H.K Jung

Microstructural characteristics and mechanical properties of hypo-eutectic and hyper-eutectic Al–Si alloys in the semi-solid forming process

Abstract The characteristics of the globularization distribution and quantitative mechanical properties in the forming process of rheologically formed parts with an arbitrary shape need to be investigated to minimize occurrence of defects and to minimize subsequent machining. However, most of relevant research that has been performed is about the effects of die temperature and applied pressure on the filling behavior of semi-solid materials. Therefore, in the present study, the microstructures and mechanical behavior of Al–Si materials with wear resistance fabricated by the rheological forming (semi-solid forming) process are evaluated in terms of globularization and surface non-uniformity. The effect of alloy compositions and heat treatment on the mechanical properties is investigated for a part fabricated using the rheological forming process. The three-step reheating process for rheological forming was performed with wrought and cast aluminum alloys. The mechanical properties of the parts so formed are obtained for several locations by the tensile test and microstructure investigation.

Induction heating process of an Al–Si aluminum alloy for semi-solid die casting and its resulting microstructure

Abstract During induction heating, the relationship between time and temperature must be controlled exactly to obtain a homogeneous temperature distribution over the entire cross-sectional area. Because the initial solid fraction in the semi-solid die casting (SDC) process is the main parameter to achieve a homogeneous flow behavior of the liquid and solid phases and to prevent macro-segregation effects in the SDC process, an accurately controllable induction heating method must be selected for the reheating process. The purpose of this work is not just to obtain the desire solid fraction, generally about 50%, but also to ensure the optimal induction heating conditions of A356 alloy to reduce the temperature gradient of a 76 mm diameter ×90 mm length billet and to obtain a fine globular microstructure without grain coarsening (resulting microstructure). This work shows that, in the case of a three-step reheating process for the SDC process, the final holding time is the most important factor to maintain a fine globular microstructure without grain coarsening.

Induction heating of semisolid billet and control of globular microstructure to prevent coarsening phenomena

An important step in the thixoforming process is the induction heating of the raw materials to the semisolid state. Using this technology, the process behavior is satisfactory from the point of view of reproducibility and temperature control. Therefore, the objectives of this study are to define the correct relationship between coil length and billet length for uniform induction heating and to present the optimal reheating conditions suitable for the thixoforming process (or to secure a fine globular microstructure without liquid segregation).The optimal inductive coil on the induction heating process of semisolid billet machined to 76 mm diameter and 90 mm length to reduce the temperature gradient of the billet and to obtain the globular microstructure was theoretically designed and the suitability of the designed coil dimensions verified by reheating experiments.The globular microstructures of semisolid billet in heating and holding processes were controlled to prevent the coarsening phenomena of Al-7%Si-0.3%Mg alloy and to apply to the thixoforming process. In addition, the effects of the history of processing and induction heating parameters such as reheating time, holding time, holding temperatures, capacity of the induction heating system, and adiabatic material size on the globularization were investigated.It was concluded that in the case of a three-step reheating process, the final holding time is the most important factor and 2 min is suitable to maintain a globular microstructure.

Numerical analysis by new proposed coil design method in induction heating process for semi-solid forming and its experimental verification with globalization evaluation

Abstract In this paper, four procedures for optimizing inductive coil design are presented to reduce computational time and lead time. First, a theoretical coil design is proposed by defining the quantitative relationship between billet length and coil length. Second, the numerical simulation of the induction heating process using a general purpose finite element analysis code, ansys , is carried out to predict the temperature distribution. Third, an objective function on the basis of the optimization process for the coil design is proposed by introducing an optimization technique. Finally, the results of the optimal coil design are also applied to the induction heating process to obtain a fine globular microstructure. The proposed objective function based on the computational techniques and numerical simulation model would contribute to producing thixoformed parts with good mechanical properties and reducing lead time.

A study on a thixoforming process using the thixotropic behavior of an aluminum alloy with an equiaxed microstructure

Alloys with an equiaxed microstructure exhibit significantly lower flow resistance in the semisolid state than alloys with a dendritic microstructure. Their thixotropic behavior (solidlike in the unperturbed state and liquidlike during shearing) has been the basis for a thixoforming process. It is accepted today that thixoforming is a new net-shaped manufacturing technology in which the billet is heated to the semisolid state with coexisting solid-liquid phases.The thixoforming process has some industrial advantages, such as the successful fabrication of high-quality components with fewer inner defects, suitable for less machining, high productivity comparable to high-pressure die casting, and being an energy-saving system without the conventional melting process. It consists of inductive coil design, a billet reheating process, billet handling, filling into the die cavity, and solidification of the thixoformed part.This work presents an overview of all the detailed stages in the thixoforming process to manufacture the net-shaped product with good mechanical properties. An air compressor part with high strength has been fabricated by the thixoforming process.

Thixoforming of an aluminum component with a die designed by process simulation

Abstract A die design to fabricate a near net shape scroll component using semi-solid-aluminum alloys is performed by the finite element method (FEM). In the case of medium and high volume fractions of solid, the distribution of strain rate is calculated by using compression test data of semi-solid materials (SSMs). By using the computational results, the overflow position to obtain a homogeneous strain rate in die design process is proposed. The effect of the overflow position in die design parameters on liquid segregation in the thixoforming process has been theoretically investigated to manufacture the scroll component with good mechanical properties. Thixoforming experiments with the designed die are carried out successfully. The die filling patterns of SSM for variation of die temperature and pressing force have been investigated. The hardness of the thixoformed scroll products is evaluated in terms of the microstructure for each position.