Development of Recycled Friendly Aluminium Alloys for Automotive and Structural Applications

Abstract

Use of aluminium alloys for structural applications in Kenya is expected to be preferred to steel; due to their high resistance to corrosion, large strength-to-weight ratio and recyclability. However, it has not been the case because; first, availability of raw material (wrought aluminium structural scrap) has not been established. Secondly, processing of standard wrought alloys using mix scrap categories is costly, since it requires dilution with primary aluminium and alloying element adjustment. Finally, the alloys produced have low strength for broader structural applications. To overcome the challenges, recycling strategies that sort aluminium structural scrap component by component were investigated. Testing performance characteristics of the recycled sorted scrap melt was carried out to ascertain its structural acceptability. Therefore, this research aimed at developement of recycled friendly aluminium alloys for automotive and structural applications. An industry survey was conducted to establish the availability and consumption of aluminium scrap in Kenya. Data was collected from scrap dealers, foundries and agencies that handled either aluminium or aluminium data using a questionnaire, site visits and interviews. A mass flow analysis (MFA) was performed on the data collected. Mass flow analysis established that 9,948.4 tons of scrap were collected in year 2017. The scrap originated from automotive, construction, household, processing and electrical industries; in proportions of 41, 29.6, 19.1, 7.7 and 2.8 % respectively. The scrap consumed locally was 7,615 tons; and consisted of 3000, 6000 and 300 alloy series in ratios of 66.2, 25.2 and 8.6 % respectively. Aluminium structural scrap originated mainly from automotive and construction industries. 2,880 tons of pure aluminium were used to dilute the scrap. Based on the MFA results, four batches of wrought aluminium structural scrap from Nairobi dealers were sorted component by component. Each batch was melted and subjected to spectro-chemical analysis. Sorting considerably reduced cost associated with melting mixed scrap, followed by dilution with primary aluminium and adjusting alloying elements composition. The batches yielded secondary wrought alloys of 6000 series that were equivalent to 6005, 6061, 6063 and 6070 standard alloys. However, impurities of iron and zinc rose slightly while magnesium and manganese faded marginally. The secondary 6061 alloy was selected for further processing through extrusion, high pressure torsion (HPT) and friction stir welding (FSW). The selection was informed by its composition that matched its equivalent standard alloy; and its broad application in manufacture of automotive and structural components. Further, its behaviour was to be generalized to the other three alloys of the same series. The processes were expected to improve its microstructure and mechanical properties for better structural performance. As extruded 6061 secondary alloy had a strength of 214 MPa which matched with standard alloy. This showed that sorting scrap component by componet was a xxiv cheaper alternative to use of mixed scrap and use of primary aluminium while developing the alloy. The extrudates were further processed by HPT at a pressure of 6.0 GPa, while varying turns from 0.25 to 10. HPT processing of the as extruded material broke down and homogenized the second phase particles. Average Vickers microhardness of HPT processed samples increased from 50 to 110 HV0.3; while tensile strength increased from 214 MPa to 381 MPa. The extruded alloy was friction stir welded at spindle speed and feed rate varying from 530 to 1320 rev/min and 40 to 100 mm/min respectively. Microstructure, micro hardness, strength and ductility of samples were analysed at every stage. Friction stir welding refined grains of the alloy in the stirred zone. Maximum average microhardness of 70 HV0.3 in stirred zone was obtained at lowest speed of 530 rev/min and highest feed of 100 mm/min. Therefore, microhardness was inversely proportional to spindle speed and directly proportional to feed rate. A maximum joint strength of 84 MPa was obtained at a speed of 915 rev/min and a feed of 100 mm/min. Through the research, it was found that available wrought Al scrap quantities can be recycled through alternative and cheaper methodology namely component by component to yield cheaper secondary alloys such as Al 6005, 6061, 6063 and 6070 alloys which are widely used for structural applications. This eliminated the need for expensive Laser assisted sorting machines which require huge investment. The mechanical properties and weldability of the extruded secondary wrought Al 6061 alloy were enhanced through HPT and FSW. However, FSW parameters including tool rotational speed,tool geometry and feed rate need to be optimized further in order to produce a much stronger joint.