Characterisation of semi-solid material mechanical behaviour by indentation test

Abstract

In many metal-forming processes, materials are loaded at high strains, high strain rates and high temperatures. Many experimental tests such as tensile, compressive or torque tests exist to determine the material rheology but are not well appropriate to achieve experiments on semi-solid alloys containing both solid and liquid. They fail due to disturbances such as necking, strain localisation, frictional effects or deformation heating in the material. In the first part of this paper, we propose a new mixed numerical–experimental methodology based on indentation tests and FEM simulations to determine the material rheology. This methodology is first applied to visco-plastic materials and then on semi-solid materials displaying a thixotropic behaviour. In particular, the influence of the microstructure and the strain rate on the semi-solid mechanical response has been studied on Sn–15% Pb alloys. It is shown that the globular morphology of the primary solid phase is really important to benefit from the thixotropic properties of the material during forming processes. In that case, the load level is lower than the one required for semi-solid materials having a dendritric primary solid phase.

Introduction

Simulations using the finite element method or the computer-aided design and computer-aided manufacturing (CAD/CAM) tools are frequently used to obtain the process sequences and to optimise the forging steps. In parallel, specific experimental tests have been developed to simulate the main different forging steps. However, they remain delicate to be performed. Similarly, various tests simulating forming processes where solid and liquid phase are mixed or when large strains undergo for example have been proposed. The optimisation of such forming steps requires to determine in particular the material rheology and the tool/material interface behaviour. For example, Mahmudi [1] proposes a novel technique for plane strain tension testing of sheet metals which directly measures stress–strain relationships. But this method eliminates friction. The tool/material interface behaviour is known to be a first-order parameter for processing and is unfortunately difficult to be characterised and modelled by the traditional tests. For these reasons, many authors have recently applied mixed numerical–experimental methods to determine the material and friction parameters [2], [3], [4], [5], [6], [7], [8], [9]. In this paper, we propose to use also a mixed numerical–experimental method based on the indentation test [10], [11], [12] to study the rheology of semi-solid materials used in thixoforming. In the first part, the methodology is explained and applied to a model visco-plastic material to study the influence of the indenter geometry. Then, in a second part, the rheology of semi-solid material is examined. Semi-solid metal forming, also called thixoforming, is a hybrid forming operation incorporating elements of both casting and forging processes. Practically, semi-solid processing can increase part forming rate, reduce the thermal shock imposed on the tools, increase tool life, reduce the force applied during forming [13]. The processing of alloys in semi-solid state and their rheological behaviours have been a lot investigated [13]. The solid fraction, the solid-phase morphology and the thermomechanical history are the parameters on which depends the semi-solid flow behaviour. Before working on steel, a high melting point material, it is interesting to validate the different experiments on low melting point alloys such as Sn–Pb or Al–Si, for example, relationships between microstructure and mechanical properties are studied on a Sn–15% Pb alloy from experimental investigation. Simulations have also been carried out to analyse the experimental results.