Solid State Phenomena Vol. 347

Abstract: The effect of Mn additions (up to 0.8 wt.%) into Al-7Si-0.3Mg alloy on the microstructure, mechanical properties and die soldering was firstly investigated in gravity die casting conditions, with the aim to obtain an optimized Mn addition level. Especially, the stirring testing on H13 die material was used to elucidate the effect of Mn additions on the Fe concentration in the Al melting as a function of melting temperature (690 °C) and holding time (up to 32 h), which can provide an indirect sign of die soldering. On this basis, the addition of 0.6wt.%Mn into Al-7Si-0.3Mg alloy was found to be beneficial for microstructure, strength and die soldering. Secondly, rheocasting of Al-7Si-0.3Mg alloy without (Mn-free alloy) and with 0.6 wt.% Mn (Mn-containing alloy) was performed. The addition of 0.6wt.%Mn into Al-7Si-0.3Mg alloy was found to have following effects: (i) solidification temperature of α-Al is increased and a higher (about 20 °C) rheocasting temperature is therefore needed, but no significant die soldering problem was observed even with increasing casting temperature, (ii) Mn-containing phase forms along grain boundaries and within Al matrix, which increases tensile and yield strength but decreases elongation in as-cast, T5 and T6 conditions, (iii) number density and volume fraction of pores increases, and (iv) die soldering slightly improves. This paper demonstrates that the addition of 0.6wt.%Mn into Al-7Si-0.3Mg alloy is beneficial for microstructure, strength and die soldering in both gravity die casting and rheocasting conditions.

Abstract: Composite materials are increasingly being used in several areas, especially in the automotive and aerospace industries, however, during regular operation their wear is one of the main causes of failure. Consequently, developing and researching new composite materials is essential to increase and improve service life. In addition, thixoforming is claimed to exhibit superior properties by reducing typical defects in casting like shrinkage and porosity. Therefore, the main objective of this study is to produce and analyze abrasion wear properties of the thixoformed aluminum matrix composite reinforced with NbC, obtained by the stir-casting method. Three different composites with 5 wt.%, 10 wt.%, and 15 wt.% of NbC were manufactured with the stir-casting method, compared with A380 alloy. The procedure involves an A380 aluminum alloy that was molten at 750 °C. In sequence, niobium carbide powder was added by mechanical stirring for 10 min; Mg was added to improve the wettability between the reinforcement and matrix. Chemical grain refinement by Al-5Ti-1B master alloy was used for non-dendritic feedstock production. Hence, the induction furnace was used for the thixoforming process, to achieve a mushy of 60 % solid fraction at 562 °C, determined by Differential Scanning Calorimetry (DSC) analysis. The holding time applied was 90s. Optical microscopy (OM) and scanning electron microscope (SEM) analyses allowed the microstructural characterization. Abrasive wear tests, according to the ASTM G65 standard, showed an improvement of the composites’ abrasion wear resistance after the thixoforming process, with a higher amount of NbC, potentially increasing the range of use of this technology and materials.

Abstract: The continuous demand for lightweighting vehicles leads to a significant increase in the use of aluminium alloys in the automotive sectors. The Al-Si-Mg aluminium alloys are widely applied in foundry processes to produce structural components, due to their high specific mechanical properties combined with good castability and corrosion resistance. Primary (from bauxite) alloys are often used to ensure the highest quality of such parts, nevertheless their production route causes high CO₂ emissions. With the goal of reducing cradle-to-grave emissions of vehicles secondary (from recycling) Al-Si-Mg alloys are often used for cast products, however their percentage of impurities is considered too high for structural components. Lately, some Al-Si-Mg recycled alloys with a low content of Fe are available on the market as an alternative to primary alloys. Alongside the traditional foundry processes, semi-solid techniques are known to be able to increase the performance of Al-Si alloys. In this paper, a preliminary microstructural and mechanical characterization of secondary A356 alloys, manufactured by the ultrasound semi-solid method, was performed. In detail, both high Fe and low Fe recycled A356 alloys were investigated in comparison to the traditional primary one. The effect of the T6 heat treatment was also taken into account.

Abstract: AlN has high thermal conductivity (TC) and low coefficient of thermal expansion (CTE), which is an ideal reinforcement phase for strengthening the thermophysical properties of magnesium alloys to be used as packaging substrate material. However, the poor wettability between AlN and magnesium melt makes the preparation of AlN/Mg composites with high AlN content very difficult. In this work, the AlN/Mg-5Zn-4.9Cu composites were prepared by semi-solid stir casting method to address this problem, and the effect of AlN particle content on the thermophysical properties of the composites was investigated. The results show that AlN can reduce the CTE of magnesium alloy significantly and weaken the TC of magnesium alloys slightly. When 20 wt.% of AlN was added, the TC of the composites was 125.1 W/(m·K) and the CTE was only 19.83×10^-6 K^-1, which was 26.7% lower than that of the matrix alloy.

Abstract: This paper investigates the influence of Cu addition on the microstructure, mechanical properties, and thermal conductivity of a semi-solid Mg-10Zn alloy produced by Thixomolding process. Microstructure of the Mg-10Zn alloy consists of α-Mg matrix and Mg₂Zn second phase. The formation of Cu₃Zn phase was found after the addition of Cu particles. Besides, the volume fraction of solid phase decreased in the Mg-Zn/Cu alloy compared with that of binary Mg-Zn alloy. The yield strength of the alloy was enhanced from 157MPa to 188MPa after Cu addition at the sacrifice of a small amount of ductility. Thermal conductivity, on the other hand, were kept the same level of 115-117 (W/m·K) in the Mg-10Zn alloy and Mg-10Zn/Cu alloy. This work demonstrates the potential for developing Mg alloys with high strength and high thermal conductivity via proper alloy design using Thixomolding process.

Abstract: Semisolid castings are usually produced using primary Al alloys to ensure significant mechanical performances. However, the need to increase the use of secondary alloys is becoming more and more urgent to reduce the carbon footprint of the manufacturing process. In fact, it is well known that the production of primary alloys is far more demanding in terms of energy and emissions than the recycling route. To extend the use of secondary alloys to semisolid processing, it is necessary to thoroughly understand their properties and how they can influence a material with peculiar properties as the semisolid one. Besides microstructural and mechanical features, the rheological behaviour also plays a major role when dealing with processing metals in the semisolid state. Therefore, in the present study, a rheological characterization of secondary AlSi7Mg commercial alloy was carried out and compared to that of the conventional primary alloy. In details, a different content of Fe, Cu and Mn was considered, as these impurities easily form primary intermetallic particles, which can remain dispersed in the liquid matrix of the semisolid metal. The aim of this work is to understand if this can affect the rheological properties of the considered semisolid alloy. Flow curves and yield stresses were obtained from the experimental results to compare the behaviour of the different alloys.

Abstract: It is easy to occur solid-liquid separation during semi-solid rheological forming, which leads to poor uniformity of microstructure and properties and limits its application in high strength and toughness parts. In this paper, a semi-solid CuSn10P1 alloy slurry was prepared by an enclosed cooling slope channel (for short ECSC). The effect of ingate length on microstructure and properties by semi-solid squeeze casting was studied. The results showed that the proper increase of the length of the ingate is beneficial to improve the uniformity of the microstructure and properties of the semi-solid squeeze casting. The microstructure uniformity and properties are the best when the ingate length is 20mm. The ultimate tensile strength and elongation of semi-solid squeeze casting CuSn10P1 alloy with 20 mm ingate length reached 419.2 MPa and 13.4%.

Abstract: In this study, microstructure evolution during cooling slope casting of Al-15Mg₂Si-4.5Si composite is studied considering a test case of melt pouring temperature of 650°C. The shear force exerted by the flowing melt on the evolving primary Mg₂Si and α-Al grains facilitates their fragmentation and subsequent spherodisation. After leaving cooling slope, the composite slurry is kept isothermally within a holding furnace for different time duration i.e., 8, 16, 24 and 32 minutes. Increasing trend in grain size values is observed with the increasing slurry holding time, whereas sphericity value decreases after initial increase till 8 minutes of slurry holding. The slurry samples are scooped from chosen locations of the slope as well as from isothermal slurry holding bath and rapidly quenched in oil to study their microstructure and mechanical properties. Simultaneous spherodisation of both the primary phases i.e., primary Mg₂Si and α-Al during cooling slope processing is evident from optical microscopy and Scanning electron microscopy of quenched slurry samples. Whereas increasing sphericity of P-Mg₂Si and α-Al phases, along the melt flow direction over the slope, is found to accompany with the increasing micro hardness values.

Abstract: The SEED (Swirled Enthalpy Equilibrium Device) process is the typical high solid fraction (with about 40-60% of solid) rheo-diecast process. The high strength Mg-12Gd-3Y-1Zn-0.4Zr magnesium alloy was successfully produced using SEED process. The microstructure and mechanical properties of rheo-diecast Mg-12Gd-3Y-1Zn-0.4Zr alloy during solution and ageing heat treatment process were studied in this paper. Optical microscopy (OM) and transmission electron microscopy (TEM) were employed to study the evolution of microstructure and mechanical properties. The cast alloy exhibited a microstructure consisting of equiaxed primary solidified grains and together with eutectics. The eutectics were partially dissolved into a-Mg matrix after solution heat treatment. The tensile properties of rheo-diecast Mg-12Gd-3Y-1Zn-0.4Zr alloy were substantially enhanced after T6 heat treatment.

Abstract: It is fundamental in thixoforming to have a refined microstructure that, when heated to the semisolid state, consists of a mixture of small spheres immersed in liquid as this ensures the best rheological properties. The present work therefore aims to analyze the rheological behaviour of aluminum-silicon-copper alloys, namely, Al5Si2.8Cu, Al6Si2.8Cu and Al7Si2.8Cu. The alloys were produced by conventional casting and then deformed by ECAP in one pass in a die containing two channels of the same cross-sectional area forming an angle of 120°. After being processed by ECAP, the alloys were heated to semisolid temperatures, i.e., temperatures corresponding to a solid fraction of 45 %, and kept in the semisolid state for 0, 30 and 90 s, after which they were subjected to hot compression tests. The structures of the three alloys had an excellent response to recovery and recrystallization mechanisms, with refined microstructures that led to the formation of very fine spheres immersed in liquid in the semisolid state. The best rheological behaviour was obtained for the Al5Si2.8Cu alloy, which had an apparent viscosity of the order of 10³ Pa.s. The findings suggest that this simple ECAP process is a promising route for the production of semisolid feedstock for use in thixoforming.