Mechanical and structural properties of bulk magnesium materials prepared via spark plasma sintering

Abstract

Field-assisted sintering is a modern approach to novel magnesium materials preparation; however, it is unclear whether it is better to sinter green compact or loose powder. This work focuses on preparing bulk materials from loose and cold-compacted magnesium powder through a field-assisted sintering technique – spark plasma sintering (SPS). Green compacts were prepared under a series of compacting pressures from 100 MPa to 500 MPa. SPS was performed at 400 °C, 500 °C, and 600 °C applying additional pressure of 100 MPa during the sintering process. Prepared materials were analysed regarding their microstructure, hardness, and microhardness and through the three-point bending test and fractography. The green compacts porosity decreased with increased cold-compaction pressure. The SPS positively affected porosity and mechanical properties only in loose powder and the lowest cold-compacted green compacts. Increasing cold compaction pressure of the green compacts above 200 MPa is therefore unfavourable for further SPS processing.

Introduction

Magnesium is a lightweight material mainly used in technical applications due to its good strength to weight ratio; however, the low ultimate tensile strength of coarse-grained cast Mg is a factor limiting its usage in the industry [1], [2], [3]. Improving a metals' ultimate strength by grain refinement is a well-known practice [4], [5], [6], [7]. However, due to magnesium's hcp structure, the methods of material grain refinement are limited. The limited number of active slip systems in the structure of Mg means that specific mechanical treatment conditions are required to ensure the improvement of mechanical properties. Severe plastic deformation (SPD) techniques resulting in intense grain refinement can significantly improve the mechanical properties of magnesium and magnesium alloys. However, the hcp structure of Mg limits the application of SPD treatment to this metal. Elevated temperatures have to be used for the SPD processing of magnesium to activate more slip systems and allow easier plastic deformation. As a result, component cracking or grain growth can occur when inappropriate processing conditions are applied. A modern approach to the grain refinement of magnesium bulk materials is powder metallurgy, which results in compact, dense and homogenous materials [8], [9], [10].

Sintering with SPS combines electrical energy with uniaxial mechanical pressure to convert powder into compacted material with the desired dimensions and density. The application of electric current is possible in pulse or continuous mode. A clear explanation of whether to apply electrical energy in pulses (tens of ms) or in continuous mode has not yet been fully presented. SPS is a rapid sintering method, with heat generated within the material by Joule heating [11]. Joule heating is responsible for a rapid heating rate because the heat is generated in the material and in the graphite die and no additional heating of the furnace is necessary. Diffusion processes (accelerated due to the heating of the material) combined with material plastic flow (due to the unidirectional mechanical pressure) are the main factors contributing to powder compaction. The plastic flow contributes significantly to the closure of pores between particles. As a result, high-density bulk material can be obtained in a shorter time and at a lower temperature when compared to the process without applied current. A short sintering time ensures only minor metallic grain growth. Degradation of the oxide layer (present on the surface of metal powder particles) due to plasma discharge was considered a significant factor in accelerating the metal sintering process. During the SPS process, plasma generation was initially assumed; however, clear experimental confirmation of the presence of plasma is still missing [12], [13], [14].

Several studies have focused on the preparation of Mg materials by SPS. Cheng et al., [15], studied the influence of magnesium particle size (from 38 to 550 µm) on the compressibility and hardness of SPS-prepared materials. The resulting porosity of the prepared materials was very close to cast magnesium; i.e., it was lower than 3% in all cases. The hardness values decreased with increasing powder particle size as follows: 42±2 HV0.1, 35±1 HV0.1, and 32±2 HV0.1. On the other hand, the greatest deformability was shown by the material prepared from the largest powder particles. Similar findings concerning metal grain size were presented in a study by Shen [16], in which rolled Mg powder was used as the initial material. The improvement in hardness corresponding to grain size refinement observed by Paraskevas [17] agrees with data presented by Cheng [15]. The high yield strength and tensile strength values were attributed to a finer microstructure. Thus the behaviour of the material corresponded well to the Hall-Petch theory [15]. Recycled magnesium chips yielded similar results, even though the metal grain sizes were very different. The materials produced by SPS had considerably finer microstructures compared to the basic materials (ingots). This refinement was the result of the intensive plastic deformation of the chips during machining. The microstructure was partially coarsened due to recrystallisation at elevated temperature; nevertheless, the resulting structure was still relatively fine-grained (600 µm) compared to the cast material (3500 µm) [17].

Besides other factors of the sintering process, the effect of sintering time on AZ91 magnesium alloy was studied by Mondet [18]. Extending the sintering time from 5 min to 60 min resulted in a reduction in material hardness from 86 HV5 to 81 HV5. Also, the compressive yield strength of the processed material decreased with increasing sintering time. In contrast, the ultimate compressive strength and ductility increased. The decrease in hardness was attributed to the combination of grain coarsening and dissolution of the Mg₁₇Al₁₂ intermetallic phase.

A wide range of SPS processing conditions for the preparation of magnesium have been studied. This article focuses specifically on sintering magnesium materials from loose powder and green compacts. The variables are sintering temperature (400 °C, 500 °C, and 600 °C) and green compacts cold compaction pressures (100–500 Mpa).