Solid State Phenomena Vols. 192-193

Abstract: The microstructural evolution and basic refinement mechanism of the semi-solid-state SKD61 tool steel fabricated by recrystallization and partial melting (RAP) method were investigated experimentally. The effects of different parameters in RAP processing were systematically clarified to achieve the grain refinement of cast SKD61 steel in the semi-solid state. Results showed that, the microstructure of RAP processed specimen were finer and more globular than the microstructure of starting material. The distribution of finer solid particles in the RAP processed specimen was affected by the distribution of strain during predeformation. Uniform and spherical solid particles inclined to locate in the regions suffered from larger strain. The specimens subjected to a plastic predeformation at a lower temperature have a more uniform and spherical semi-solid microstructure than those specimens undergoing predeformation with the same reduction at higher temperatures. In addition, the grain refinement mechanism of SKD61 tool steel processed by RAP method was discussed.

Abstract: Thixoforming is an alternative forming method, by which intricate and complex-shaped products can be manufactured using a single production step. This technology allows a material’s microstructure to be altered profoundly. Typical microstructure of steels processed in this manner consists of quasi-polyhedral austenite grains embedded in a ledeburite-carbide network. This type of microstructure was produced by processing the experimental material in this study: the X210Cr12 steel. Since austenite is a metastable component depending on oversaturation with a number of elements, its thermal and mechanical stability needs to be known. This information is required for further modification and enhancment efforts. In previous experiments, the thermal stability was tested by thermal exposure. In the present work, the behaviour of austenite was explored under mechanical load at room temperature in a micro-compression test. A single block of austenitic material was used for making a test specimen with the dimensions of 2.4×2.2×4.9 µm. Its mechanical properties were measured and deformation stability was investigated using compressive deformation.

Abstract: The use of a specially designed hypoeutectic cast iron as a potential raw material for the thixoforming process is described in this paper. Thixoforming technology normally uses aluminum-silicon alloys such A356 and A357 as raw materials. Iron-based alloys are less common, despite the lower cost of the raw material. The paper describes the semi-solid behavior and corresponding final microstructure of a hypoeutectic gray cast iron after thixoforming tests. The Fe-2.6wt%C-1.5wt%Si alloy was prepared via conventional casting in sand molds. Samples were heated to the semi-solid state at 1160 and 1180oC and held at these temperatures for 0, 30, 90 and 120s, and then subjected to compression tests. Two-platen compression tests were carried out in an instrumented eccentric press in order to determine the semi-solid behavior. The holding time in the semi-solid range simulates the industrial heating process that is time-controlled rather than temperature-controlled. The semi-solid behavior indicated that the semi-solid cast iron behaves like aluminum-silicon alloys, presenting a stress of up to 24MPa under 80% strain and a corresponding apparent viscosity of up to 1.5*105 Pa.s at 1180oC. The final microstructure after compression testing was essential in determining the material’s morphological evolution. Tests revealed that heating up to the semi-solid range followed by thixoforming changes the material’s graphite morphology from type A to B (or E), but does not significantly affect the interdendritic arm spacing between graphite lamellae. The resulting structure is composed of fine graphite and pearlite.

Abstract: Flexibility of the Council for Scientific and Industrial Research’s Rheocasting System (CSIR-RCS) and its rheo-high pressure die casting (R-HPDC) technology is again demonstrated, as with aluminium alloys, by processing and shape casting of three different magnesium alloys (AM50A, AM60B, AZ91D) in a first attempt. All as-cast microstructures are characterised more by rosette shaped globules of the primary-(Mg) phase together with Mg17Al12 as evidence of non-equilibrium cooling rates. Surface liquid segregation is observed in the as-cast microstructure for all three alloys. Minor alloy additions of Mn, in composition specifications, results in the formation of Al8Mn5 intermetallic phase particles dispersed throughout the microstructure. All alloys were homogenised at 415 °C for 16 hours for the T4 condition. The Mg17Al12 phase dissolves with homogenisation while the Al8Mn5 intermetallic phase does not dissolve. The resulting tensile properties of all three alloys in the as-cast and T4 conditions are reported.

Abstract: A comparison of corrosion resistance of die-cast and semi-solid cast AZ91, AM60 and AM50 magnesium alloys was performed in different corrosive media by measurement of the open circuit potential, potentiodynamic scans and weight loss tests. Before testing, a heat treatment was carried out onto semi-solid cast alloys. Electrochemical measurements have shown that the semi-solid cast alloys have a different corrosion rate compared to the die-cast ones. The results could be correlated to the different microstructures of the samples produced by the different processes.

Abstract: In this paper, chip recycling technology combined with SIMA method which is called CR-SIMA method was adopted to prepare semi-solid billets. AZ91D magnesium alloy was refined by Er and its microstructural evolution was investigated during semi-solid isothermal treatment. The results show that Er can improve the feature of cast structure and decrease the grain size. Moreover, the γ-Mg₁₇Al₁₂ phase is well refined and disperses in the α-Mg matrix. A semisolid microstructure with small and spheroidal primary particles can be obtained after partially remelting. With increasing heating temperature, the dissolution of eutectic Mg₁₇Al₁₂ phase first took place, resulting in the primary dendritic grains coarsening into interconnected non-dendritic grains. With heating continuously, the residual interdendritic γ-Mg₁₇Al₁₂ at the edges of the primary grains melted in succession and the primary grains separated into small polygon grains. During the semi-solid isothermal treatment, the amount of liquid increased until the solid-liquid system reached its equilibrium state. At the same time, owing to the decreasing of interfacial energy, the grains gradually spheroidized and began to grow with a further increasing of the holding time.

Abstract: Microstructural evolution of ZK60 Mg alloy during partial remelting was investigated in this paper. The results show that ZK60 Mg alloy semi-solid billets were successfully produced by recrystallisation and partial remelting (RAP) process. Following partial remelting in each case, grain coarsening had occurred in the semi-solid state with increasing holding time. Liquid was present, during partial remelting all four temperatures, in the form of isolated intragranular droplets. These liquid droplets were produced by intragranular eutectic phase and entrapped liquid. Intragranular liquid droplets migrated and merged into some large rounded liquid droplets. After the spheroidization was completed, coarsening and coalescence occurred. When temperature is relatively low (the thickness of liquid is relatively thin), grains contacted with each other, which produced irregular-shaped grains.

Abstract: Rheological properties of semi-solid alloys are closely knit to the solid-phase microstructure. Parameters such as particle size distribution are commonly determined by 2D cross section analysis. The determination of mechanisms such as particle deagglomeration with increasing shear rate however, requires information on the 3D spatial distribution. By means of synchrotron radiation tomography and SEM on AlCu samples, particle size distributions and the not yet microscopically observed interrelation of shear rate and particle agglomeration in thixo-material is investigated.

Abstract: Several rheocasting processes have been developed or applied in the world. One of the new rheocasting processes is the limited angular oscillation (LAO), in which the molten metal is rapidly cooled and slightly mixed during initial stages of solidification. Squeeze casting (SQC) using semi-solid slurry produced by LAO (Rheo-SQC) has been developed. Microstructure and mechanical properties of squeeze cast semi-solid slurries have been investigated. Complete parts with little defects have been produced. The ultimate tensile strength and elongation of semi-solid cast samples are higher than those of the liquid cast samples. It can be concluded that the rheo-SQC is a feasible process.

Abstract: A standard method used to determine material properties of semi-solid slurries is the squeeze flow experiment in which a fixed amount of material is squeezed under constant force or velocity. The relation between the force and the displacement provides information about the rheology of the slurry. The thixotropy and the time response of the sample however is rarely, if ever, taken into consideration. In this work we study how thixotropy affects the flow characteristics and consequently the predicted material properties. We show that depending on the method of compression and the thixotropic constants the flow can be significantly different. Therefore the predicted material constants can vary and hence cannot be unique.