Powder Metallurgy Progress, Vol.8 (2008), No 3 258

THE INFLUENCE OF PARAMETERS OF HOT CONSOLIDATION AND EXTRUSION ON PROPERTIES OF PM Al–Si–Fe–Cu MATERIALS

M. Wojtaszek, S. Szczepanik

Abstract Semi-product designed for extrusion was produced by hot consolidation of Al17Si5Fe3Cu1.1Mg0.6Zr powder in a closed-die. Consolidation was realized at 500°C, at a pressure of 150 MPa and with stamp pressing time of 5 minutes. The semi-products were forward-extruded at several temperatures, with the extrusion ratios 4.1 and 13.7 and with the tool speed of 0.1 mm⋅s-1. The values of the forces which appear during extrusion as a function of punch displacement were registered. The extrusion at temperature 400°C with extrusion ratio of 13.7 required a very large force. For materials after hot consolidation and hot extrusion the relative density and hardness were estimated. For hot-extruded materials, flow curves were constructed at ambient and elevated temperatures, and structures were observed. Processing of Al17Si5Fe3Cu1.1Mg0.6Zr powder, with application of hot consolidation, leads to highly compacted materials. Hot extrusion did not change the relative density. The properties of extruded products can be controlled by the selection of extrusion temperature and extrusion ratio. Increasing the extrusion temperature results in a decrease of hardness. For hot extruded samples compression and bend strengths were evaluated. Increasing the compression test temperature to 150°C resulted in decrease of maximum values of true stresses only by about 100 MPa, regardless of the assumed extrusion parameters. The application of powder of small particle size to the processing, as proposed, leads to products showing fine-grained and regular structure, without disadvantages such as pores or primary boundaries of powder particles. It can be concluded that the proposed technology leads to pore-free materials showing properties which allow use in the production of structural elements. Keywords: powder metallurgy, forming, aluminium alloy powder, consolidation, hot extrusion, physical properties, mechanical properties, microstructure

INTRODUCTION Alloys of type Al-Si-Fe-Cu are characterized by low density, favourable

mechanical properties and stability in elevated or variable temperature conditions. These alloys find their application in automotive industry, e.g. for engine heads or pistons [1,2]. Currently, these products are usually manufactured by foundry engineering. However, the disadvantage of aluminium-silicon alloy castings is thick-acicular eutectic, and in case of hypereutectic high-silicon aluminium alloys – large acicular precipitates of primary silicon, Marek Wojtaszek, Stefan Szczepanik, AGH - University of Science and Technology, Faculty of Metals Engineering and Computer Sciences for Industry, Cracow, Poland

Powder Metallurgy Progress, Vol.8 (2008), No 3 259 which decrease ductility and make machining difficult [3,4]. This problem can be solved by the application of powder metallurgy to the manufacturing of these alloys, which makes it possible to obtain material with a fine-grained structure [5]. However, the disadvantage of this technique, when applied conventionally, i.e. basing on pressing and sintering processes, is porosity, which impairs properties. This can be avoided by combining powder metallurgy and plastic working technologies. The proper design of the process and its parameters allows us to eliminate the disadvantages of both technologies and to combine their advantages, which makes it possible to obtain materials showing the properties competitive to castings. One of the methods is forming of pre-compacted semi-finished products. Hot consolidation of powder leads to obtaining materials of high relative density.Depending on the shape, destination, predicted operating conditions and required product quality, the manufactured element can be applied as a final product or destined for further processing, e.g. forging or extrusion. Extrusion process makes it possible to obtain shaped products of structure and properties depending on process temperature and velocity, extrusion ratio and shape of deformation zone. The field of application of the technology of extrusion of hot-consolidated alloy powder of type Al-Si-Fe-Cu covers the manufacturing of light, high-strength structural elements, mainly for transport industry.

EXPERIMENT

Objective and initial material The objective of the investigations was the evaluation of the effect of applied

parameters on selected properties and structure of materials obtained by means of plastic forming of aluminium alloy powder. Hot consolidation of powder and hot extrusion of consolidated semi-products were employed. As an initial material the atomized Al17Si5Fe3Cu1.1Mg0.6Zr powder was used. The size of powder particles fraction was lower than 50 μm.

Preparation of material Semi-product was produced by hot consolidation in a closed die of

Al17Si5Fe3Cu1.1Mg0.6Zr powder. The process was realized at 500°C, at a pressure of 150 MPa and with stamp pressing time of 5 minutes. The semi-products were forward-extruded at temperatures 400, 450 and 500°C, with the extrusion ratios λ = 4.1 and 13.7 and with tool speed of 0.1 mm⋅s-1. The values of forces during extrusion as a function of punch displacement were registered. The results of the measurements are shown in Fig.1. Since the extrusion at temperature 400°C with extrusion ratio of 13.7 required very large force, the tests realized under these conditions were discontinued. The investigations of the material were limited to the determination of its density and hardness.

The relative density and hardness were determined for semi-products after hot consolidation and for products obtained by hot extrusion. For hot-extruded materials, flow curves were constructed at ambient and elevated temperatures (150°C), and the structures were observed.

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Fig.1. The values of the extrusion forces as a function of punch displacement.

RESULTS

Density The measurements of density were performed by applying the Archimedes

method. It was found that the processing of Al17Si5Fe3Cu1.1Mg0.6Zr powder, with application of hot consolidation, leads to highly compacted materials. The relative density after consolidation amounted to 99.66 % ± 0.3 %. No significant differences were found between the density of materials after consolidation and after extrusion. Hot extrusion caused processing without changing relative density value.

Hardness measurement Brinell hardness number was determined in accordance with a standard. The

results of hardness measurements of investigated materials are presented in Fig.2. The hardness of material after hot extrusion at 400°C, with extrusion ratio λ = 13.7, was the same as for the semi-product after hot consolidation. Increasing of extrusion temperature results in a decrease of hardness.

Fig.2. The effect of manufacturing method and extrusion parameters on the hardness of

materials obtained by forming of Al17Si5Fe3Cu1.1Mg0.6Zr powder.

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Stress-strain curves in compression The behaviour of materials after extrusion was analysed in an uniaxial

compression test. Tests were performed by applying a strain rate of 0.01 s-1 at ambient temperature and at 150°C. The relationships of true stress (σeffective) as a function of strain (ε) in compression are presented in Fig.3. The highest strain hardening during compression tests, performed at both ambient temperature and 150°C, was obtained after hot extrusion at 450°C, with extrusion ratio 13.7. In the case of this material, the character of stress-strain curve at 150°C was comparable with the curve constructed for materials extruded at 500°C at ambient temperature. Increasing the test temperature to 150°C resulted in a decrease of maximum values of true stresses only by about 100 MPa, regardless of the assumed extrusion parameters.

Fig.3. The influence of extrusion parameters on the character of stress-strain curves

obtained in compression at the temperatures 20°C (A) and 150°C (B), for samples obtained by extrusion of Al17Si5Fe3Cu1.1Mg0.6Zr hot-consolidated semi-products.

The observation of structures of extruded materials was realized by optical microscopy, on longitudinal sections, after preparation of etched metallographic specimens. Example microstructures are shown in Fig.4. The obtained structures are fine-grained and neither pores nor primary boundaries of powder particles were observed on their surfaces. In the case of the material extruded at 450°C with extrusion ratio of λ = 4.1, certain orientation of structure is observed, consistent with flow direction (Fig.4a). This effect was not observed in the case of materials extruded at 500°C, regardless of the extrusion ratio (Figs.4b,c).

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Fig.4. Structures of materials obtained by forward extrusion of semi-finished products

manufactured by consolidation of Al17Si5Fe3Cu1.1Mg0.6Zr powder at 500°C. Extrusion temperature: A - 450°C, B and C - 500°C. Extrusion ratio: A and B – 13.7, C – 4.1.

Longitudinal sections, after etching.

CONCLUSIONS It was found that the consolidation of Al17Si5Fe3Cu1.1Mg0.6Zr alloy powder at

500°C results in a product showing the density of a solid material. Its further processing through extrusion makes it possible to give a shape to the product and to modify its structure and properties. Since the alloy being investigated is a hard-deformable material, its extrusion realized at too low temperature (400°C for extrusion ratio of 13.7) requires large forces to be applied, which reduces tool life and limits the attainable degree of deformation. The properties of extruded products can be controlled by the selection of extrusion temperature and extrusion ratio. Increasing the test temperature results in lower hardness of products. The application of powder of small particle size to the processing, as proposed above, leads to products showing fine-grained and regular structure, without disadvantages such as pores or primary boundaries of powder particles. Orientation of structure is observed in case of materials extruded at 450°C. Increasing the temperature to 500°C eliminates this effect. In the light of obtained results, it can be concluded that the proposed technology leads to pore-free materials showing properties which allow use in the production of structural elements.

Acknowledgements The research work was financed within the AGH-UST project no. 10.10.110.864.

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