C.G. Kang

The effect of strain rate on macroscopic behavior in the compression forming of semi-solid aluminum alloy

Abstract The behavior of alloys in the semi-solid state depends strongly on the imposed stress state and the morphology of the phase, which can vary from dendritic to globular. For the optimal net shape forging of semi-solid materials (SSM), it is important to have knowledge of the material behavior with variation of strain rate. Therefore, to investigate the effect of the compression velocity of the deformation of aluminum alloy with a globular microstructure, the compression test for semi-solid aluminum alloy with a controlled solid fraction is performed by material test system provided with a furnace. The behavior of the semi-solid aluminum alloy is discussed for various solid fractions and die speeds. The material constants in the stress–strain relationship are also proposed.

Die design optimization for axisymmetric hot extrusion of metal matrix composites

Abstract Strain rate sensitive materials such as Ti alloys, superplastic materials and metal matrix composites (MMCs) can be deformed only in very narrow range of strain rate. In this work, a new process design method for controlling strain rate in workpiece during hot extrusion process is proposed. In this approach, a coupled numerical approach of finite element analysis and optimization technique to optimal profiled die which yields more uniform strain rate distribution in the deforming region is applied to the hot extrusion process of MMCs. Extrusion die profiles are defined by Bezier curves, and FPS (flexible polyhedron search) method is used as optimization technique. The change of relative deviation of strain rate, the progressive development of die profiles with increase of iteration for optimization and the corresponding strain rate distributions are investigated. In addition, the die profiles by optimization scheme for different extrusion ratios are compared with those by analytical solution.

Finite element simulation of die design for hot extrusion process of Al/Cu clad composite and its experimental investigation

Abstract A clad composite material is a variant of the typical composites which is composed of two or more materials joined at their interface surface. The advantage of clad material is that the combination of different properties of materials can satisfy both the need of good mechanical properties and the demand of user such as electrical properties simultaneously. This paper is concerned with the direct and indirect extrusion process of copper-clad aluminum rod. Extrusion of copper-clad aluminum rod was simulated using a commercially available finite element package, DEFORM which can be used to simulate metal forming and heat transfer, welding and machining processes. The simulations were performed for copper-clad aluminum rod to predict the distributions of temperature, effective stress, effective strain rate and mean stress for various sheath thickness, die exit diameters and die temperatures. From the simulations, it was found that the larger the die outlet diameter and the thickness of the sheath material are, the better results can be acquired. It was found that the experimental result for lubricant material with carbon oil has good agreement with extrusion force from finite element analysis.

An investigation of flow characteristics considering the effect of viscosity variation in the thixoforming process

Abstract The semi-solid forming process (thixoforming, rheocasting) is a novel forming process that has some advantages compared to conventional die casting, squeeze casting and hot/cold forging. In this study, the thixoforming process was selected for analysis in terms of billet handling and ease of the automation process. The thixoforming process consists of a re-heating process of the billet, billet handling, filling into the die cavity and solidification of the SSM part. In the filling process, two rheological models, i.e., a Newtonian and a non-Newtonian model (Ostwald–deWaele), were verified with experimental results. The Ostwald–deWaele model shows good agreement to the real flow and the filling phenomena in the die cavity. To give a boost to the economical efficiency of the thixoforming process and to ensure good forming results, a re-heating device coupled to die set was proposed and the initial billet temperature for the system was found by experimental results. This study presents an over-view of the application of numerical analysis for the simulation of a semi-solid metal forming process to reduce the lead time for the development of a manufacturing part in the industry field.

The effect of die shape on the hot extrudability and mechanical properties of 6061 Al/Al2O3 composites

Abstract Metal matrix composites (MMCs) fabricated by the compocasting process show a homogeneous distribution of the reinforcing fiber in the matrix. Microstructural observation of hot extruded MMCs reveals that as the extrusion ratio increases, the fiber alignment becomes improved, but fiber fracture occurs more severely. The mechanical properties of hot extruded MMCs are better than those of the matrix metal, with the exception of the elongation at failure, and are not influenced significantly by the extrusion temperature. The tensile strength and hardness of MMCs are improved to a greater degree by hot extrusion using a constant-strain-rate die. Also, there exists a critical extrusion ratio that gives maximum strength, which is 5.44 in this study. Additionally, fractograph of the tensile specimen of extruded MMC indicates the ductile fracture behavior of the matrix.

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.

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.

Model experiments for the determination of the heat-transfer coefficient and transition thermal analysis in the direct rolling process

Abstract The direct rolling process of steel is characterized by two rotating water-cooled rolls receiving a steady supply of molten metal that solidifies onto the rolls. A solidification analysis of the molten metal considering phase transformation and thermofluids is performed using the finite-difference method with curvilinear coordinates to reduce computing time. An enthalpy–specific-heat method is used to handle relaxation of latent heat during the phase change. The computed flow field is used to analyze solidification in the molten metal. The temperature distribution in the cooling roll is obtained using a two-dimensional finite-element method, because of the complex roll shape due to the cooling holes in the rolls and secure improvement in the accuracy of the calculation results. The energy equation of the cooling roll is solved simultaneously with the conservation equation of the molten metal in order to consider heat transfer from the molten metal. The calculated roll temperature is compared to experimental results and the heat-transfer coefficient between the cooling roll surface and the cast material is also determined from comparison of the measured roll temperature and the calculated roll temperature.

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.