Chapter 1 Introduction

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CHAPTER 1. INTRODUCTION

With the legislative requirement for the auto industry to decrease both fuel

consumption and emissions, the demand for light metals and alloys has increased. This

exigency has caused a decrease in the quantity of ferrous-based components in modern

cars (see Table 1.1). These parts have largely been replaced by light metals, most

commonly aluminium, and plastics. Economic restrictions within the auto industry

require the replacement of a material to be cost effective. Since P/M processing can

often be used to produce low cost components(1), superimposed on the decrease in

iron-based alloys is an increase in the use of (ferrous) powder metallurgy parts. Despite

the increase in both P/M components and light alloys, there has been no coupling of

the two, and the utilisation of Al-based P/M alloys in the auto industry has been

negligible.

Material 1980 (kg) 1996 (kg) % Change

Ferrous (all) 1123.8 983 -12.5

Plastics 88.5 111.1 25.5

Aluminium 59.0 88.7 50.3

Copper and Brass 15.9 20.4 28.3

P/M parts 7.7 13.4 74.0

Zinc die castings 9.1 7.0 -23.1

Mg castings 0.7 2.5 257.1

Fluids and lubricants 80.7 89.6 11.0

Rubber 59.4 63.1 6.2

Glass 37.9 42.6 12.4

Other materials 42.9 45.1 5.1

TOTAL 1525 1466.5 -3.8

Table 1.1. Comparison of the amount of material in a typical family car, from

(2).

Chapter 1 Introduction

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The limited application of Al powder alloys in the auto industry may be a result of the

inferior properties of commercially available alloys (see Table 1.2); the compositions of

which are very similar to wrought alloys. Since a material is designed as much for its

processing route as it is for its intended application, cast alloys are very different in

composition to wrought, directionally solidified or rapidly solidified alloys. This maxim

has been applied to ferrous powder metallurgy (P/M), where a series of Fe-Cu alloys

has been developed specifically for press and sinter processing. The same can not be

said for Al P/M alloys. There is no evidence that these commercial alloys were

designed specifically for sintering, and this may be the reason that the obtained

properties fall far below their wrought counterparts. As a result, more complex and

expensive methods have been employed to increase the properties of these alloys. This

includes the use of pre-alloyed powders(3) and hot working(1).

Density T6 Properties

Alloy Nominal Composition (g cm-3) (%) UTS (MPa) Strain (%)

201AB Al-4.4Cu-0.8Si-0.5Mg 2.58 92.9 323 0.5

2014 Al-4.4Cu-0.8Si-0.5Mg-0.8Mn 2.8 100 483 13

601AB Al-1.0Mg-0.6Si-0.25Cu 2.52 93.7 232 2

6061 Al-1.0Mg-0.6Si-0.3Cu-0.2Cr 2.7 100 310 12

Table 1.2. Comparison between properties of wrought and powder aluminium

alloys, from (4).

To gain more widespread use, especially in the automotive industry, there is a need to

improve the properties of Al powder alloys. This, therefore, constituted the broad aim

of this thesis.

To achieve this aim, two different techniques were used. The first uses German and

Rabin's(5) phase diagram characteristics of ideal sintering systems to select an ideal

alloy system. The selection of this system was not to be constrained by conventional

wrought alloy design strategies. The second technique involved the judicious use of

selected trace elements to improve the sintering characteristics of Al powder alloys.

Chapter 1 Introduction

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Chapter 2 is a critical analysis of the literature on powder metallurgy in general and Al

P/M specifically. The oxide layer formed on Al, trace element effects and rapid

prototyping are also considered. Chapter 3 provides the experimental detail for the

remaining sections. In Chapter 4, the first of two trace element effects is presented and

discussed. Small additions of Mg are essential to the sintering of Al and its alloys. The

sintering of Al with Sn (as an ideal sintering system) is contained in Chapter 5.

Knowledge from these two chapters is transferred into Chapter 6 which is the

development of an alloy suitable for use in freeform fabrication. The second trace

element effect is the basis for Chapter 7, while Chapter 8 takes this knowledge to

produce an alloy with strength exceeding that of currently available commercial alloys.

Chapter 9 presents the conclusions and future work for this thesis.