Processing techniques for functionally graded materials

Abstract

An overview of the achievements of the German priority program “Functionally Graded Materials (FGM)” in the field of processing techniques is given. Established powder processes and techniques involving metal melts are described, and recent developments in the field of graded polymer processing are considered. The importance of modeling of gradient formation, sintering and drying for the production of defect-free parts with predictable gradients in microstructure is discussed, and examples of a successful application of numerical simulations to the processing of functionally graded materials are given.

Introduction

In a functionally graded material (FGM) the properties change gradually with position. The property gradient in the material is caused by a position-dependent chemical composition, microstructure or atomic order. In the case of a position-dependent chemical composition the gradient can be defined by the so-called transition function c_i(x, y, z) which describes the concentration of the component c_i as a function of position. Already in 1972, the usefulness of functionally graded composites with a graded structure was recognized in theoretical papers by Bever and Duwez [1], and Shen and Bever [2]. However, their work had only limited impact, probably due to a lack of suitable production methods for FGMs at that time. It took 15 more years until systematic research on manufacturing processes for functionally graded materials was carried out in the framework of a national research program on FGMs in Japan. Since then, a major part of the research on FGMs was dedicated to processing of these materials and a large variety of production methods has been developed [3], [4], [5], [6].

The manufacturing process of a FGM can usually be divided in building the spatially inhomogeneous structure (“gradation”) and transformation of this structure into a bulk material (“consolidation”). Gradation processes can be classified into constitutive, homogenizing and segregating processes. Constitutive processes are based on a stepwise build-up of the graded structure from precursor materials or powders. Advances in automation technology during the last decades have rendered constitutive gradation processes technologically and economically viable. In homogenizing processes a sharp interface between two materials is converted into a gradient by material transport. Segregating processes start with a macroscopically homogeneous material which is converted into a graded material by material transport caused by an external field (for example a gravitational or electric field). Homogenizing and segregating processes produce continuous gradients, but have limitations concerning the types of gradients which can be produced.

Usually drying and sintering or solidification follow the gradation step. These consolidation processes need to be adapted to FGMs: processing conditions should be chosen in such a way that the gradient is not destroyed or altered in an uncontrolled fashion. Attention also has to be paid to uneven shrinkage of FGMs during free sintering. Since the sintering behavior is influenced by porosity, particle size and shape and composition of the powder mixture, these problems must be handled for each materials combination and type of gradient individually referring to the existing knowledge about the sintering mechanisms [7]. It has been demonstrated that the introduction of a controlled grain size gradient in addition to the composition gradient can balance different sintering rates [8]. In general, this optimization results in unjustifiable limitations for the gradients design. Additionally, variations in the particle packing density can lead to deformation of the part. To overcome some of these limits, the superposition of the internal driving forces for sintering with an external pressure by hot pressing [9], [10], [11] or hot isostatic pressing [12] is possible. If the temperature regions for densification are very different, application of a temperature gradient during consolidation [13], liquid-phase sintering [14], laser assisted sintering [15], and spark plasma sintering [16], [17] have been proposed. Although many of these methods for functionally graded materials were already developed in the early 1990s, there were still limitations concerning material combinations, specimen geometry and cost in 1995. One of the goals of the German priority program “Functionally Graded Materials” was thus the improvement of existing and the development of new processing techniques for functionally graded materials. Besides the well established powder metallurgical techniques other production processes suitable for metals and polymers with a low melting point were investigated in the priority program. Particular emphasis was also placed on modeling of production processes. Process simulations may allow the prediction of suitable processing parameters for FGMs in the future and reduce the considerable amount of experimental effort which is still necessary to produce a graded material free of macrodefects. In the following sections on powder metallurgy, melt and polymer processing and modeling, the achievements of the priority program are described. Each field is introduced with basic background information and literature, but a comprehensive literature review of the international literature is not attempted.