of FSP pure Al. For sample in which the Al-TiO2reaction took place completely during VHP, subsequent FSP blunted the polygonal Al3Ti particles and distributed the Al2O3homogeneously in the matrix. However, the tensile strength of the FSP sample in which the Al-TiO2reaction took place completely during VHP was lower than that of FSP samples in which Al-TiO2reaction took place hardly or partly during VHP.For FSP sample in which the Al-TiO2reaction did not take place during VHP, the volume fraction of reinforcing particles increased with decreasing the traverse speed, however, the matrix grain size was not influenced by the traverse speed of FSP. Thus the strength of FSP samples increased as the traverse speed of FSP decreased. When the traverse speed was25mm/min, increasing tool rotation rate from1000to2000rpm caused little influence on the microstructure and mechanical properties of the in situ composites. Additional2pass FSP in water did not change the size and distribution of reinforcing particles, but refined the matrix grains, resulting in the increase in the tensile strength and the decrease in the elongation of the in situ composite.更多还原