S.W Youn

The effect of test specimen size and strain-rate on liquid segregation in deformation behavior of mushy state material

Abstract For optimization of net shape forging process with semi-solid materials (SSMs), it is important to predict the deformation behavior of material. In this study, compression experiments have been performed to investigate the deformation behavior of SSM with variation in processing parameters such as test specimen size and strain-rate. Stress–strain curves and microstructures of SSM were investigated. The characteristics of flow between solid and liquid phase considering liquid segregation is examined through the above experiments. Moreover, porosities were measured to evaluate the effects of experiment parameters on liquid segregation quantitatively.

Mechanical properties of particulate reinforced metal matrix composites by electromagnetic and mechanical stirring and reheating process for thixoforming

Abstract Both mixing processes of electromagnetic stirring and mechanical process technique were used to fabricate particulate metal matrix composites (PMMCs) for variation of particle size. The PMMCs were tested for their tensile test with and without heat treatment with T6. PMMCs fabrication processing conditions for both electrical and mechanical processes are also suggested. In order to study thixoforming of PMMCs, fabricated billet are reheated by using the optimal coil designed as a function of length between PMMC billet and coil surface, coil diameter, and billet length. The effect of reinforcement distribution on billet temperature variation is investigated with calculated solid fraction theory proposed as a function of matrix alloy and volume fraction of reinforcement.

Horizontal reheating of aluminum alloys and semi-solid casting for a near net shape compressor component

Abstract A semi-solid casting will have a higher internal integrity, mechanical properties and dimensional accuracy than a conventional castings. This process can reduce the manufacturing costs and finished weight for critical components. Semi-solid castings are capable of greater dimensional repeatability, this affording considerable savings when extensive machining, salvage and scrap are key variables in the current automotive product. The most important factors regarding the semi-solid die casting process are the reheating method of the raw materials to the semi-solid state, the specifications of the semi-solid die casting machine, the optimal injection conditions and die design. Thus, in the present work, a horizontal-type induction heating system to obtain the optimal reheating conditions for the semi-solid die casting process was designed and manufactured. The mechanical properties of semi-solid die castings for varying die temperatures, injection velocity and applied pressure were investigated, and the relationship between applied pressure and stroke to assess semi-solid die castability was proposed. Semi-solid die casting experiments to replace heavier cast iron compressor products with lightweight aluminum castings were also carried out. The possibility of their use for compressor parts was verified.

Experimental investigation of semi-solid casting and die design by thermal fluid-solidification analysis

Abstract In the semi-solid casting process, various defects are strongly affected by the die shape rather than by other fabrication parameters because the semi-solid materials have to travel a relatively long distance to fill the die cavity through a gate during the filling time. To obtain high quality component with the semi-solid casting process, it is important to obtain a homogeneous distribution of solid particles without liquid segregation until the die is completely filled. A gate shape with a different runner system was investigated to produce a near-net-shape compressor component called a cylinder block. Die design by computer simulation has some advantages compared with the conventional process that is performed by the designer’s experiences and trial-and-error. The experiment of semi-solid casting was performed with the conventional die design concept. Various defect shapes were found on the surface of the semi-solid cylinder block parts. Evaluation of die gate system to obtain part without defects was realized by computer simulation. MAGMAsoft performed the die filling and solidification analysis of the semi-solid casting process with the add-on module ‘thixo’. A designed new gating system considering thermal fluid and solidification phenomena was suggested by the filling simulation.

The effect of globular microstructure size on the mechanical properties in reheating process of aluminum alloys

One of the important steps in semi-solid forming is the process of reheating raw materials to the semi-solid state. This process is not only necessary to achieve the required semi-solid state of the billet, but also to control the microstructure of the billet. In the reheating process, the globule size is determined by the holding time of the final reheating step. Therefore, some experiments to investigate the relationship between the mechanical properties and the holding time in the last heating step were performed. The alloys used in this experiment were 357, 319, and A390 alloys. The experiments of reheating were performed using an induction heating system with a capacity of 50 kW. This article shows the evolution of the microstructure according to the holding time of the last reheating stage. Furthermore, to evaluate the effect of globule size as determined by holding time of the final reheating step, uniaxial tension tests were performed. The stress-strain curves were plotted according to the holding time, and a relationship between the microstructure and the flow stress of semi-solid material was formulated.

Mechanical characteristics evaluation of hollow shape part with metal matrix composites fabricated by thixoforging process

Abstract Thixoforming process has been accepted as a new method for fabricating the net shaped metal matrix composites (MMCs) with lightweight and wear resistance. In this paper, the effect of volume fraction and reinforcement sizes on mechanical properties in cylinder liner part of MMCs has been investigated with processes parameters such as pressure and velocity. Moreover, the methods to obtain the thixoforged composites cylinder liner with high quality has been proposed. To evaluate the composites cylinder linear fabricated at the conditions proposed, mechanical properties of fabricated composites cylinder linear were compared with those of commercial composites cylinder linear.

Reheating Process of Metal Matrix Composites for Thixoforming and Their Inductive Coil Design

In this work, the fabrication processes of particulate metal matrix composites (PMMCs) with a homogeneous distribution of reinforcement and their reheating for thixoforming were studied. Electromagnetic stirring was used to fabricate PMMCs to vary particle size. PMMCs were tested before and after the reheating process using a tensile test with and without heat treatment. The combined process conditions for fabricating the PMMCs are also suggested for a variety of particle sizes. For the thixoforming of PMMCs, fabricated billets are reheated by using an induction heating system with a maximum capacity of 20 kW. The effects of the dispersion state of the reinforcements on the reheating temperature and microstructural morphology were investigated.