Abstract—Effective heat treatment conditions to obtain maximum aluminium swarf recycling are investigated in this work. Aluminium swarf briquettes underwent treatments at different temperatures and cooling times to investigate the improvements obtained in the recovery of aluminium metal. The main issue for the recovery of the metal from swarfs is to overcome the constraints due to the oxide layers present in high concentration in the swarfs since they have a high surface area. Briquettes supplied by Renishaw were heat treated at 650, 700, 750, 800 and 850 ℃ for 1-hour and then cooled at 2.3, 3.5 and 5 ℃/min. The resulting material was analysed using SEM EDX to observe the oxygen diffusion and aluminium coalescence at the boundary between adjacent swarfs. Preliminary results show that, swarf needs to be heat treated at a temperature of 850 ℃ and cooled down slowly at 2.3 ℃/min to have thin and discontinuous alumina layers between the adjacent swarf and consequently allowing aluminium coalescence. This has the potential to save energy and provide maximum financial profit in preparation of swarf briquettes for recycling. Keywords—Aluminium, swarf, oxide layers, recycle, reuse. I. INTRODUCTION ANAGING swarf from production sites has been a problem that is addressed since the early stages of metal processing in industry due to the complications that come with it in the working area. Complications range from; generation of mass swarf that reduces efficiency in production, large amount of heat in the swarf making it unsafe to handle, significant amount of cooling lubricant contaminating the swarf and wasting the resources. Aluminium processing and manufacturing businesses produce this swarf in high volumes i.e. Toyota in 2010 produced 1,600 tonnes a month aluminium swarf which were transported offsite [1], while in the other hand Hubco Forgings supplies foundries with 400 tons of compacted swarf [2], [3]. The problem persists still nowadays even though some research has been performed to offer possible solution as shown from [4]. It is important to find an optimised methodology for the maximum re-use of the swarfs and to solve this problem the focus should be on the fundamental behaviour of the swarfs as they are treated for recycling. This paper aims to provide results on the bonding and coalescence between swarfs after heat treatment in different conditions. J. Uka, B. McKay, T. Minton, O. Adole and L. Anguilano* are with the Brunel University London, United Kingdom (*corresponding author, e-mail: lorna.anguilano@brunel.ac.uk). R. Lewis and S. J. Glanvill are with the Renishaw, Gloucestershire, United Kingdom. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. A. Aluminium Oxide Layer Alumina is the main barrier making it significantly difficult to recycle the swarf in solid billets using 100% swarf. As such it is important to study the behaviour of the swarf (alumina layer and aluminium) at high temperatures to find the optimal conditions to bring this material back to life without mixing it with virgin aluminium. In particular the fundamental behaviour of the aluminium/alumina interaction are the focus of this study to evaluate how to best break the alumina layers forming the external “crust” of each swarf within the briquette and impeding the coalescence of aluminium metal. For this purpose, there are two experimental hypotheses: (1)- Aluminium trapped inside the alumina envelope is going to expand in volume with an approximate linear expansion 24 x 10 -6 m/m℃ [5] as the temperature increases and begin to melt, the forces generated by such expansion can support the breakage of the oxide layer. Fig. 1 shows thermal expansion coefficient for aluminium from different sources collected in one graph. This graph displays that aluminium thermal expansion increases as the temperature increases. When the temperature range is between 600-900 K corresponding to 327-627 ℃ the gradient of the curve is steeper showing a higher increase in thermal expansion of aluminium. In the conditions of heat treatment, once temperature has reached the melting point of aluminium, the aluminium trapped inside the alumina is assumed to have melted, forming in this way a liquid mass inside the still solid alumina walls. These alumina walls can stand a certain pressure and after that alumina layer cracks and allows aluminium to sink towards the bottom of the vessel. As such there should be a point at which force due to thermal expansion of aluminium surpasses the force of alumina grains stuck together. As such, (2)- Alumina during heat treatment is going to change in different parts of polymorphs which are going to result in structural change and ease the cracking of the alumina walls of the swarf. When gamma alumina which forms at around 450 ℃ goes through few transformations to δ, θ and finally α as the temperature increases from 600 ℃ to 1000 ℃, the volume of alumina decreases significantly forming cracks in the alumina layer. It should be this range of temperatures at which liquid aluminium can get through the alumina cracks and sink. So, to find a way to break the alumina layer, it is important to study polymorphs of aluminium oxides and conditions at which it forms. That information is then used to find the right conditions to treat aluminium swarf in order to recover the maximum aluminium. Therefore, in order to find out the properties of the alumina polymorphs, the coexistence/stability of aluminium and alumina was traced back in the literature. It An Evaluation of the Oxide Layers in Machining Swarfs to Improve Recycling J. Uka, B. McKay, T. Minton, O. Adole, R. Lewis, S. J. Glanvill, L. Anguilano M World Academy of Science, Engineering and Technology International Journal of Chemical and Materials Engineering Vol:15, No:2, 2021 29 International Scholarly and Scientific Research & Innovation 15(2) 2021 ISNI:0000000091950263 Open Science Index, Chemical and Materials Engineering Vol:15, No:2, 2021 publications.waset.org/10011819/pdf