studies on the microstructure and mechanical properties of recycled cast aluminium scrap

Mbuya TO. studies on the microstructure and mechanical properties of recycled cast aluminium scrap .; 2003.


The aluminium casting industry in Kenya is strongly dependent on aluminium scrap recycling. There is, however, little organised information on appropriate recycling procedures and the expected chemical composition of the resulting secondary alloys. Furthermore. little is also , known on the expected mechanical properties of castings produced from the secondary alloys . under different processing conditions. Consequently, the quality of locally produced castings is poor compared to their imported counterparts as most Kenyan foundries rely on chance to make good castings. The main objectives of this preliminary study were: (a) to determine the chemical composition of the secondary alloys obtained from recycling various cast aluminium scrap components available in Kenya; and (b) to determine the microstructure, tensile strength. percent elongation and hardness obtainable from these alloys using both green sand and permanent mould casting processes and under different process conditions. The process parameters for both green sand and permanent mould casting processes, whose influence on the said properties was investigated . are: the type of mould filling system, melt handling and pouring temperature. The influence of the thickness and initial temperature of permanent moulds on these properties was also investigated. It was found in this study that the common types of scrap components available in Kenya are automotive engine parts. Most of these components were found to be pistons, cylinder heads, gearbox housings and to a lesser extent, rear axle housings. Pistons and cylinder heads were classified as individual groups while gearbox and rear axle housings were grouped together. The rest of the scrap items, most of which could not be identified, were lumped together to form another separate group. Samples were randomly picked from these scrap groups and • individually remelted to obtain secondary alloys. Other secondary alloys were also prepared by • blending the above scrap groups in predetermined proportions. • The resulting secondary alloys from these groups of scrap •c9mponents were all hypoeutectic aluminium-silicon-copper (AI-Si-Cu) based alloys. The samples in similar scrap groups were v \. < fairly equivalent in chemical composition although some minor variations were observed. Some samples from different groups were also found to be fairly equivalent In their chemical composition. Furthermore, all the samples were also fairly equivalent. In their chemical composition, to several common commercial hypoeutectic AI-Si-Cu based alloys. Permanent mould casting resulted in castings with higher tensile strength, percent elongation and hardness compared to green sand casting in all scrap samples tested. Both quiescent melt handling and quiescent bottom filled mould systems yielded castings with higher aforementioned properties than turbulent melt handling and turbulent top filled mould systems . . respectively. In addition, these mechanical properties were found to decrease with increasing pouring temperature and mould preheat. Increasing the mould thickness was, however, found to increase these mechanical properties. Heat treatment of some of the alloys to the T6 condition increased their tensile strength and hardness, but decreased their percent elongation. The microstructure of all the recycled alloys was typically similar and contained cc-aluminium matrix, eutectic silicon particles, iron-bearing and copper-bearing intermetallics. The tensile strength and percent elongation of the recycled alloys did not correlate well with those of their respective commercial counterparts. This was particularly so with top filled green sand castings. The mechanical properties of bottom filled samples were. however. closer to those of their commercial counterparts albeit falling on the lower end in the range of reported . mechanical properties of their respective equivalent commercial alloys. The hardness values of the samples, however, correlated well with those of their commercial counterparts. It is inferred from the results of this study that using the type of scrap component as the sorting criterion is of limited usefulness because some samples cast from different scrap components were fairly equivalent in their chemical composition. Furthermore. some scrap items are either too few to recycle separately and/or difficult to identify. However, in the absence of appropriate facilities to determine the alloy chemistry on-line and make appropriate melt corrections. this sorting criterion together with the chemical compositions of the samples studied in this study can be a useful guide. Also inferred is that blending scrap samples is not useful because no . definite trend in the chemical composition was observed with blending. Finally the processing conditions strongly influenced the mechanical properties of the samples . . It is therefore important to choose the right casting process and to closely control the mould design, molten metal processing and other process parameters like pouring temperature and initial mould temperatures in permanent mould casting. In particular. permanent mould casting should be preferred to green sand casting if maximising mechanical properties is the major factor for process selection. In addition, mould filling systems should be skilfully designed to quiescently introduce molten metal into the mould cavity preferably via bottom filling. The pouring temperature should also be as low as possible.


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