Solid State Phenomena Vol. 347

Abstract: A semisolid slurry of Mg_98.5Ni_0.5Y_1.0 alloy was prepared by UV (ultrasonic vibration) method, then the slurry was cast under different squeeze pressures, in which the pressure varied from 0 MPa to 400 MPa. The results show that the primary α-Mg phase and LPSO structure (Long Period Stacking Ordered structure) of the alloy can be refined significantly. The solid solubility of Y and Ni elements in magnesium matrix can be increased with higher pressure, and the Mg₂Ni particles is also precipitated out in LPSO structure under pressure. The grain size is decreased with the enhancement of squeeze pressure, which improves the strength. The alloy squeeze cast under pressure of 100 MPa has the highest cost performance. Its yield strength, tensile strength and elongation are 115 MPa, 243 MPa and 13.5%, and they are increased by 21.5%, 9.0% and 16.4% respectively compared with the alloy without applied pressure.

Abstract: Most magnesium alloys are produced by die casting due to their formability issues, but the thixomolding process has recently drawn considerable attention in manufacturing magnesium alloy casings for 3C electronic devices with lower porosity levels. This study compared the mechanical properties and microstructure of thixomolded AZ91 and ultra-light LAZ771 magnesium alloy for thin laptop cases. The experimental results firstly show that the microstructure of as-thixomolded samples contains fine αMg grains and a divorced eutectic network. After solution heat treatment, the ductility is improved due to the dissolution of the non-equilibrium secondary phases. Secondly, microstructural characterization for samples after aging treatment shows that the Mg₁₇Al₁₂ precipitates in AZ91 can be classified into continuous (CPs) and discontinuous precipitates (DPs) according to their morphologies. On the other hand, the AlLi precipitates in LAZ771 after aging treatment are spherical and incoherent with the αMg matrix. Those AlLi precipitates obstruct basal slip of dislocations in αMg matrix more effectively than Mg₁₇Al₁₂, resulting in higher yield strength of peak-aged LAZ771 specimens than heat-treated AZ91 specimens.

Abstract: One of the main functions of semisolid metal alloy forming processes, notably rheocasting and thixoforming, is the manufacture of parts, casings and frames for mechanical assemblies and systems. These parts not only must have the required minimum mechanical properties in terms of yield strength and elongation, but also must be able to withstand cyclic tensile and compressive forces. However, there is little fatigue strength data for the materials used for these parts. The present work seeks to fill this gap by determining the fatigue limit at 10⁷ cycles of Al-6.0wt%Si-2.5wt%Cu alloy, or simply Al6Si2.5Cu, thixoformed in a pneumatic press at 585 °C (the temperature corresponding to 40 % solid fraction) with isothermal treatment times of 30 and 60 seconds. The parts were also subjected to T6 heat treatment, for which they were solution heat treated at a temperature of 520 °C for 4 hours followed by aging at 180 °C for 10 hours. For all the conditions tested, the microstructures were characterized to determine the grain size, appearance and shape of the silicon particles, in addition to the residual porosity. For the best conditions observed, 30 s holding time and T6 heat treatment, the grain size varied between 100 µm and 130 µm; the shape factor was around 0.60, indicating an excellent degree of roundness; and there was low residual porosity of around 0.3 %, resulting in a yield strength of up to 240 MPa with 4.5 % elongation. The average fatigue strength was estimated by the staircase method and was between 95 MPa and 98 MPa for 10⁷ cycles.

Abstract: A strip caster was used to cast two types of aluminium alloys: an almost eutectic Al-Si alloy (Al 10 Si) with contaminants of 0.6 Fe, 0.2 Pb, and 1.4 Sn, which exhibits a polyphase microstructure upon solidification, and recycled aluminium beverage cans made from Al 0.8 Mn alloy containing 0.4 Si, 0.5 Fe, and 0.1 Mg, which has a monophase microstructure upon solidification. The molten materials were poured at 650 oC (Al 10 Si) and 670 oC (Al 0.8 Mn, 0.4 Si, 0.5 Fe, 0.1 Mg) on a cooling slope specially designed to obtain a semisolid material. This semisolid material was then dragged between rolls at a rate of 0.2 m/s to obtain high-quality metal strips. There was less eutectic modification with larger Al-α grains in the middle region of the sheet between the fine eutectic layers, indicating a lower cooling rate. However, during the recycling of aluminium beverage cans, large grains were formed with a columnar structure at the interface of the rolls, and semisolid melts with cracks were formed between the columnar grain boundaries owing to the compression of the rolls. The middle of these grains contained smaller equiaxial grains that were subjected to dragging. The as-cast specimens were submitted to homogenisation heat treatment at 560 oC for a period of 10 h and cooled to room temperature before being cold rolled (Temper H18) and recrystallised (Temper O) to examine the effect of these treatments on the tensile mechanical properties. During cold rolling (Temper H-18), grain alignment occurred with a yield stress, maximum stress, and elongation of 209.5 MPa, 210.1 MPa, and 2.8%, respectively. The strength decreased (yield stress of 58 MPa and maximum stress of 120.2 MPa) under recrystallisation conditions (Temper O), but the ductility increased (8.9%). This is in contrast to the Al-Si (Al 10Si) strip, which exhibited a yield stress, maximum stress, and elongation of 103.3 MPa, 128.7 MPa, and 1.9%, respectively. The A1 10Si strips also fractured during cold rolling, indicating high material fragility.

Abstract: Rheocasting is a semisolid casting process allowing to obtain near-net shape parts. Through the Rheocasting process, it is possible to achieve aluminium castings having a low grade of porosity if compared to traditional die-casting methods, encouraging the production of automotive frame parts. However, casting processes, as commonly known, may cause tensile residual stresses inside the parts. On the other hand, compressive stresses inside castings can significantly increase the life of components: residual compressive stresses increase the material's resistance by counteracting crack initiation and propagation. The cracks propagate when the material is under tensile stress, while the Rheocasting technique seems to promote compressive stresses inside the castings. This work aims to analyse an aluminium rheocasted frame component for race cars in both the as-cast and heat-treated conditions. First, the mechanical properties of the components were evaluated in terms of tensile tests and microhardness. Then, residual stresses were measured at specific points of the casting. Finally, the evolution of the residual stresses inside the component before and after heat treatment led to assessing the effect of the Rheocasting process condition and the heat treatment, proving the marked advantage of using such a technology.

Abstract: Aluminium matrix composites (AMCs) offer improved mechanical and tribological properties compared to monolithic materials and therefore provide great potential for various applications. This particularly applies to particle-reinforced AMCs revealing comparatively high contents of reinforcement particles. However, these types of composites are difficult to manufacture due to their abrasive characteristics as well as their complex rheological material behaviour. An approach to produce such AMCs is semi-solid powder processing, combining powder pressing and sintering in one step in order to produce fully dense composites with currently only cylindrical shapes. Therefore, the tools as well as the powder mixture are first heated into the semi-solid temperature range of the aluminium powder and subsequently formed using low pressures under 200 MPa. Due to the shear thinning behaviour of the semi-solid aluminium matrix the porous structure of the pressed powder is filled during compaction, resulting in homogenous particle distributions in the component. However, this process results in high process times as well as energy costs, due to the heating inside of the die. In contrast to the semi-solid powder processing in one step, in this paper, a novel process route combining cold uniaxial compaction of particle reinforced aluminium powders having up to 50 vol.% SiC with subsequent semi-solid forming is presented. Here, a particle reinforced and cylindrically shaped green body is utilized as raw material, in order to produce complex components through semi-solid forming. The parts produced in this way are featuring varying wall thicknesses and are used in order to determine the process limits for manufacturing particle reinforced components having up to 50 vol.% SiC. Thereby, the influence of reinforcement particle size as well as particle loading on the homogeneity of the resulting particle distribution of an academic component is investigated. Future main objective of the process route is the manufacturing of complex parts with homogenously distributed particles.

Abstract: High-pressure die-casting (HPDC) can be a productive process for high-quality cast aluminium alloy components. However, it is also a process prone to generate defects, such as gas porosities and incomplete fillings, resulting in rejections. One way to reduce the reject rate is to employ Semi-Solid Metal processing with HPDC. The most important advantages of Semi-Solid alloys are reduced shrinkage defects, fewer gas porosities, and fewer chances of filling-related problems. To take full advantage of a semi-solid metal slurry, the casting process must be controlled meticulously to reach homogeneous casting quality and high process repeatability. A study has been conducted on cast parts composed of two-dimensional symmetrical cavities. From the mechanical tests, unexpected differences emerged in both tensile strength and fracture elongation, which were confirmed by differences in the microstructure. The paper investigates the reasons for the asymmetry in the proprieties to avoid similar problems in future studies and maximize the effectiveness and repeatability of the high-pressure die-casting process.

Abstract: Two casting technologies were introduced to fabricate the Al-7Si-0.6Mg alloy, including high pressure die casting (HPDC) and semi-solid die casting (SSM). The microstructure and mechanical property were studied by optical microscopy (OM), scanning electron microscopy (SEM), X-Ray diffraction (XRD), microhardness and tensile test. The electrical conductivity was also introduced to evaluate the thermal conductivity of the alloys. Results show that the as-cast microstructure of the HPDC alloy and SSM alloy both consists of primary α-Al, eutectic Si and Mg₂Si phase. The primary α-Al phase in HPDC alloy possesses fine equiaxed dendrite structure with an average grain size of 29.4 μm. While the SSM alloy forms abundant coarse and globular structure α-Al phase, and the average grain size reaches 98.9 μm. This is because of the lower cooling rate during slurry preparation promotes the formation of a relative coarse microstructure in the SSM alloy. Furthermore, the increased content of eutectic Mg₂Si phase was also detected in the SSM alloy. Therefore, the average yield strength and ultimate tensile strength decreases from 148 MPa and 275 MPa of the HPDC alloy to 140 MPa and 254 MPa of the SSM alloy, respectively. However, the elongation and electrical conductivity of the SSM alloy both increases from 5.8±1.3% and 20.0±0.1 MS/m of the HPDC alloy to 7.5±1.2% and 20.6±0.1 MS/m, respectively.

Abstract: In this paper, both thixocasting and rheocasting of Al-7Si-0.6Mg alloy (EN AC 42200) for the same part was performed. It was found that rheocasting of Al-7Si-0.6Mg alloy show a smaller primary Al grain size and significant improvement of cast defects compared with thixocasting of Al-7Si-0.6Mg alloy. This paper demonstrates that rheocasting of Al-7Si-0.6Mg alloy is more beneficial in terms of microstructure and cast defects compared with thixocasting of Al-7Si-0.6Mg alloy.