Thermomechanical treatment of A380 aluminum alloy for globular thixo feedstock

The main aim of this paper is to present and evaluate thermomechanical treatment for semi-solid feedstock production of A380 (AlSi₉Cu₃(Fe)) aluminum alloy. Equal channel angular pressing (ECAP) is used to heavily deform and refine as cast dendritic microstructure with needle like eutectic appearance. The deformed microstructure is easily changed to globular microstructure by recovery and recrystallization mechanisms during heating at the semi-solid temperature range which is a crucial factor for quality semi-solid feedstock preparation. The microstructural evolutions investigations during semi-solid heating at different holding times is carried out utilizing optical microscope and scanning electron microscope accompanied by energy dispersive x-ray spectroscopy. Homogeneous distribution of the silicon particles and intermetallic compounds around globular primary aluminum phase is observed. The used approach is appropriate and seems highly promising for preparation of the quality semi-solid feedstock.     

The erosion of H21 tool steel in molten A380 alloy

The chemical interaction between H21 tool steel and molten A380 alloy was investigated at 700°C under dynamic conditions. Cr₂O₃-coated H21 samples were also investigated under the same condition for comparison. Samples were rotated at 300 rpm. The test time was varied between 1 and 36 hours. The reaction zone was characterised by SEM and EDS analysis. It was found that, from A380 alloy to H21 steel, FeAl₃, Fe₂Al₅ and FeAl₂ intermetallic compounds formed in sequence. Major alloying elements from both H21 steel and A380 alloy were present in those intermetallic compounds. FeAl₃ compound was porous, while Fe₂Al₅ and FeAl₂ compounds were compact. The thickness of Fe₂Al₅ layer kept constant in the range of 7–10 m during the tests, while the thickness of FeAl₃ and FeAl₂ compounds increased with increasing test time. Under the dynamic conditions, the high erosion rate of H21 sample in molten A380 alloy may be attributed to dissolution, swift melt agitation and poor protective effect of the reaction zone. It was also found that although Cr₂O₃-coating could protect H21 steel to some extent from erosion in molten A380 alloy, it can only act as a diffusion barrier because the coating itself was attacked by molten Al-alloy through chemical dissolution.     

The role of yttrium modifying A357 alloy with sand casting

The microstructure of A357 alloy modified by yttrium (Y) and its modification mechanism was investigated. The results show that under a sand casting condition, there was small or even no solution strengthening effect of Y on the A357 alloy. Y did not change the morphology of eutectic silicon but made it shorter, and Y increased the nucleation cores during its nucleation processes rather than promoting the formation of twinning during the growth processes. The solidification mode of many eutectic silicon phases changed from nucleating on the dendrites to heterogeneous nucleation of eutectic grains in the interdendritic liquid. The number and area of eutectic silicon clusters increased with increasing Y. A bright Al₂Si₂Y phase appeared even though Y was very low.     

Improved properties of A319 aluminum casting alloy modified with Zr

The beneficial effects of 0.15 wt.% Zr addition on mechanical properties and wear resistance of A319 aluminum casting alloy were investigated. The cast alloys were given a solutionizing treatment followed by artificial aging in the temperature range 175 to 235 °C for different period of times. Mechanical properties and wear behavior of the Zr-containing material were determined and compared to those of the base A319 alloy in both as-cast and age-hardened conditions. It is shown that minor addition of Zr results in the precipitation of Al₃Zr particles in the aluminum matrix. These particles are stable upon heating due to the low solubility of zirconium in aluminum matrix. The main effects of such particles are an increase in hardness, strength, quality index and wear resistance. This is very promising where these aluminum cast alloys are to be used at relatively high temperatures.     

Silicon segregation in aluminium casting alloy

Abstract

The objective of the present study was to find an explanation for the observation made by earlier researchers that the distribution of silicon across dendrite branches in an aluminium–silicon casting alloy is sometimes anomalous, in that the concentration gradient is in the opposite direction to that predicted by solidification theory. Small specimens of aluminium alloy A356 were solidified to give a similar dendritic microstructure. One specimen was quenched from a temperature just above the eutectic temperature, giving the silicon distribution expected from theory, while a second specimen was cooled more slowly to give the anomalous silicon distribution, suggesting that it is caused by something occurring in the solid state. It was noted that there is a rapid decrease in solubility of silicon in aluminium with decreasing temperature below the aluminium–silicon eutectic temperature, so that a substantial amount of silicon is expected to come out of solution at temperatures above 500°C on cooling. An estimate of the diffusion rate of silicon in aluminium showed that, for normal cooling rates, this can occur by diffusion of silicon to the interdendritic silicon particles formed during the final stages of freezing, thereby removing silicon directionally from the dendrite branches and producing the observed anomalous silicon distribution.     

铸造铝铜合金的冲击韧性

研究了铝铜系铸造合金的冲击韧性与显微结构和拉伸性能之间的关系 .结果表明 ,显微结构对冲击韧性和拉伸性能有着相似的影响规律 ,即晶粒尺寸越小 ,冲击韧性值越大 .在本文实验条件下 ,当强度比 (σ0 .2 /σb)小于 0 .5 5 ,延伸率大于 1 5 %时 ,合金表现出高的冲击韧性 .     

Skin layer of A380 aluminium alloy die castings and its blistering during solution treatment

Skin layer is a characteristic microstructure of aluminium die castings, which would effect the surface blistering during solution treatment. In this study, the microstructures of skin layer were investigated by the methods of optical microscope (OM), scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA). High resolution X-ray CT was used to compare the skin layer with normal surface before and after solution treatment. With the aid of computational fluid dynamics (CFD), the formation mechanism of the skin layer was discussed based on microstructure distribution, solute segregation, porosity distribution and surface blistering. The results suggested that the skin layer is related to a succession of complex processes before the filling process finished. Pore clusters or laminar defects would be formed in skin layers during solution treatment and cause severe surface blistering.     

用喷射铸造法制备CuCr25电触头合金

用喷射铸造法制备了CuCr25触头合金,研究了工艺参数(雾化压力和沉积距离)对合金组织形态和性能的影响.结果表明:采用优化的工艺参数和锻造过程制备出的CuCr25合金致密、组织良好,合金中Cr粒子平均直径为5～10μm,且均匀、弥散地分布于铜基体中.细小的Cr粒子提高了合金作为电触头材料的性能.经800℃锻压后,材料的密度＞8.4 g/cm3,电导率为28.5×106Q-1·m-1,硬度为108 HB.     

异型薄壁Cu-Cr-Zr系合金铸件的离心铸造及组织

根据异型薄壁Cu-Cr-Zr合金精密铸件的结构特点，制定了切实可行的工艺路线，确定了熔炼温度、浇注温度、离心转数等相互匹配的工艺参数，获得了表面及内部质量均合格的铸件。     

Quantitative relationship between microstructure characteristics and fatigue parameters of A319 casting alloy

This paper describes a microstructure‐based uniaxial strain‐controlled fatigue life prediction model applied to A319 aluminum alloy which is widely used in automobile industry. The materials made with different casting conditions are characterized and quantified in terms of secondary dendrite arm spacing (SDAS), size, and aspect ratio of eutectic Si particles. Uniaxial low cycle fatigue tests have been performed on four groups of A319 alloy under different casting conditions in which cooling rate and Sr addition are variables. It is shown that the effect of various degrees of microstructure on the fatigue life and fatigue behavior is obvious. The first part of the paper is quantitatively characterizing the microstructure of samples to identify the influence of different casting conditions. With regard to mechanic properties, the tensile properties and fatigue behavior of samples are analyzed combining with microstructure. Finally, a microstructure‐based Manson‐Coffin‐Basquin model is proposed to predict fatigue life of Al‐Si alloy.     

铸造Ti46A18Nb合金拉伸过程中的损伤

研究了热等静压处理后铸态Ti46A18Nb合金拉伸过程中的损伤．结果表明：1）消除了疏松后，Ti46A18Nb合金拉伸强度的数据分散性仍然很大，表明疏松不是导致数据分散的唯一原因．在拉伸过程中不同强度试样的声发射行为有很大差异，说明其具有不同的损伤特征．断口分析和预拉伸着色实验结果证明，拉伸强度的数据分散性是合金晶粒粗大及其片层结构拉伸行为明显的各向异性所致；2）将预拉伸后样品表面去除约300μm，在随后的第二次拉伸过程中声发射Kaiser效应不再出现，说明在拉伸加载初期（低应力下）该合金的损伤主要集中于近表面．     

Cooling rate inference in aluminum alloy squeeze casting

An indirect method for the evaluation of cooling rate of a squeeze cast aluminum alloy is proposed. Cooling rate is inferred by simple local mechanical tests such as microhardness or indentation. A predictive model is used to correlate cooling rate and mechanical properties. An aluminum alloy (EN-AB46000) was squeeze cast. Four cylindrical samples were fabricated at different values of the squeezing pressure (from 0 to 100 MPa). Samples were cut longitudinally at the middle height and microhardness was measured along the sample radius as well as dendrite size. Moreover yielding stress was evaluated by means of FIMEC test in the centre of the sample section. Mechanical properties can be related to dendrite size and used for cooling rate inference.     

Thermal stresses in aluminium alloy die casting dies

The aim of this research is to analyze the influence of Aluminium Alloy die casting parameters, die material, and die geometry on in-service tool life. An innovative immersion testing apparatus is developed, at which Aluminium Alloy die casting is simulated. It enables controlled thermal fatigue cycling. Special specimens with different edge geometry and specimens with maraging steel welds deposited by Gas Tungsten Arc (GTA) welding are prepared. They are subjected to cyclic heating in bath of molten Aluminium Alloy 226 and cooling in bath of water-based lubricant. The specimens are continuously internally cooled with cold water. The microstructure, hardness profile, and the surface cracks developed are periodically analyzed after completion of a particular number of cycles. Temperature transients at different locations of the specimens are measured and used in calibration of finite element model (FEM). The computation of transient stresses is performed by developed FEM. The influence of immersion test parameters, material, specimen edge geometry, and thickness of maraging steel surfacing welds on thermal stresses is studied. To improve thermal fatigue testing efficiency, a specimen of particular geometry and immersion test parameters are developed based on finite element analysis. The results showed significant differences in produced thermal stresses for analyzed materials, test parameters, and edge geometries. Maraging steel is found to be superior material for die casting dies, due to generation of lower stresses.     

基于遗传算法的低压铸造铝合金车轮工艺优化

为解决低压铸造铝合金车轮质量控制难度大的问题,采用遗传算法对工艺参数进行优化.基于铸造数值模拟结果,利用BP人工神经网络建立了铸造工艺参数与质量控制目标缩松缺陷和凝固时间的非线性关系,采用遗传算法实现了铸造工艺参数的优化.以某型低压铸造A356铝合金车轮为例,对浇注温度、上模温度、下模温度、侧模温度、模芯温度5个参数进行优化,得到的最佳工艺组合,可有效控制缩松缺陷和凝固时间.利用数值模拟结果、建立神经网络模型,采用遗传算法优化的方法,获得近似最优解,有助于优化低压铸造工艺.     

介孔材料应用于污水处理的研究进展

本文综述了近年来国内外利用介孔材料吸附水体中的氨氮、重金属、有机污染物的研究现状,阐述了其作用机理和污染物的去除效果,并展望了介孔材料在污水处理领域中的应用发展方向。