Thermal exchange effects on steel thixoforming processes

Steel thixoforging is an innovative semi-solid forming process. It allows the manufacturing of complex parts and minimises the forming load. This work aims to identify and characterise the main feature zones of a thixoforging part. The material flow and the forging load are dependent on the thixoforging speed, the tool temperature and the initial temperature of the slug. The data are obtained for C38 thixoforging steel. A specific extrusion tool was designed that integrates the heating of the tool and the slug. This tool was set up on a high-speed hydraulic press. This work highlights the effects of heat exchange on the microstructure, the internal flow and the mechanical characteristics of thixoforging material. These heat exchanges depend primarily on the working speed and tool temperature. The internal flow is composed of three distinct zones. Among them, only semisolid zone is observed during working. The microstructures of thixoforming C38 steel consist of ferrite, pearlite and bainite

Personal Bankruptcy Law, Fresh Starts, and Judicial Practice

We explore the ways French judges respond to the possibility of discharging personal debts in exchange for liquidation of debtors’ assets. We present empirical results on the determinants of judicial selection between debtors whose debts are wiped out and those who have to reimburse them. We find that French judges tend to disqualify debtors with multiple creditors from debt discharge, and are sensitive to regional labor market conditions. These empirical results help us understand better how French personal bankruptcy laws perform compared to other national systems. Finally, our results serve to fill the gap between bankruptcy rules and judicial practice.Personal bankruptcy, over-indebtedness

INFLUENCE OF EXPERIMENTAL CONDITIONS ON MATERIAL RESPONSE DURING THIXOEXTRUSION PROCESS

The behavior of aluminum alloy to deformation is strongly influenced by the morphology of the microstructure. This paper illustrates several experimental research activities for thixoextrusion of 7075 aluminum alloy being carried out within the “Arts et Métiers ParisTech” of METZ. Inductive re-heating of the aluminum billet is the method to enable the desired liquid fraction for thixoextrusion. Experimental results show that after induction reheating the solid particles obtained has globular shape and homogeneous dimension. A sample obtained from a direct extruded bar is inserted in a die and thixoextruded after reheating in semisolid state. In laboratory experiments the temperature it was directly controlled applying thermocouples for temperature measurements in the slug and also in the die. The experimental tests were made for two different working speeds on the mechanical eccentric press. The preliminary experimental results on extrusion load and microstructure evolution of the product are reported. These results will contribute to implementing the specific boundary condition of the manufacturing process to the future numerical analysis of the current project activities

Impact of experimental conditions on material response during forming of steel in semi-solid state

Semi-solid forming is an effective near-net-shape forming process to produce components with complex geometry and in fewer forming steps. It benefits from the complex thixotropic behaviour of semi-solids. However, the consequences of such behaviour on the flow during thixoforming, is still neither completely characterized and nor fully understood, especially for high melting point alloys. The study described in this paper investigates thixoextrusion for C38 low carbon steel material using dies at temperatures much lower than the slug temperature. Four different process parameters were studied: the initial slug temperature, the die temperature, the ram speed and the presence of a ceramic layer at the tool/material interface. The extruded parts were found to have an exact shape and a good surface state only if the temperature was below a certain value. This critical temperature is not an intrinsic material property since its value depends on die temperature and the presence of the Ceraspray©layer. Two kinds of flow were highlighted: a homogeneous flow controlled by the behaviour of the solid skeleton characterized by a positive strain rate sensitivity, and a non homogeneous flow (macro liquid/solid phase separation) dominated by the flow of the free liquid. With decreasing ram speed, heat losses increase so that the overall consistency of the material improves, leading to apparent negative strain rate sensitivity. Finally, some ways to optimise thixoforming are proposed