The quality of a powder compact depends on the uniformity of the particle size distribution throughout its volume and the retaining of a refined microstructure during the final consolidation process. This paper deals with the creation of uniform green compacts from powders and the dynamic consolidation process. An overview of basic issues and problems concerning the behaviour and handling of granular materials is presented, emphasising recent results on particle size segregation induced through vibrations and occurring in granular flows. The implications of segregation on the production of green compacts are introduced followed by a discussion of their formation via ultrasonic vibrations. The dynamic (shock) consolidation process to form dense compacts with metastable or refined microstructures from green compacts is described. A critical review and discussion of the kinetics and energetics in the shock consolidation process, including particle bonding and shock energy distribution, is presented.

Manufacture of powder compacts

The microstructures of rapidly solidified Al–10Mg alloy powders have been observed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. A technique is described which enabled individual powder particles to be made electron transparent. The alloy powders were then consolidated by cold compaction followed by hot extrusion using a wide range of temperature compensated strain rates. The microstructures evolved are reported and it is shown that there is a correlation between the microstructures and the process parameters. It is also shown that the initial powder structure is an important variable. Magnesium is retained in solid solution in the extrudate but is metastable at room temperature leading to decomposition over long periods of time. However, the very fine grain size observed (1–3 μm) is thermally stable up to 300°C and aging processes may be utilized to obtain acceptable structures.

Microstructural aspects of extrusion of rapidly solidified Al–10Mg alloy powder

Thermomechanical Processing of Aluminum Alloys

The present paper describes two innovative enhancements of the common direct extrusion process: on the one hand, curved profile extrusion for the production of curved profiles with improved properties in comparison to common bending processes and, on the other hand, composite extrusion, in which continuous reinforcements like steel wires are extruded together with conventional billets. A further innovation is the combination of both methods to manufacture curved reinforced aluminum profiles. Due to an unsymmetrical feeding of the reinforcing elements, the curvature generation is influenced by the changed material flow. Additionally, the curvature generation influences the position of the steel wires in the profile cross section. The investigation of these two effects, which have an impact on the quality of the process combination, is presented.

Combination of curved profile extrusion and composite extrusion for increased lightweight properties

A study on powder extrusion is attempted to extend the understanding of the extrusion process and the interrelationship between process variables and forces involved. The extrusion characteristics of a promising Al-Si alloy produced from rapidly solidified powder, and the effects of such variables as temperature, reduction ratio, and die geometry on extrusion pressure and friction force have been investigated. Results show that extrusion starts prior to the attainment of a peak pressure. The friction force between the billet and lubricated container occupies only about 4.5% of the peak pressure. This friction is therefore not principally responsible for the pressure drop of about 50% from a peak value to a steady state occurring during extrusion—a finding which is in conflict with the traditional understanding of extrusion. It is also found that temperature-compensated strain rate Z can satisfactorily reflect the combined effects of the process variables on the pressure requirement. The relationship between the Z parameter and the pressure required for the extrusion of the powdered metal (PM) alloy has been formulated, and thus together with the consideration of press capacity and product quality, the extrusion process can be well controlled.

Extrusion of an aluminum alloy prepared from rapidly solidified powder

The extrusion of atomised aluminium alloy compacts and composites

The mechanism of powder consolidation in the extrusion chamber is analysed and it is shown that theoretically the specific pressures required to extrude initial powder compacts will be lower than an equivalent cast billet. The hot working range of metals is thus enlarged and inherently stiffer materials may be extruded. The extrusion of Al alloys from air atomized powders is described and the conditions required for meaningful increases in properties elaborated. The production of Al alloys reinforced by glass is also described and it is shown that although strength levels may be enhanced this type of reinforcement does not lead to a product having a combination of properties significantly more attractive than the cast and wrought product. Consideration is given to other forms of reinforcement. The extrusion of rapidly solidified AI10Mg powder shows that this non-heat treatable alloy has properties comparable with the strongest heat treatable Al alloy. PMj0257

Production of High Strength Aluminium Alloys by Extrusion of Atomized Powders

This communication reports work completed on the extrusion of powder compacts prepared from rapidly solidified Al–10 Mg powder. It is shown that mechanical properties of the extrudate are related to the temperature compensated strain rate obtaining during the extrusion process. Pressure requirements are much lower than are necessary for the extrusion of cast products of Al–7Mg and Al–5Mg and the corresponding increase in heat generation considerably widens the extrusion limits. In the as-extruded condition, the properties are comparable with a peak aged conventional 7050 alloy but the Al–10Mg is extremely unstable and requires an aging treatment to correct this instability. It is concluded that improvement in this alloy system would be obtained either by decreasing the Mg content or by developing a lower %Mg ternary alloy. PM/0280

G. H. Tan

Papers

Microstructural aspects of extrusion of rapidly solidified Al–10Mg alloy powder

The extrusion of atomised aluminium alloy compacts and composites

Production of High Strength Aluminium Alloys by Extrusion of Atomized Powders

Extrusion and Properties of an Al–10Mg Alloy Prepared from Rapidly Solidified Powder