Recycling of Aluminium scrap in Kenya: A Survey of Foundry Enterprises and Mechanical Properties of Resultant Alloys

Abstract

Small and medium scale foundry enterprises occupy an important position in the economic growth and development in
our societies. They are the main force behind a large number of innovations and contribute significantly to the nation’s growth
through employment creation, investments and exports. Their contribution to poverty reduction and wealth creation is enormous.
This study investigated the aluminium scrap recycling practices through a survey of 45 foundry enterprises in Kenya; and evaluated
the effectiveness of scrap segregation, melt cleanliness and post cast processing in reducing failures of load bearing products using
commonly used cylinder head scrap. Survey data was acquired through a questionnaire, onsite observations and individual
interviews with the foundry men. Three alloys were developed from carefully segregated cylinder head scrap; which include BA
(base alloy), BA + 0.6%Fe and BA + 0.6%Fe + 0.3%Mn. Analysis of microstructure and mechanical properties was done on as
cast and T6 heat treated alloys. The foundries involved in the survey were classified into ‘‘jua kali’’(micro), small, medium and
large enterprises based on capital investment, number of employees, volume of castings produced and level of technology. It was
found that ‘‘jua kali’’ enterprises were majority at 44%. Their operations were limited by low capital base, low level technology
and inadequate skills; forcing them to operate below 40% capacity. Further, the approaches adopted for sorting scrap were found
to lead to unpredictable chemical composition and uncertainties in mechanical performance. Use of alloying additives to adjust the
alloy chemistry and improve properties of load bearing castings was utilized by 10% of the enterprises. This resulted in production
of low quality and unreliable castings. BA had composition similar to that of parent cylinder head alloys. A small increase of Fe
by 0.2 % as an inclusion in the as cast alloy, resulted in a drop of strength and ductility by 7 and 20 % respectively. Mn mitigated
the harmful effects of Fe by increasing strength and ductility by 5 and 12 % respectively. T6 heat treatment of as cast alloy improved
the strength and decreased ductility by 40 and 18 % respectively. It was shown that sorting separately the aluminium scrap
component by component, observation of melt cleanliness, use of additives, and post cast processing can yield good mechanical
performance of the resulting alloy. International market requirements have set quality standards, which local foundries must fulfill
through adoption of scrap segregation, casting and melt control procedures in order to ensure high quality products.