Solid State Phenomena Vols. 217-218

Abstract: The thixocasting technology is chosen to produce the turbocharger impellers as it is capable of producing castings with high precision dimension, free of oxide and shrinkage porosity defects and therefore long service life. The thixocasting turbocharger impellers have the similar mechanical property to those produced by forging and machine processes, but much less costs. In the thixocasting process, the semisolid slurry with certain solid fraction is injected into mould at a high speed. Since high viscosity and thixotropic characteristics, the semisolid slurry reveals unique flow condition and characteristics in the filling process. So, its very desirable for the die design, process optimization, and defect control to visualize the high-speed filling process of semisolid slurry by numerical simulation. In this paper, several commonly used viscosity models for semisolid slurry are analysed. The Power law cut-off model (PLCO) model is selected to simulate the filling process in the thixocasting process of the impellers of 319s alloy, compared with actual results by partial filling testing. The causes of the casting defects are also analysed, indicating that the simulation results can accurately reproduce the filling process of semisolid slurry, and PLCO model is a successful choice for simulating the filling process of semisolid slurry with high solid fraction.

Abstract: Two objectives were targeted: 1) compare the high cycle fatigue behavior of rheocast aluminum alloy 357 prepared by the swirl enthalpy equilibration device (SEED) and by the Council for Scientific and Industrial Research (CSIR) process, and 2) study the effect of surface liquid segregation (SLS) on the fatigue behavior of the CSIR material. Rectangular hourglass specimens machined from rheocast plates were tested at four stress amplitudes in axial fatigue with a stress ratio of R = -1 and a frequency of 20 Hz. Results obtained for SLS free specimens show that the SEED and the CSIR processes produce rheocast materials with comparable high cycle fatigue properties, 115 MPa at 10⁷ cycles. In order to study the influence of surface liquid segregation, slightly polished specimens with a remaining SLS of nearly 750 microns thick were also tested. According to the results, the SLS reduces the average fatigue strength by approximately 5% (110 MPa vs. 115 MPa at 10⁷ cycles). For SLS free specimens, the fatigue crack initiated at shrinkage cavities, oxide films or in the alpha globules. On the other hand, for specimens with SLS, no crack initiation in the alpha globules was observed. The main crack initiation mechanism was identified to be a deformation incompatibility between regions characterized by higher silicon content compared to nominal eutectic regions. The originality of the work is provided by the rigorous comparative analysis of the fatigue performance of components produced in two different rheocasting facilities, but tested in a single laboratory. It is also the first fundamental research published on the mechanical effect of surface liquid segregation. It confirms that SLS should be removed in critical areas in order to optimize the fatigue resistance of rheocast components.

Abstract: Mechanical properties of all metals depend predominantly on the type and morphology of their microstructure. Microstructure attributes can be altered by various heat treating and thermomechanical treatment procedures. One of the advanced techniques profoundly affecting the microstructure evolution is semi-solid processing. It can produce unconventional microstructures even in conventional steel types. Moreover, subsequent heat treatment can also deliver a wide range of microstructures and correspondingly varied mechanical properties. In the present experimental programme, the X210Cr12 ledeburitic tool steel was studied. Its initial annealed microstructure consisted of ferritic matrix, chromium carbides and globular cementite particles. The semi-solid processed structure, on the other hand, contained polyhedral austenite grains embedded in carbide-austenite network. The austenite volume fraction exceeded 95 %. This microstructure was then altered by subsequent heat treatment or thermomechanical treatment. The experimental programme comprised three stages. At the first stage, the effects of the rate of cooling from the semi-solid region to the ambient temperature on the nature and morphology of the ledeburitic network and the austenitic grain size were explored. The second stage was aimed at the impact of tensile and compressive deformation applied after transition through semi-solid state on the microstructure evolution and, in particular, on grain size. Once suitable processing conditions and parameters were identified, the treatment led to a recrystallized austenitic microstructure with an average grain size of less than 3 μm. As high volume fractions of austenite were obtained, the third stage involved exploring the effects of thermal exposure. The stability of austenite and the decomposition of austenite into other microstructure constituents were mapped. Metallographic observation revealed a resulting wide range of microstructures from fine pearlite to martensite, depending on the heat treating schedule.

Abstract: The effects of annulus gap width, stirring power and stirring frequency on the microstructure of the semi-solid A356 aluminum alloy slurry have been investigated by the annulus electromagnetic stirring (AEMS) technology The results show that narrow annulus gap , strong stirring power and high stirring frequency are advantageous to obtain the small spherical primarymicrostructure, the smaller the annulus gap width is, the bigger the stirring power is, and the higher the stirring frequency is, the more uniform, the smaller and the more spherical the microstructure is. So the high stirring frequency, narrow annulus gap, strong stirring power are beneficial to obtain the fine and spherical semisolid microstructure in AEMS. Also the results indicate that the primary particles are globular, small and distribute homogeneously in the AEMS.

Abstract: Anodising of aluminium alloy components can be used for decorative purposes and to enhance the corrosion and wear resistance. In this research, four different Al-alloy series namely Al-Cu-Mg (2xx/2xxx), Al-Si-Mg (3xx), Al-Mg-Si (6xxx) and Al-Zn-Mg-Cu (7xxx) were used to produce plates by rheo-high pressure die casting (R-HPDC). Castings produced by R-HPDC suffer from a surface liquid segregation (SLS) phenomena. This surface layer is enriched with alloying elements and it is shown to have different characteristics as compared to the bulk material. The study focused on the anodisability of plates from the four series produced by R-HPDC with and without the SLS. The samples were anodised at a commercial facility. The oxide layer thickness and homogeneity were studied using optical microscopy.

Abstract: In this paper, a new method for preparing semi-solid Al-25%Si alloy slurry named as Rotating-Rod Induced Nucleation (RRIN) was adopted. In this method, liquid metal (velocity 0.66 kg/s) flew toward a vertical stainless steel rod (45 mm in diameter) which was self-revolving on its axis. Molten alloy was spread around the rod surface under the effect of rotating, which caused supercooling and induced nucleation of primary Si in the molten metal. After spread around the rod, molten alloy flew off the rod and down to a graphite crucible. The influence of rotating speed on the microstructures in the semi-solid slurry was studied. The refining mechanism of primary Si was also analyzed according to the change of the spreading state, the number of primary Si particles (PSPs) in per unit area (N), and the distribution of PSPs size under a series of different rotating speeds (from 100 r/min to 1000 r/min, and at intervals of 100 r/min). The results showed that, under the lower rotating speeds such as 100 and 200 r/min, the molten alloy flew downward along the rod. With the rotating speed increasing, a molten alloy film around the rod was formed under the effect of the tangential force caused by the rotating rod, and the film thickness thinning, the spreading area and the N was increased. The uniform supercooling in the whole molten metal film was obtained under the condition of higher rotating speeds such as 500 and 600 r/min, which promoted nucleation of the PSPs significantly. It is called Rotating-Rod Induced Nucleation (RRIN). After spreading around the rod during short time, the molten metal film was thrown away the rod rapidly under the effect of centrifugal force, which prevented further supercooling and growth of PSPs. Under the condition of too higher rotating speed of the rod such as up to 700 r/min, the molten melt flew off from the rod apparently before spreading around the rod surface, molten metal film as mentioned above was destroyed, RRIN was weakened. The microstructure with fine PSPs (27 μm) was distributed uniformly in the semi-solid slab obtained under the condition of rotating speed of 600 r/min and pouring temperature of 785 °C in this experiment. Keyword: high silicon aluminum alloy; Rotating-Rod Induced Nucleation; Si phase; refinement.

Abstract: The 6xxx series alloys are well known for desirable combinations of high strength, weldability, corrosion resistance and formability. This paper investigates the influence of chemical composition on the solution heat treatment parameters of rheo-high pressure die cast (R-HPDC) 6xxx series aluminium alloys. The presence of copper in the 6xxx series aluminium alloys affects the solution heat treatment by promoting incipient melting. The incidence of incipient melting is investigated for the R-HPDC alloys using Differential Scanning Calorimetry (DSC) and optical microscopy. R-HPDC is known to produce surface liquid segregation and centre-line liquid segregation when processing the alloys and these areas are the most susceptible to incipient melting. The applicability of single and multiple step solution heat treatments are investigated. The alloys used for this study include the Cu-free alloy 6082, as well as the Cu-containing alloys 6013 and 6111.

Abstract: Continuous metal matrix composite strip casting (MMCS-ing) composed of six 0.3-mm diameter annealed bare copper wires in a eutectic SnPb matrix was manufactured by a two-roll melt dragged processing (TRMD-ing) method at a rate of 0.3 m/s. The wires were dragged through a semisolid pool with a fibre contact time of approximately 0.2 s. The required gap between rolls to thixoforge the semisolid material around the wire filaments was approximately 1.4 mm. A successful continuous composite strip casting was achieved with a notably good degree of wire alignment. No cracks were observed at the copper wire/matrix interface. However, regions of porosity occurred in the matrix; their possible formation mechanisms are discussed. The solidification structure of the matrix was analysed, and the analysis results indicated the formation of small globular grains measuring approximately 3 μm in diameter. The specimens were evaluated for their tensile properties and compared with the rule of mixtures. The surface fracture analysis indicated a good matrix/fibre union. MMCS-ing is an economically viable process and has significant advantages over other metal matrix composite (MMC) fabrication methods.

Abstract: Turbocharger impellers undergo extreme cyclic speed conditions and thus fatigue fracture in service. Any kind of defects, even defects with sizes of tens of micrometres, can make significant impact on service life of the impellers. The alloy used for thixocast impellers is the aluminum alloy 319s. T61 heat treatment is used to improve mechanical properties. However, the solution treatment in the standard T61 heat treatment of the 319s alloy can induce incipient melting defects. This paper carried out systematic study of formation of defects caused by the incipient melting during solution treatment in the thixocast and heat treated impellers, using optical and scanning electron microscope. Based on the study, the critical temperature of incipient melting is detected. An optimized two-stage solution treatment is therefore proposed.

Abstract: Semi-solid forming processes can lead to hot cracking. The process parameters responsible for hot cracking are still not well known. The paper aims at contributing to a better understanding of the process parameters promoting this phenomenon. Finite element simulations of thixoextrusion tests were carried out to analyse the thermal and mechanical fields. Comparisons between experimental and calculated results enable to propose a hot cracking criterion.