细化剂对ZA27合金触变组织的影响

采用半固态等温处理法研究了细化剂Zr加入量对ZA2 7合金在半固态等温热处理中组织的影响。结果表明 ,只要达到0 0 4%Zr时 ,就能获得触变成形组织 ,随Zr量的增加 ,初生相由发达的树枝晶逐渐演变为细化的短轴晶和等轴晶 ,但Zr含量不宜超过 0 2 % ;在半固态等温处理后发现初生相细化得愈好 ,所得到的组织粒度愈小 ,圆整度愈好。

SIMA法制备7A04合金半固态触变组织的演化

研究了二次加热工艺参数对SIMA法制备7A04合金半固态触变组织的演化规律。结果表明:随着加热温度的升高和保温时间的延长,晶粒的形状逐渐趋于球形。当加热温度低于610℃时,晶粒尺寸不断长大,而后逐渐减小。在保温的初始阶段,共晶液相没有完全浸润晶粒边界,晶粒的聚结长大处于主导地位。当保温时间达到4 min左右时,共晶液相几乎充满了所有的晶界,Ostwald长大扩散机理处于主导地位。晶粒长大规律符合动力学方程。

AZ91D镁合金触变铸造件热处理前后组织与性能研究

采用低过热铸造技术与半固态触变成形技术相结合,直接在高压铸造机中成功制备了摩托车发动机支架镁合金件。试验结果表明:半固态成形件试样与热处理试样中的未熔α-Mg相细小、圆整而且分布均匀,微观组织没有气孔和缩松;与没有热处理的半固态压铸成形件相比,固溶态试样的硬度和抗拉强度分别降低了4.5%和3.1%,而固溶时效态试样的硬度和抗拉强度分别提高了30.1%和14.0%,两种热处理工艺均使得材料的伸长率分别比没有热处理的试样提高17.6%和40.5%。

Microstructure evolution and segregation behavior of thixoformed Al-Cu-Mg-Mn alloy

A commercial wrought Al-Cu-Mg-Mn alloy（2024） was thixoformed based on the recrystallization and partial melting（RAP） route, and the microstructure evolution and segregation behavior during the indirect thixoforming process were studied. The results show that fine spheroidal microstructures can be obtained by partial remelting of commercial extruded 2024 alloys without additional thermomechanical processing. During the indirect thixoforming, the stress distribution can be optimized by increasing the thickness of base region. Under three-dimensional compression stress state, the microstructures are homogeneous among different regions with no evidence of liquid segregation and micro-porosities, and the grains in the columns are deformed plastically. The distribution of tensile mechanical properties is consistent with the microstructures. Moreover, the distribution of deformation mechanism was discussed, and a technical method for improving the stress distribution was proposed.

半固态材料制备的研究现状和发展趋势

半固态加工的优越性完全依赖于半固态材料所需具备的特殊组织结构,即细小球状固态颗粒均匀悬浮于液态基体中而形成液固共存的半固态组织.对现有的半固态材料的制备工艺进行了总结,主要有机械搅拌法,电磁搅拌法,促进凝固过程形核,外加晶粒细化剂,SIMA工艺和雾化成型工艺.还结合半固态加工的工艺路径:流变成形和触变成形,提出上述半固态材料制备方法的发展趋势.

Microstructural evolution during solution treatment of thixoformed AM60B Mg alloy

The microstructural evolution and kinetic characteristics were studied during solution treatment of AM60B Mg alloy prepared by thixoforming. The results indicate that the microstructural evolution includes two stages： the first stage involves rapid dissolution of eutectic β （Mg 17 Al 12 ） phase, homogenization and coarsening, and the second stage is regarded as normal grain growth consisting of primary α-Mg particles （primary particles） and secondary α-Mg grains （secondary grains）. In the first stage, the dissolution completes in a quite short time because the fine β phase can quickly dissolve into the small-sized secondary grains. The homogenization of Al element needs relatively long time. Simultaneously, the microstructure morphology and average grain size obviously change. The first stage sustains approximately 1 h when it is solutionized at 395 ℃ Comparatively, the second stage needs very long time and the microstructure evolves quite slowly as a result of low Al content gradient and thus low diffusivity of Al element after the homogenization of the first stage. The growth model of primary particles obeys power function while that of the secondary grains follows the traditional growth equation in the first stage. In the second stage, both of the primary particles and secondary grains behave a same model controlled by diffusion along grain boundaries and through crystal lattice.

Microstructural evolution and phase transformation during partial remelting of in-situ Mg_2Si_p/AM60B composite

The microstructural evolution and phase transformations during partial remelting of in-situ Mg2Sip/AM60B composite modified by SiC and Sr were investigated. The results indicate that SiC and Sr are effective for refining primary α-Mg grains and Mg2Si particles. After being partially remelted, a semisolid microstructure with small and spheroidal primary α-Mg particles can be obtained. The microstructural evolution during partial remelting can be divided into four stages： the initial rapid coarsening, structural separation, spheroidization and final coarsening, which are essentially caused by the phase transformations of β→α, α＋β→L and α→L, α→L, and α→L and L→α, respectively. The Mg2Si particles have not obvious effect on the general microstructural evolution steps, but can slower the evolution progress and change the coarsening mechanism. During partial remelting, Mg2Si particles first become blunt and then become spheroidal because of melting of their edges and corners, and finally are coarsened owing to Ostwald ripening.

Effect of ball milling on microstructural evolution during partial remelting of6061 aluminum alloy prepared by cold-pressing of alloy powders

The effect of ball milling on the microstructural evolution was investigated during partial remelting of 6061 aluminum alloy prepared by cold-pressing of atomized alloy powders.The results indicate that the microstructural evolution of 6061 aluminum alloy can be divided into three stages,the dissolution of eutectic phases and the coarsening and growth behavior of the resulting grains,structural separation and spheroidization of primary particles,and the final coarsening behavior of the particles.Compared with the alloy without ball milling,ball milling accelerates the first stage of microstructural evolution due to the energy stored in the powders,but the latter two stages are slowed down because of the formation of large-sized powders.Moreover,the finer the as-cold-pressed microstructure is,the smaller and more spherical the primary particles in the final semisolid microstructure are.Furthermore,properly elevating the heating temperature is beneficial for obtaining small and spheroidal particles.

Microstructure and high temperature tensile properties of Al-Si-Cu-Mn-Fe alloys prepared by semi-solid thixoforming

The differences in the microstructure and elevated temperature tensile properties of gravity die cast,squeeze cast,and semi-solid thixoformed Al-Si-Cu-Mn-Fe alloys after thermal exposure at 300℃were discussed.The results demonstrate that the elevated temperature tensile properties of semi-solid thixoformed alloys were significantly higher than those of gravity die cast and squeeze cast alloys,especially after thermal exposure for 100 h.The ultimate tensile strength(UTS)of semi-solid thixoformed alloys after thermal exposure at 300℃for 0.5,10 and 100 h were 181,122 and 110 MPa,respectively.The UTS values of semi-solid thixoformed alloys were higher than those of heat resistant aluminum alloys used in commercial applications.The enhanced elevated temperature tensile properties of semi-solid thixoformed experimental alloys after thermal exposure can be attributed to the combined reinforcement of precipitation strengthening and grain boundary strengthening due to thermally stable intermetallic phases as well as suitable grain size.

Corrosion behaviour of thixoformed and heat-treated ZA27 alloys in NaCl solution

The influence of corrosion on the microstructure of thixoformed and heat-treated ZA27 alloys was investigated. The microstructure of ZA27 alloy was affected by heat treatment. The process of electrochemical corrosion occurs in both ZA27 alloys through the area of r/phase. According to the results of immersion test and electrochemical measurements, the corrosion rate of the thixoformed ZA27 alloy is at least 50% lower than that of the thixoformed and thermally processed alloy. This indicates the unfavourable influence of applied heat treatment （T4 regime） on the corrosion resistance of the thixoformed ZA27 alloy.

Microstructure and mechanical behavior of Ti−Cu alloys produced by semisolid processing

The mechanical properties of Ti−Cu alloys processed via thixoforming were evaluated.Ti−Cu(25,27,and 29 wt.%Cu)ingots were produced via arc melting,homogenization at 950℃ for 24 h,and hot-forging at 900℃,followed by thixoforming at a speed of 8 mm/s after isothermal heat treatment at 1035℃ for 300 s.The thixoformed alloys exhibited good mechanical strength,limited plasticity under tensile loading,and reasonable plasticity under compressive loading.The mechanical strength and plasticity decreased as the Cu content increased as a result of the increasing volume fraction of the peritectic Ti_(2)Cu phase(transformed liquid),which exhibited a lower strength and plasticity than theα+Ti_(2)Cu regions(transformed solid).These findings indicated that the trade-off between the mechanical properties and semisolid processability is largely governed by the Cu content.