Papers by Author: Kai Kun Wang

Abstract: Sustainable development is increasing in importance with restrictions on emission and carbon footprint. Similarly, both energy and resources efficiency are required, and at the same time, cost-efficiency is required. The current paper is focusing on carbon footprint, energy usage and material use efficiency of semisolid metal casting. A detailed analysis is made on the RheoMetal process, which is benchmarked to conventional HPDC casting. The analysis includes the gating system and the importance of the use of primary or secondary material. It furthermore includes a discussion of process yield and benefits based on process capability.

Abstract: Due to the excellent thermal conductivity and high strength of aluminum alloy and unique advantages containing shock absorbing, vibrations dampening and corrosion resistant of magnesium alloy, Al/Mg double-layer tubes have been widely utilized in household appliances, automobiles, aerospace industries, and high-speed trains in recent years. Conventionally, double-layer tubes are produced by welding, extrusion, hydroforming, magnetic pulse cladding. These processes are either complex or highly energy-consuming. To improve efficiency and reduce energy consumption, a new technology, thixo-co-extrusion (TCE), is put forward for production of double-layer tubes. In this paper, the thixo-co-extrusion of a 7075/AZ91D double-layer tube is investigated by numerical simulation. This study assesses the influences of extrusion velocity, the thickness ratios of two layers, the reheating temperatures of billets and the preheating temperature of die on the flow behaviors. The results show that the extrusion velocity and the thickness ratios significantly influence the contact pressures on the interface as well as the velocity fields. Besides, the reheating temperatures of billets and the preheating temperatures determine the distribution of temperature fields. Under the contact pressures and a certain temperature, atomic diffusion takes place on the interface between inner and outer layer, which leads to metallurgical bonding of the interface. The paper contributes to a better understanding of the thixo-co-extrusion technology for the production of double-layer tubes with desirable mechanical properties.

Abstract: To investigate the influence of refined grains on the microstructure of 7075 aluminum alloy in semi-solid state, a new strain induced melting activation (SIMA) method was put forward containing two main stages: pre-deformation with equal channel angular pressing (ECAP) method and isothermally holding in the semi-solid temperature range. The breaking up and growth mechanisms of the grains and kinetics of equiaxed grains coarsening during the semi-solid holding were investigated. The results showed that the average grain size after ECAP extrusion decreased significantly, e.g., microstructure with average globular diameter less than 5μm was achieved after four-pass ECAP extrusion. Obvious grain coarsening had been found during isothermal holding in the semi-solid state and the roundness of the grains increased with the increasing holding time. The proper microstructure of 66.8μm in diameter and 1.22 in shape factor was obtained under proper soaking condition (at 590°C for 15 min). Two coarsening mechanisms, namely, coalescence in lower liquid fraction and Ostwald ripening in higher liquid fraction contributed to the grain growth process.

Abstract: The problem of an uncompressible power-law fluid has long been the challenge in semi-solid forming area. In this paper, the flow of a power-law fluid film on an unsteady stretching surface is analyzed by the means of Boubaker Polynomials Expansion Scheme (BPES). Analytic solutions were given and compared with the numerical results for some real power-law index and the unsteadiness parameter in wide ranges. The good agreement between them showed BPES could be used effectively to solve the flow of nonlinear power-law fluid in semi-solid state.

Abstract: An inventive method of flexible thixo-extrusion technology was put forward, in which the stress status on flow front surface was controlled during thixo-forming. A complex part with different dimensions in three axis directions was designed. The velocity of sliding die was controlled by spring. This kind of die construction could make sure that the flow front of the slurry was always in compressive stress status. The velocity field, effective strain fields, effective stress fields were simulated and analyzed. The results showed that the thixo-extruded complex part was produced under compressive forming state, which forecast good mechanical properties in the practical experiment.

Abstract: In the heavy machinery and energy industry, forged connecting rods have been used more and more widely. Due to the complex process and large-size, it is a challenge to ensure the quality of large forgings. Different kinds of defects were produced during forming process. In this study, the forming process of S20S forged connecting rod was first analyzed. Then SEM and energy spectrum were conducted. The results showed that inclusions of sulfide and oxide were the main defects during forging of S20S connecting rod.

Abstract: Semi-solid forming technology is a short metal processing method between the liquid casting and solid forming. Many forming factors have been found concerning the quality of the forming part. In this study, the influences of slurry filling sequence were investigated in semi-solid forming one complex aluminum part. Two different slurry filling methods were designed. Method one is thixo-upsetting + compound extrusion, method two is thixo-upsetting + extrusion. The experimental results showed that the optimised method of thixo-upsetting + extrusion could be the best strategy in improving the part quality during thixo-forming of aluminum alloy A356.

Abstract: Thixo-forming is in the forefront of metal processing technology in the 21st century. The research on thixo-co-extrusion of multi-layer tube as extension and development of the semi-solid forming technology is a completely new processing method for the composite material forming and is of great significance, in which different semi-solid billets (slurries) are extruded at the same time to form multi-layer tubes. In this study, different sizes of column-shaped and ring-shaped billets of Al/Mg alloys were firstly prepared by using specially designed molds. Then they were reheated by electric-resistance furnace，microstructures from different heating laws were investigated. Lastly, FEM simulation on thixo-co-extrusion of double-layer tube with A356/AZ91 was conducted.

Abstract: Transmission electron microscopy (TEM) was used to assess the microstructural changes accompanying semisolid molding of a creep resistant Mg-5%Al-2%Sr alloy. It was revealed that the processing created thixotropic morphologies with globular solids of α-Mg, having well-developed sub-structures. The quickly-solidified liquid fraction, enriched in Al and Sr, led to the formation of increased volumes of a lamellae type Al4Sr phase, surrounding the secondary α-Mg. Traces of Mg17Al12 still existed, predominantly as irregular shape compounds located mainly in grain interiors. Both the primary solid and high melting point precipitates of Mn-Al modified with Sr, acted as nucleation substrates for secondary α-Mg grains.