铝硅合金凝固过程中的力学行为和热裂倾向性

利用自制的准固态力学性能测试装置,对铝硅合金的准固态力学性能进行了较为系统的测试,得出了反映不同含硅量铝硅合金的准固态力学行为参数(强度、断裂应变、准固态区间等),并分析了这些参数与热裂倾向性的关系。     

Hot Shortness of Al 4.5% Cu Alloy

Synopsis

The effects of the addition of iron, copper, silicon and titanium at two levels on the hot shortness of the aluminium 4.5% copper casting alloy was studied. The compositions used were within the limits of the British Standard Specification BSS L92. The hot shortness was measured using the ring casting die technique¹ and the effect of compositional changes determined using the analysis of variance. Two compositions of high and low level of hot shortness sensitivity were then retested for hot shortness in a die of another design which confirmed the results of the first series of tests. Thermal analysis, optical and scanning electron microscopy were used for examining the freezing behaviour and the microstructure of the two alloys of extreme cracking behaviour. Two arrest temperatures, 644°C and 544 or 550°C indicate freezing of the α solid solution and the Al-Cu eutectic respectively. The 544°C arrest occurred with the hot short alloy and that at 550°C with the less hot short. In addition, the less hot short alloy shows arrests at 612°C and 586°C. The constituents are iron rich phases which separate out at these latter temperatures, freezing before the eutectic, and forming a framework which isolates the eutectic into pockets increasing the ability of the test pieces to resist constraint stresses imposed by the testing die. Tensile tests on chill cast test bars show that the UTS of both alloys are above the specified minimum but the elongation of the less hot short alloy is lower than specified.     

Al-Mg-Si合金半固态触变成形研究

以水泵盖为目标零件,在自行建立的半固态触变成形试验线上使用A357合金和新开发的半固态专用铝合金Al-6Si-2Mg进行了半固态触变压铸试验研究.对这两种合金在半固态坯料制备、二次加热及半固态压铸中的显微组织及工艺性进行了比较.结果表明,Al-6Si-2Mg合金在触变成形过程中均表现出更好的工艺可控性,其半固态压铸件热处理后的性能为σb=335MPa,σs=305MPa,δ5=3%,强度高于A357,伸长率与铸态A357合金相当.试验最终获得了充型完好、性能优异、组织均匀的半固态压铸件.     

High-quality aluminum turbocharger impellers produced by thixocasting

The thixocasting process was chosen to produce the impellers as it is capable of producing castings with extremely high internal quality. Slugs cut from the DC cast bars are re-heated to the semi-solid casting temperature, and a specially-designed runner and gating system is used to prevent oxide from the surface of the slugs from becoming incorporated into the impeller. The technology used to produce the semi-solid impellers is described in detail. The semi-solid cast impellers, produced from an Al-Si-Mg-Cu alloy, are heat treated to the T6 temper. Results from testing are presented demonstrating that the impellers are free of porosity and other internal defects. Both mechanical property and fatigue data are presented showing that the semi-solid impellers have better properties than impellers produced by conventional casting and similar properties to forged and machined impellers. A short study is also described which identified suitable processing parameters to minimize hot tearing in the complex-shaped turbocharger impellers semi-solid cast from alloy 201. The surfaces of the impellers were examined using penetrant testing, and the results of modifying processing parameters on the propensity for surface cracking are presented. An aging study was performed to identify optimum mechanical strength.     

流变压铸成形镁合金AZ91D的显微组织与性能

采用双螺杆机械搅拌法制备半固态镁合金浆料,对比研究了液态压铸与半固态流变压铸成形镁合金AZ91D的组织与性能,探讨了半固态成形镁合金铸件的热处理强化机制.试验结果表明,流变压铸成形铸件与液态压铸成形铸件在铸态时的力学性能相当;由于半固态成形减轻了铸件内部气孔、偏析等铸造缺陷,因而T4和T6热处理均明显地提高了半固态成形镁合金铸件的力学性能.研究还得出,液态压铸成形镁合金铸件经过固溶处理之后,其抗拉强度和伸长率也有所提高,但是,时效处理明显地恶化了铸件的力学性能.     

ZL101过流冷却转移法半固态压铸工艺及性能研究

采用倾斜管过流冷却-转移法生产半固态流变压铸件,研究了压铸工艺对ZL101铝合金半固态流变压铸件性能的影响,以及半固态压铸件经T6热处理之后性能的改变.对比研究了液态与半固态压铸件的力学性能.结果表明,在浇注温度为595℃、压射速度为1.8 m/s时,压铸件性能最佳,此时较浇注温度为630℃的液态压铸件的抗拉强度提高了11％.经热处理的半固态压铸件抗拉强度与伸长率都得到改善.液态与半固态压铸件试样的拉伸断口为准解理断裂,经热处理的半固态压铸件试样的拉伸断口为韧性断裂.     

Experimental characterisation of a semi-solid A356 alloy during solidification and remelting

Abstract

Within the general framework of hot tearing research in casting processes the tensile behaviour of the semi-solid aluminium alloy A356 under remelting and solidification conditions is studied. In order to measure true-stress versus true-strain curves of the semi-solid metal, a new experimental technique has been developed to perform hot tensile tests in the mushy regime between the coherency and the solidus temperatures. The local true-strain in the semi-solid region of the tensile specimen is measured optically with a laser speckle extensometer (LSE). During the solidification tests measurement is done by using a transparent aero-gel mould. The resulting stress–strain curves and the corresponding microstructure are compared for the remelting and solidification condition.     

Strengthening technology and mechanism for semi-solid die casting of aluminum alloy

Combined with theoretical evaluation,an optimized strengthening process for the semi-solid die castings of A356 aluminum alloy was obtained by studying the mechanical properties of castings solution treated and aged under different conditions in detail,then,the semi-solid die castings and liquid die castings were heat treated with the optimized process.The results show that the mechanical properties of semi-solid die castings of aluminum alloy are superior to those of the liquid die castings,especially the strengthening degree of heat treated semi-solid die castingsis much greater than that of liquid die castings with the tensile strength more than 330 MPa and the elongation more than 10％,and this is mainly contributed to the non-dendritic and more compact microstructure of semi-solid die castings.The strengthening mechanism of heat treatment for the semi-solid die castings of A356 aluminum alloy is due to the dispersive precipitation of the second phase(Mg2 Si) and formation of GP Zone.     

Design and evaluation of an enhanced constrained rod casting mould for experimental characterisation of hot tearing in aluminium casting alloys

The most popular mould configuration for hot tearing evaluation of aluminium alloys, the Constrained Rod Casting (CRC) design, was evaluated via computer modelling. Filling patterns, heat flows, and shrinkage areas were assessed using the existing published design, and improvements in the design were generated. A mould of the improved design or Enhanced Constrained Rod Casting (ECRC) was fabricated and successfully used to evaluate the hot tearing tendencies of A206.2 and A380 die casting alloys. The ECRC mould reduced fill time and increased uniform filling of the constrained rods, which resulted in more uniform solidification rates for all the constrained rods. The ECRC mould and novel measurement technique predicted hot tearing in real time of A206.2 and A380 die casting alloys at initial pouring temperatures of 700, 760, and 800 °C. The A206.2 alloy exhibited hot tearing characteristics at the measurement rod while A380 did not show any hot cracking defects.     

半固态加工Al-Zn-Mg合金的组织演化

选用SIMA法制备的LC4合金及液相线凝固法制备的LC9合金为原料，研究了半固态加工Al-Zn-Mg合金的组织演化。由于这两种方法制备半固态坯料的机理完全不同，因此这两种材料加工后的组织演化规律截然不同：LC4合金的晶粒尺寸由于聚结而长大，而LC9合金的显微组织没有明显的变化。借助于扫描电镜和能谱分析，对半固态加工前后的显微组织进行成分分析，结果表明晶界处由共晶区、低熔点的富Cu区和杂质区三部分组成，且LC9合金的再加热组织的液相薄膜厚度大于CL4合金的；半固态加工后制件各部分的成分基本一致，证明半固态加工形成过程中材料的流动方式为层流。     

Cu 含量对Ti-Cu合金半固态可锻性及锻造后力学性能的影响

通过镦锻试验和模锻实验研究了Ti-Cu系合金半固态锻造行为,并对锻材进行了拉伸试验,讨论了Cu含量对半固态可锻性及力学性能的影响。结果表明：1000 °C至1150°C半固态锻造较常规锻造具有较小的顶锻压力;其中,1000 °C至1050°C间半固态锻造的Ti-Cu系合金均表现出较好的可锻性,在75%的锻造变形量下无明显缺陷。分析认为,Ti-Cu系列合金中含有较多的低熔点Ti2Cu相,随着半固态温度升高或Cu含量的增加,材料中的液相含量增加,增加的液相含量对变形起到润滑作用,减少了固相变形引起的应力集中,有效的降低了变形抗力,改善了成形性。力学性能研究表明：半固态锻造Ti-Cu系合金较常规锻造合金强度升高,塑性降低。随着Cu含量的升高,合金的强度明显提升,塑性降低。分析认为：力学性能的变化主要是由于Ti2Cu相析出含量、形态和分布相关,随着Cu含量和半固态温度的升高,更多Ti2Cu相在晶内和晶界析出,引起析出强化作用,同时,晶界析出的针状Ti2Cu相形成了偏析带,降低了合金塑形。     

SSTT和RAP方法得到的ZK60镁合金的组织变化和力学性能

研究了分别由半固态热成形方法（SSTT）和部分重熔再结晶方法（RAP）得到的 ZK60镁合金在一定温度下显微组织随等温时间的变化和触变成形试样的力学性能。结果表明,合并长大机制在SSTT合金的组织变化中占主导地位,熟化机制在RAP 合金的组织变化中起主要作用。在相同的等温条件下,与SSTT方法相比,RAP方法可以得到更细小的半固态显微组织,RAP合金组织比SSTT合金组织更圆整。由SSTT方法和RAP方法得到的ZK60镁合金触变成形后均得到了较理想的成形件,触变成形工艺提高了材料的力学性能。与 SSTT 合金相比, RAP合金具有更优越的力学性能。     

Effect of external forces on microstructural evolution and mechanical properties of high pressure die cast AA5754 alloy

The effects of ultrasonic treatment (UT) and electromagnetic stirring (EMS) forces, individually and in combination, prior to high pressure die-casting of AA5754 alloy were assessed. The liquid of alloy was subjected to UT, EMS and the combined style and then transferred to the shot chamber. The grain size, mechanical properties, gas content and hot tearing susceptibility were analyzed. The results suggest that the application of each process enhances grain refinement and decreases the porosity of the specimens. UT reduces the grain size from 140 to 82 μm and decreases the porosity from 5.5% to 1.4%. EMS reduces the grain size to 107 μm and the porosity to 3.3%. The combination of UT and EMS decreases the grain size and the porosity to 65 μm and 1.1%, respectively, which are the lowest grain size and porosity ever reported for this alloy achieved via physical processing. Intensive cavitation and stirring not only resulted in a refined microstructure but also significantly decreased the hot tearing susceptibility and improved the mechanical properties. All of the aforementioned characteristics are due to cavity formation, during each process. By combination of techniques, EMS could promote the nucleation process by providing more exogenous particles in the strong cavitation field of UT. Ultrasonic plays a major role in grain refining, decreasing the hot tearing susceptibility and the gas content by introducing a strong cavitation field.     

水轮机下环铸件凝固过程热应力模拟分析

采用数值模拟方法对水轮机下环铸件凝固过程中的应力场进行了模拟和分析。模拟中采用接触单元法处理铸件与砂芯之间力的作用,改进了传统的铸件/铸型边界条件处理方法。文中重点分析了下环凝固过程中准固相区的受力情况,根据准固相区受力特点建立了适当的热裂判据,对下环凝固过程热裂倾向性进行预测。实际生产结果与预测结果相符合。     

Rendering wrought aluminium alloys castable by means of minimum composition adjustments

Wrought alloys have low fluidity and are prone to hot tearing, which make them difficult to cast. The presence of eutectic-forming elements in the alloy composition lessens these effects. For this reason, the constituents of casting alloys tend to include a eutectic portion. Typically, silicon is added to aluminium alloys to provide casting ability by forming the aluminium–silicon eutectic. However, the presence of silicon in aluminium alloys is associated with a number of issues that do not allow these alloys to reach their full potential. In this publication we report results of our investigation of three alternative eutectics: Al–6.1Ni, Al–1.8Fe, and Al–1.75Fe–1.25Ni. Our results indicate that these eutectics have satisfactory fluidity and resistance to hot tearing and higher strength than the aluminium–silicon eutectic. We also found that introducing these eutectic compositions into 7075 wrought alloy results in a castable composition with yield strength comparable to that of the wrought alloy.     

Effect of alloying elements on structure and mechanical properties of semi-solid processed cast iron

Semi-solid metal processing of alloys is one of the key technologies for producing advanced materials. Through semi-solid processing, it is possible to produce high quality cast components from grey cast iron. A series of experiments were carried out to clarify the effect of the alloying elements copper, chromium, molybdenum, and nickel on the properties of cast iron. A comparison was made of the microstructure and mechanical properties in semi-solid processed cast iron and ordinary cast iron. This showed that an increase in the level of alloying elements in cast iron gave a higher level of hardness. However, the tensile strength of alloyed semi-solid cast iron did not exceed that of grey iron, for every composition, as a result of the characteristic microstructure produced by semi-solid processing. Thus, the alloying elements had little effect on the tensile strength of semi-solid cast iron, but did have an effect on elongation. The tensile strength of semi-solid cast iron, however, still depends on the cooling rate.     

齿轮半固态成形工艺研究

论述了用ZA27合金采用半固态模锻工艺生产齿轮的实验研究,对工艺参数进行了选择,分析了半固态工艺模锻圆柱齿轮成形性能,并对不同工艺生产的齿轮力学性能进行了比较。结果表明:半固态模锻工艺生产的齿轮不仅形状精度高,而且力学性能明显高于其它两种工艺的制件。     

Evolution of microstructure in semi-solid slurries of rheocast aluminum alloy

Semi-solid metal processing is being developed in die casting applications to give several cost benefits. To efficiently apply this emerging technology, it is important to understand the evolution of microstructure in semi-solid slurries for the control of the rheological behavior in semi-solid state. An experimental apparatus was developed which can capture the grain structure at different times at early stages to understand how the semi-solid structure evolves. In this technique, semi-solid slurry was produced by injecting fine gas bubbles into the melt through a graphite diffuser during solidification. Then, a copper quenching mold was used to draw some semi-solid slurry into a thin channel. The semi-solid slurry was then rapidly frozen in the channel giving the microstructure of the slurry at the desired time. Samples of semi-solid 356 aluminum alloy were taken at different gas injection times of 1, 5, 10, 15, 20, 30, 35, 40, and 45 s. Analysis of the microstructure suggests that the fragmentation by remelting mechanism should be responsible for the formation of globular structure in this rheocasting process.     

自蔓延技术在模具上的应用研究

论述了黑色金属半固态加工用模具的现状和应用陶瓷存在的问题;介绍了自蔓延高温合成技术的特点;应用自蔓延技术制备了带有陶瓷内衬涂层的模具,并应用该模具进行了简单的半固态铝合金加工实验。实验结果表明:应用自蔓延技术制备陶瓷模具的方法是可行的,而且适合金属半固态加工的应用。     

Design of a new hot tearing test apparatus and modification of its operation

This study introduces a new apparatus for quantitative assessment of hot tearing tendency in casting alloys. In this method, molten metal is cast in a T-shaped mold cavity. Each arm of the T-shaped casting is hooked onto a rigid flask which restrains its free contraction during solidification. One of the hooks connects to a load cell which enables the real-time measurement of the contraction load during the solidification process as well as the load-time curve of each experiment. Temperature-time curves are also plotted using K-type thermocouples placed in the casting hot spot and connected to a data-acquisition system. Through the use of this set up, it is possible to estimate the solid fraction at which hot tearing occurs. Experiments were conducted with Al-9 %Si alloy to investigate the accuracy of the apparatus and modify its operation. The influence of the mold thermal gradient on the load-time curve and the hot tearing severity were investigated. Microscopic study of the observed hot tear regions was also performed to characterize the hot tearing characteristics.