Abstract: Based on the green and saving concept, a complex process preparing semisolid alloy slurry was developed, which was composed of the low superheat pouring and low frequency electromagnetic stirring. The semisolid A356-La slurry was prepared by the complex process, and the microstructure of the semisolid A356-La alloy was researched under the different preparing parameters in the complex process, such as the pouring temperature, electromagnetic stirring frequency, stirring time and micro-addition of La. The results indicated that it was feasible to reduce the addition of La and consumption of energy during the preparation of semisolid alloy slurry by optimizing the preparing parameters in the complex process. The suitable preparing parameters were obtained by the experiment, in which the pouring temperature was 630 °C, the frequency of electromagnetic stirring was 30 Hz, and the stirring time was 8 s. When semisolid A356 alloy slurry added 0.3 wt% La was prepared by the suitable preparing parameters in the complex process, the refining effects on microstructure in the semisolid A356-0.3La alloy was indistinguishable with that of the conventional addition amount of 0.6 wt% La in semisolid A356 alloy prepared by low superheat pouring.
Abstract: Melt-SHS (Self-propagating High-temperature Synthesis) was used for the preparation of Al–5Ti–1B master alloy. The quality ratio of Ti powder/TiO2 in initial powder mixture was varied from 0:1 to 1:0. The AES, XRD and SEM were applied to evaluate the microstructure and phase componet. The results showed that the Al-5Ti-1B master alloy could be successful produced by the reaction of Al powder, TiO2 and H3BO3 in Al melt, while the reaction rate was slow. The microstructure mainly presents the TiAl3 particles with long strip shape. A significant improvement was noted both in reaction rate and in the grain refining efficiency when Ti powder was added to the reactants and the optimized ratio of Ti powder/TiO2 was 2:3. The TiAl3 particles were reduced and the grain refining efficiency turned bad when Ti powder was totally used to supply Ti
Abstract: Al-Si alloy was widely applied in automobile engine parts to realize weight reduction. The influence of casting temperature on the microstructure of die casting Al-Si-Cu-Mg alloy was studied in this paper. Based on ZL 101 alloy, the strength was improved with addition of 0.8% Cu element. The influence of pouring temperature on microstructure was investigated using optical microscope and electron probe micro-analysis (EPMA), and T1 heat treatment was optimized. The primary α-Al was more coarsened when the pouring temperature rose from 660 ̊C to 690 ̊C. The solid solubility of Cu in α-Al was 0.2406 wt%, analyzed by EPMA. Considering the solid solubility of Cu and avoiding porosity at high-temperature, T1 heat treatment was reasonable and affective. The micro-hardness reached to a peak value of 114 HV during aging at a temperature of 175 ̊C after 10 h. Therefore, 175 ̊C×10h aging was the most appropriate heat treatment process.
Abstract: It is well known that the presence of phosphorus in hypoeutectic Al-Si alloys causes coarsening of Al-Si eutectic even when the phosphorus concentration is as low as a few ppm. The reason is that phosphorus reacts with aluminum to form AlP, the particles of which serve as potential nuclei for silicon due to a very similar lattice parameter with Si. The purpose of this study is to investigate the possibilities for reducing the nucleation potency of AlP by changing the alloy chemistry. The experiments include melting and solidification of a number of aluminum alloys under different conditions. After that, the samples are examined using SEM and TEM microscopy to elucidate the presence of phosphorus-containing compounds and to explain their formation mechanism. The experimentally obtained data are discussed on a systematic basis of thermodynamic calculations and compared with the corresponding data from the relevant literature. Particularly, the results reveal that such elements as Mg, Ca and Sr are potentially capable of neutralizing the nucleation activity of AlP although their effect is dependent on the presence of other chemical elements in the aluminium alloy.
Abstract: This study investigated the effects of the Si content on the tensile properties and machinability of 6xxx−series aluminum alloys for application in hydrogen container valve housing. Four types of extruded Al−Mg−Si−Cu−Mn−Cr alloy specimens with different Si contents of 0.6−3.0 mass% were prepared. The effects of the Si content on characteristics such as microstructural features, tensile strength, and drill machinability were examined and compared with those of a commercial A6061 alloy. These specimens were found to exhibit different microstructures, tensile strengths, and chip segmentation behaviors after T6 heat treatment. These results were attributed to the interaction of the amount of second phase particles and solid-solute Mn and Cr with changes in the Si content.
Abstract: Quenching is a critical step during the strengthening age hardening of Aluminium alloys. To obtain optimal technological results, parts should be quenched with the upper critical cooling rate. The precipitation behaviour of Al alloys during cooling from solution annealing and thereby the critical cooling rates are typically investigated by in-situ measurements with differential scanning calorimetry (DSC). Conventional DSCs are limited at cooling rates below 10 Ks-1. Unfortunately, medium to high strength Al alloys typically have critical cooling rates between 10 and some 100 Ks-1. Recently it was shown that dilatometry is generally able for in-situ detection of precipitation in Al alloys. Dilatometry allows controlled cooling up to some 100 Ks-1 and therefore covers the cooling rate range relevant. In this work, we aim to show up and discuss possibilities and limitations of dilatometric detection of quench induced precipitates in 2xxx, and 7xxx Al alloys. The basic method will be presented and results will be compared with DSC work.
Abstract: In the present research, a comprehensive study on the effect of the homogenization treatment on the characteristics of dispersoid and recrystallization behavior in an Al-Zn-Mg-Cu-Zr alloy has been conducted by means of optical micrography, scanning electron micrography and transmission electron micrography. The influence of three process parameters of the homogenization treatment, first stage holding temperature, holding time and heating rate, on the dispersoid characteristics has been throughly studied. The result shows that holding at 400°C for sufficient time is highly beneficial for obtaining fine and uniformly distributed Al3Zr particles. Compared with the high heating rate treatment, the slow one apparently leads to significantly smaller Al3Zr dispersoids, the dimension of the dispersoids decreases from 35 nm to 22 nm, and the number density of Al3Zr particle in the center of the grains increases from 13/μm2 to 35/μm2. The percentage of recrystallized grains of the alloy is found to be affected by dispersoid distribution.
Abstract: Quenching is an important step during age hardening of aluminium alloys. It significantly influences on microstructures, properties, residual stresses and component distortion. Due to high heat transfer, thermal stresses occur in quenched components. I.e., all premature precipitates during undercritical quenching form on condition of mechanical stresses. Opposite, quench sensitivity investigations, e.g. recording of continuous cooling precipitation diagrams, are usually performed in stress-free conditions and may therefore be incomplete. We have developed a new method of thermomechanical analysis and calorimetric reheating to investigate stress induced precipitation during quenching of aluminium alloys. For aluminium alloy 2024, it has been shown for the very first time that mechanical stresses during quenching also influence on quench-induced precipitation reactions.
Abstract: The effects of Ni content and intermediate annealing conditions on recrystallization behavior during brazing heat treatment of Al–Fe–Ni–Si alloys fin stocks of automotive heat exchangers were studied by means of microstructural analysis. Following brazing heat treatment, coarsening of recrystallized grain structure was observed in both 0.5% Ni and 1.0% Ni fin stocks, which were intermediately annealed in a salt bath. This coarsening was particularly marked in 1.0% Ni fin stocks. The results of the microstructural observations indicated that fine dispersoids were heterogeneously dispersed in 1.0% Ni fin stocks that were annealed at 550°C for 10 s in a salt bath. It was proposed that the preferential recovery and recrystallization during the brazing heat treatment in the low density area of the fine dispersoids contributed to the larger recrystallized gain structure. The coarse recrystallized grains obtained by controlling the salt bath annealing conditions are expected to contribute in improving the properties of Al–Fe–Ni–Si alloy fin stocks of automotive heat exchangers.
Abstract: Upsetting tests on two newly developed Mn and Cr-containing Al-Mg-Si-Cu alloys with various Mn contents were carried out at a speed of 15 mm/s under upsetting temperature of 450 °C after casting and subsequent homogenization heat treatment using a 300-Tone hydraulic press. STEM experiments revealed that Mn and Cr-containing α-Al (MnCrFe)Si dispersoids formed during homogenization showed a strong pinning effect on dislocations and grain boundaries, which could effectively inhibit recovery and recrystallization during hot deformation in the two alloys. Recrystallization fractions after solution heat treatment following hot deformation were measured by EBSD technique. It was found that the recrystallization fractions of the two alloys were less than 30%, giving rise to lower recrystallization fraction in the alloy with higher amount of Mn, which had higher number density of dispersoids. This implied that the finely distributed α-dispersoids were rather stable against coarsening and they stabilized the microstructure by inhibiting dislocation recovery and recrystallization during elevated temperature exposure. Increasing the content of Mn could increase the number density as well as the aspect ratio of the dispersoids, and more significantly, the effect of retardation on recrystallization were further enhanced.