This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg) Nanyang Technological University, Singapore. An analysis of solid‑state recycling of aluminium alloy AA6061 from controllable hot extrusion processes Shamsudin, Shazarel 2018 Shamsudin, S. (2018). An analysis of solid‑state recycling of aluminium alloy AA6061 from controllable hot extrusion processes. Doctoral thesis, Nanyang Technological University, Singapore. https://hdl.handle.net/10356/87457 https://doi.org/10.32657/10220/46734 Downloaded on 25 Sep 2021 14:20:11 SGT ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library AN ANALYSIS OF SOLID-STATE RECYCLING OF ALUMINUM ALLOY AA6061 FROM CONTROLLABLE HOT EXTRUSION PROCESSES SHAZAREL SHAMSUDIN School of Mechanical & Aerospace Engineering A thesis submitted to the Nanyang Technological University in partial fulfilment of the requirement for the degree of Doctor of Philosophy 2018 ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library ABSTRACT Solid-state recycling of machining chip wastes via hot extrusion has nowadays been seen important to reduce energy consumption and greenhouse effect, perceived more important than the recycling by remelting. Characteristics of machining chips that are micro in size and large area-to-volume ratio in nature, melting can cause a massive molten metal vaporization leading to huge material losses. The hot extrusion process has long been recognised as the most outstanding direct recycling method which can result in much better material recovery. However, the high quality weld bonds among chips must be well developed during the process to ensure the mechanical performance of the recycled product is comparable to as-cast material. The optimum weld bond quality often relies on the selection of accurate process parameters to be combined with the appropriate die design. Nevertheless, the use of conventional approach of trial and error to complete the process parameter selection offers an unrealistic implementation, time consuming, and a costly process cycle. This conventional method was identified as the key shortcoming of the current solid-state recycling process. This research provides an investigation into the optimum homogenisation condition for chip-based billet, a mathematical model development which allows the prediction of weld strength recycled materials and a proposed heat treatment for performance improvement. For homogenisation aspect, a series of hot extrusion experiments were performed at different settings of temperature (400 °C - 550 °C) and preheating time (1 - 6 hours). Each extrudate profile was then tested for its tensile strength performance. To further investigate the effect of temperature in homogenisation via damage evolution within the extrudate structure, a numerical simulation using a finite element method i ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library was implemented. The normalized Cockcroft and Latham (C & L) fracture criterion was applied. Besides, a mathematical model for tensile bond strength prediction was developed using both theories of thin film bonding and diffusion. All related deformation variables for model calculation were numerically obtained via the finite element simulations, undertaken by the DEFORM 3D software. The model’s validation was done by a series of tensile tests on gear and rectangular profiles, where the predicted strengths were compared with the strengths obtained experimentally. Finally, the heat treatment process was carried out at T6 condition for all samples. For chip-based billet homogenisation, both experimental and numerical findings showed that the billet temperature must be kept higher than 400°C while for preheating time, minimum two hours was required to obtain a satisfactory tensile strength under the extrusion ratio of 11.22 or lower. The derived weld strength model was fairly accurate in predicting the tensile bond strength of extrudate. The average error resulting from the gear profile was 12 % while 23 % was calculated for the rectangular profile, both compared between the predicted and experimental results. The sources of error in the analysis were mainly contributed by the extreme state of strain-rate and deviations in the shear area estimation. The research shows that a great amount of heat or induced normal stress or shear stress was demanded to reduce the threshold strain of weld creation, thus promoting a sound weld bond. The greater magnitude of the normal stress (perpendicular to the extrudate surface) than the yield strength of material increases the real contact area of chips to result in a better cohesion and a delay in the contact time between die and billet enhances diffusion. Finally, the heat treated AA6061-T4 aluminium chips at T6 condition resulted in 105.66 HV, inferior just by ≈ 5 % hardness of as-received AA6061-T6, indicates that the mechanical properties of the direct recycled material can be successfully enhanced for practical applications. ii ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library ACKNOWLEDGEMENT I would like to express my greatest gratitude and appreciation to my supervisor, Assoc. Prof. Dr. Zhong Zhaowei, and research collaborator Prof. Dr. Mohd Amri Lajis for their support, guidance, encouragement and effort throughout my research. I believe, without them, I would not be able to contribute in this research. I would like to express my gratitude towards my family for the encouragement and kind help in the completion of this project. My beloved and supportive wife, Hasnah who always stood by my side to support me during hard times and lovely children who served as my inspiration to pursue this undertaking. I would also acknowledge the help of Dr. Ahmed Syed Adnan, Dr. Mohd Faizal Mohideen Batcha, Dr. Periyasamy Manikandan and Mr. Shanmugasundaram Durairaj for their valuable support. I always seek support from them whenever I am in need. My sense of gratitude also conveyed to all other individuals, directly or indirectly, who have helped me out with their abilities throughout this venture. iii ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library Contents TABLE OF CONTENTS ABSTRACT ................................................................................................................... i ACKNOWLEDGEMENT ......................................................................................... iii TABLE OF CONTENTS ........................................................................................... iv LIST OF TABLES ................................................................................................... viii LIST OF FIGURES ..................................................................................................... x LIST OF ABBREVIATIONS ................................................................................. xvii LIST OF SYMBOLS ............................................................................................. xviii CHAPTER 1 : INTRODUCTION .............................................................................. 1 1.1 Background ..................................................................................................... 1 1.2 Problem Statement .......................................................................................... 4 1.3 Research Motivation ....................................................................................... 8 1.4 Research Objectives ........................................................................................ 9 1.5 Research Scope ............................................................................................... 9 CHAPTER 2 : LITERATURE REVIEW ............................................................... 12 2.1 Introduction ................................................................................................... 12 2.2 Overview of Aluminum Recycling ............................................................... 13 2.3 Experimental Studies of Solid-state Recycling of Aluminium ..................... 17 2.3.1 Effects of Chips and Billet Preparation ................................................. 18 2.3.2 Effects of Reinforcing Phase and Mixture of Different Aluminum Alloys ................................................................................................................ 20 2.3.3 Effects of Die Geometry ........................................................................ 24 2.3.4 Miscellaneous Methods in Direct Recycling Technique ....................... 28 2.3.4.1 Extrusion and Rolling Method ....................................................... 28 2.3.4.2 Equal Channel Angular Pressing (ECAP) Method ......................... 29 2.3.4.3 Continuous Extrusion Process (Conform) ...................................... 30 2.3.4.4 Friction Extrusion ........................................................................... 31 2.3.4.5 Forging............................................................................................ 31 2.3.5 Effects of Processing Parameters ........................................................... 33 iv ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library Contents 2.3.5.1 Effects of Temperature and