Solid State Phenomena Vols. 217-218

Abstract: An application of semisolid processing to magnesium alloys is discussed emphasizing both the fundamental and applied research activities aimed at better understanding the microstructure-property relationship. The reduced temperature of semisolid processing, providing common benefits of longer tool life, tighter dimensional tolerances and better process consistency is of special importance for magnesium alloys due to their high affinity to oxygen, requiring an expensive protection and leading otherwise to ignition and burning. However, the reduced temperature resulting in higher part integrity does not create beneficial microstructural characteristics converting to substantially improved mechanical properties. Major microstructural factors controlling properties of magnesium alloys after semisolid processing are discussed.

Abstract: Martensitic stainless steel was chosen for a semi-solid processing due to high mechanical properties and resistance to high temperature surface oxidation. Modified H18 martensitic steel (1.1 % - C, 18.9 % - Cr, 0.1 % - V, 0.7 % - Mo, 0,9 % - Si, 2.0 % - Mn, balance Fe, all in weight %) modified with addition of 0.01 % boron was applied as a feedstock for semi-solid range temperature experiments. The samples were heated up to 1330°C to obtain about 26% of the liquid phase followed by rapid quenching in water. The microstructure of the samples consisted of austenitic globular grains (average grain size 42 μm, about 78 % of volume) surrounded by a eutectic mixture (ferrite, and M₇C_3, M₂₃C₆ carbides as identified by X-ray and electron diffraction). The initial hardness of as-cast sample was 357 HV₅ and that after quenching from liquidus solidus range was 422 HV₅. The X-ray analysis confirmed the presence of 16% - α-Fe, 80% - γ-Fe and 4% - M₇C₃ carbides in rapid quenched sample. The EDS analysis of a eutectic mixture was as follows: 6.2 % - C, 31.7 % Cr, 0.1 % - Mn, 0.6 % - Si, 61.2 % Fe. The chemical composition analysis of globular grains confirmed the presence of 2.0% C, 16.3 % - Cr, 1 % - Si, 1.7 % - Mn, 0.5 % - Mo, 78.5% Fe.

Abstract: It is necessary to well understand the microstructure evolution during high speed heating and forming for steel thixoforging, since it determines the thixotropic flow behavior of materials in the semi-solid state. A new in situ technique - high temperature Confocal Laser Scanning Microscopy (CLSM) - was developed and used for studying the microstructure evolution directly at high temperature where the microstructure in the semi-solid state could not be preserved by quenching experiments for conventional 2D characterization. Several steel grades (C38LTT, 100Cr6 and M2) were investigated during heating from the as-received state to the semi-solid state and finally cooled to the solid state).It has been found that there is a significant difference in diffusion rate of alloying elements between these grades during heating and cooling. In M2, thanks to the high content of alloying elements and their low diffusion rate, the semi-solid temperature range is greater and its microstructure in the semi-solid state could be preserved by quenching or even at a low cooling rate, which means the microstructure of M2 in the semi-solid state can be characterized in room temperature on quenched M2 samples. On the contrary, the microstructure of other steel grades 100Cr6 and C38LTT in semi-solid state can only be revealed by CLSM at high temperature because of the lower volume fraction of alloying elements and their high diffusion rate. It is very interesting to use high temperature CLSM to in situ investigate the microstructure evolution in the semi-solid state, especially at low liquid fraction.

Abstract: There has been a great debate whether grain refinement is more effective when shearing above or below the liquidus. In this work, examination of the microstructural evolution in AA7075 alloy has been performed, after stirring above & below the liquidus. The results suggest that shearing above the liquidus is more effective. The outcomes indicate that at temperatures below the liquidus, the convective forces produced by electromagnetic forces cannot break the dendrites due to low velocities, whilst at shearing above the liquidus through a cavitation-enhanced nucleation mechanism grain refinement is promoted. It appears that by applying magnetic forces above the liquidus temperature, shearing effects result in finer grain sizes and consequent enhancement of properties. Keywords: Electromagnetic Casting, Grain Refinement, Cavitation Mechanism

Abstract: In the present study, 7050 supplied in extruded state was heated to different temperatures below solidus or the semisolid state and microstructural evolution and coarsening were investigated. The results showed that complete recrystallisation only occurs after soaking for 5 minutes at 545°C, which is characterised by formation of a lot of fine equiaxed grains. RAP is suitable for fabricating high-quality semisolid billet of 7050 aluminum alloy with an average grain size ranging from 47.4 um to 70.5 um and a roundness ranging from 1.3 to 1.7. Grain growth occurs as the samples were soaked at 610°C and 615°Cfor prolonged soaking time. When the isothermal temperatures were 610°C and 615°C, the coarsening rate constants were 359.5μm3s-1 and 470.5μm3s-1, respectively, indicating an increase of coarsening rate constant (K) with the increasing isothermal temperature. Coarsening tends to occur via Ostwald ripening and coalescence. Ostwald ripening plays an important role during the whole coarsening process, but the grain coalescence only contributes to coarsening after soaking for 12 minutes. 625°C is an optimal temperature to keep cylinder shape of the sample due to collapse of the sample above this temperature, leading to difficult clamping.

Abstract: Hypereutectic Al-Si alloys have low thermal expansion coefficients, excellent wear resistance, and high hardness. To replace cast iron with hypereutectic Al-Si alloys, the hypereutectic Al-Si alloys must still have the originally desired mechanical properties. The addition of Fe and Mn to the hypereutectic Al-Si alloys can form Fe-rich phases, which can improve thermal stability. In this paper, the effect of manganese (0 %, 0.99 %, and 1.36 %) on the Fe-rich phases of hypereutectic Al-22Si-2Fe (% w/w) alloys was studied. The results showed that primary Si particles (PSPs), needle-like Fe phases, a coarser fishbone-shaped α-Al₁₅(Fe, Mn)₃Si₂ phases, and eutectic Si could be refined by cooling slope. With such a CS process, the intermetallic compounds in the alloys with different Mn/Fe ratios were examined with an optical microscope, scanning electron microscope, and X-ray diffraction. Moreover, the blocky α-Al₁₅(Fe, Mn)₃Si₂ phases were analyzed by transmission electron microscopy. Through the analysis of the SADP, the lattice structures of α-Al₁₅(Fe, Mn)₃Si₂ phases were identified with tetragonal structure and body centre cubic.

Abstract: A general rule in conventional liquid metal casting processes is that smaller size would produce better metallurgical quality and mechanical properties. The conclusion that semi-solid thixocasting process doesnt follow this rule has been made recently. The microstructure and mechanical properties of semi-solid thixocastings are much less dependent on casting size and cooling rate than the liquid castings. The step casting in the previous study is specially designed and simplified. The practical castings, e.g. turbocharger impellers, are more complex than the step castings. In this work, the turbocharger impeller is used to study the impact of section size representing the casting size on the microstructure and mechanical properties of semi-solid thixocastings, compared with the step casting. Section thickness decreases from 50 mm to 3.5 mm. In addition, the impact of casting thickness on the eutectic phase is also presented. The results reveal that the size of primary α-Al particles is insensitive to the casting thickness in semi-solid thixocasting. The cooling rate has a notable impact on the size and geometric characteristics of the eutectic silicon particles, but the impact is reduced by the following T61 heat treatment. The association between the casting thickness and casting defects is discussed, in order to further understand the impact of casting size on durability and reliability of real casting components.

Abstract: Owing to the phase segregation occurred during semisolid process, the ferroalloys products manufactured by semisolid process always exhibit inhomogeneous microstructure and poor mechanical properties. In this study, the post heat treatments including quenching treatment and tempering treatment with various processing parameters were carried out to improve the quality of Cr-V-Mo steel (JIS SKD61, AISI H13, DIN 1.2344) which processed by RAP (recrystallization and partial melting) processed. The microstructural characteristics (phase transformation, morphology of carbides, and distribution of alloying elements) and mechanical properties (hardness, tensile strength, elongation, impact toughness, and resistance to high-temperature wear) of specimens processed by RAP and heat treatments under various experimental conditions were investigated experimentally. Being quenched from 1050 °C after isothermal holding for 480 s and then tempered twice at 560 °C for 2 h, the microstructural evolution took placed in both former solid-phase and liquid-phase regions of the RAP-processed Cr-V-Mo steel specimen. During this post heat treatment strategy, the weakening of phase segregation, the redistribution of carbides, and the release of residual stress occurred and resulted in the improvement of microstructure and a good combination of mechanical properties.

Abstract: Near-net shape casting of wrought aluminium alloys has proven to be difficult due to a tendency towards hot tearing during cooling. Rheo-high pressure die casting (R-HPDC), has been shown to be an effective method of producing near-net shape wrought aluminium alloy castings. Limited information is available regarding the mechanical properties of age-hardenable wrought Al-castings produced by semi-solid metal forming. The purpose of this study is to investigate the effects of chemical composition and natural pre-ageing on the hardness and mechanical properties of rheo-HPDC 6xxx series Al-Mg-Si-(Cu) alloys in the T6 temper condition. The effects of the addition of Cu, as well as the (Mg+Si) content and Mg:Si ratio of the alloys are quantified. Alloys that are included are Cu-free 6004 and 6082, as well as Cu-containing 6013, 6111 and 6066. It is shown that the addition of Cu and excess Si result in higher hardness and strength. Natural pre-ageing has a significant effect (positive for 6004 and negative for the others) on the T6 properties. Good strength values can be achieved, but ductility is dependent on factors such as hot tearing during casting and incipient melting during solution heat treatment.