Processing of a porous 7075 Al alloy by bubble expansion in a semi-solid state
Introduction
Porous materials are super-light materials exhibiting unique properties such as high energy absorption [1]. The applications of porous materials are in a wide range of impact energy absorbers, constructional light materials and so on [2]. A variety of porous metals have been developed [2], [3], [4], [5], [6], [7]. For example, Miyoshi et al. [3] fabricated a porous Al with the closed cell structure by a casting method. A high porosity of 93% with a pore size of about 3–4.5 mm was attained by the casting method. The porous Al showed excellent impact energy absorption ability [8]. Recently, Queheillalt et al. [9] fabricated a porous Ti–6Al–4V alloy by expansion of gas bubble within a creeping matrix (creep expansion method). In this case, the pore size was rather small (less than about 80 μm). Development of such porous metals with a small pore size leads to new applications, for example, super-light thin sheets. However, there are still some problems for the creep expansion method, for example, a long time is required for pore formation and the porosity is relatively low. Elzey and Wadley [10] analyzed the limit of porosity by creep expansion in a solid state.
More recently, Davis et al. [11] showed that creep expansion is accelerated by superplastic flow in titanium. Superplastic expansion is effective to enhance pore formation. Another method for acceleration of pore formation is gas bubble expansion in a semi-solid state [10]. The aim of the present paper is to fabricate a porous Al alloy by bubble expansion in a semi-solid state. In the present investigation, effects of expansion temperature and time on pore formation are investigated.
Section snippets
Experimental procedure
7075 Al alloy powder with the size less than 50 μm was prepared from TOYO ALUMINIUM K.K. The chemical composition of the powder is shown in Table 1. The Al alloy powder was canned with Ar gas whose pressure was 1 MPa, and then it was sintered at 673 K with 80 MPa hydrostatic pressure for 30 min. The sintered billet was then extruded at 673 K with a ratio of 8:1. The extruded specimens were heated at 733, 823, 873 and 903 K for 24–72 h in air. The onset temperature of melting for the extruded
Results and discussion
The microstructure of the specimen prior to heating is shown in Fig. 1. Pores were not observed in the specimen prior to heating by the optical microscope, indicating that Ar gas was compressed into pore too small to be observed by the optical microscope.
Microstructures of the specimens heated for 24 h are shown in Fig. 2, where the heating temperature is (a) 733 K, (b) 823 K, (c) 873 K and (d) 903 K, respectively. The microstructure of the specimen heated at 733 K was almost the same as the
Conclusions
Effects of expansion temperature and time on pore formation by bubble expansion in a semi-solid state were investigated to fabricate a porous 7075 Al alloy. The results are as follows:
- (1)
Pore formation by bubble expansion for 24 h was negligible in a temperature range below the onset temperature of melting (= 733 K). On the other hand, pore formation was observed in a temperature range above the onset temperature of melting (= 823–903 K), namely, in a semi-solid state.
- (2)
In a semi-solid state, pore
Acknowledgements
The authors are grateful to K. Yokoe of TOYO ALUMINIUM K.K. for the supply of 7075 Al alloy powder.
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