Materials Science Forum Vols. 546-549

Abstract: The wear resistance, microstructures, worn surface morphology, TiAl3 phases, eutectic Si particles and the high density of dislocations of the Al-Si-Ti piston alloys have been studied by means of SEM and TEM. The electrolytic low-titanium aluminium alloys were used to make the Al-Si-Ti pistons alloy, the experiment results showed that the wear resistance of the Al-Si-Ti alloy was 1.49 times of that of the Al-Si alloy, and the wear mechanism was mainly plastic deformation wear. The relationship between wear resistances and the yield strength of the Al-Si-Ti piston alloys were established, and the wear mechanism investigated by mean of plastic-elasticity theory. When Ti content is reached to 0.12%, the yield strength of the Al-Si-Ti piston alloy increased from 255 Mpa to 358 Mpa.

Abstract: By means of FISICO test, the cracking susceptibility of the 6061 aluminum was investigated, the experimental results showed that it is prone to form crater crack under restraint condition comparison with that of the 3003 aluminum. So the crater fill-up technology need to be employed accordingly during the field welding. The strength, elongation percentage and hardness of the as welded joint was lowered 29.7%
40%42% respectively, the former two is recovered to 73.4% and 81.7% of the base material respectively after natural aging 30days. This indicate that the natural aging treatment can remedy part of the property loss during the welding.

Abstract: Heavy deformation plus micro alloying could be an effective way to obtain ultrafine grain structure of metals. In the present work, an Al-Cu-Mg alloy was microalloyed with Zr to obtain homogeneous precipitates and then heavily deformed by conventional forging at high temperature. The possible refining processing routes were studied and the superplasticity behaviors of the alloy was investigated. Results show that the micro alloyed alloy can be stably refined to 3-5μm under conventional processing routes. The Al-3Zr precipitates act both as additional sites to enhance recrystallization nucleation rate and pins to impede grain growth to increase the thermal stability of the fine grain structure. However, as the Al3Zr precipitates remains along grain boundaries, the superplastic capability of the material is not high. At 430°C with 1×10-4S-1 strain rate, the elongation obtained was 260%.

Abstract: Two Al-Mg-Mn alloys having similar compositions but with and without Zr addition were subjected to equal channel angular pressing (ECAP) at 350°C using route BC and a 90° die, followed by water quenching or air cooling. A series of annealing experiments were conducted at various temperatures from 400°C to 460°C on the water-quenched alloys. Fine structures with grain size of approximately 1~2μm were obtained in these alloys after 6 passes. The water-quenched alloy containing Zr exhibited finer structure compared with the Zr-free alloy in the same cooling condition, which was due to the existence of Al3Zr dispersoids. And in the air cooling condition, it was found that in the Zr-free alloy static recovery occurred, and in partial regions some small grains without dislocation inside appeared suggesting the occurrence of static recrystallization. This demonstrated a fairly restored structure. However, the microstructure of the alloy containing Zr kept stable during the air cooling process. In addition, in the annealing experiments, secondary recrystallization took place in the Zr-free alloy annealed at 410°C for 1h, while the alloy containing Zr kept stable up to 460°C. TEM observation showed that the Al3Zr dispersoids pinned the motion of the grain boundaries, thereby the secondary recrystallization and grain growth were inhibited.

Abstract: The aim of this work is to study the effect of a low frequency alternating magnetic field on morphology and distribution of A3F2 in the Al-2.89 wt.% Fe alloy. At the cooling rate of 0.05 °C/s, only Al3Fe phase was observed in the iron-containing intermetallics. It was noteworthy that, compared with the conventional solidification, the primary Al3Fe phase was refined and accumulated towards the center of the sample by applying the alternating magnetic field. This phenomenon is considered as the result of the larger Lorentz force acting on the Al3Fe phase than the Al matrix.

Abstract: Effects of titanium refining methods on the microstructure and mechanical properties of A356 alloys were studied. The results showed that the A356 alloys refining by electrolytic low titanium Al alloys had excellent grain refinement effect and high age-hardening behavior comparing with the traditional A356 alloys. By means of DSC analyses, the solidification characteristics and the age-hardening behavior were discussed. The results showed that the E-A356 alloys had more powerful nucleation potency than that of the M-A356 alloys. The exothermic peaks of β// phase was shifted to a slightly lower temperature, and the rate of the continuous transition from GP zone to metastable phases increased.

Abstract: The alloying behavior of RE element of Er in Al-Cu-Mg-Ag alloys with addition of Er were investigated in this paper. During solidification ,most of element Er segregated at grain boundaries in the form of primary Al8Cu4Er phase and part of Er dissolved in the α-Al matrix. After rolling, Al8Cu4Er phase could be fragmentated into dispersed particle which leaded to an increase of the grain boundary strengthening . TEM observation and tensile strength test showed that small addition of Er retarded the precipitation of Ω phase and prolonged the time corresponding to the peak aging,with a remarkable increase of ductility.

Abstract: Al-Ti-C-P master alloy has been successfully fabricated by SHS-melting technology. Microstructures and phase constituent of Al-Ti-C-P master alloy were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and EDS. The results show that Al-Ti-C-P master alloy comprises TiAl3, TiC, AlP and α-Al matrix. After adding 2.0 wt% Al-Ti-C-P master alloy, microstructure and mechanical properties (after T6 treating) of ZL117 alloy has been improved dramatically: The average grain size of primary silicon decreases from 260 to 35 μm, edges and angles of primary silicon are passivated, the morphology of eutectic silicon changes from large needle-like one to fine rod-like or particle-like one, and the microstructures are noticeably refined; Room temperature tensile strength changes from 168 to 260 MPa while elevated temperature tensile strength (at 300
) varies from 88 to 125 MPa. Impact toughness and macro-hardness(HB) increases to 17 J/cm2 and 97 from the original 6.5 J/cm2 and 92.

Abstract: The influences of low frequency electromagnetic field on microstructures and macro-segregation in direct chill casting process were investigated in the experiments，Al-19.2%Si alloys were semi-continuously cast into ingots with 100 mm in diameter. Microstructures and macrostructures of samples taken from different part of the cast with different electromagnetic field conditions were characterized by optical microscopy. The results showed that low frequency electromagnetic field refines the microstructures and reduces macro-segregation. Unlike casting without electromagnetic field, the primary Si grains were homogeneous with fine dimensions and the morphology of the primary silicon exhibited small blocky structures or near-spherical structures. Further more, decreasing frequency is beneficial to the improvement. In the range conditions in the experiments, the optimum frequency is found to be 15Hz. During casting, the temperature of the liquid metal in the sump was monitored. The results shown, under LFEC the width of the liquid-solid region became narrowed and the temperature field in the sump also became homogeneous.