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Improved Selective Laser Melting Processing of Aluminium-Silicon

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Improved Selective Laser Melting Processing of Aluminium-Silicon IMPROVED SELECTIVE LASER MELTING PROCESSING OF ALUMINIUM-SILICON-MAGNESIUM ALLOYS Thesis submitted in accordance with the requirements of The University of Liverpool for the degree of Doctor in Philosophy By Donal Lynch February 2019 Abstract This thesis details the investigation of the cracking phenomenon of aluminium alloy AA6061 processed by Selective Laser Melting (SLM) and proposes a modified alloy composition which was proven to eliminate cracking. High strength aluminium alloys are of commercial interest to the aerospace and automotive industries, in order to manufacture lightweight structural components. Current alloys that are regarded as processable through SLM are compromised by low strength, high cost or weight and this has led to an increased interest in developing high-medium strength aluminium alloys specifically for the SLM process. A thorough literature review was conducted to consider how different aluminium systems may perform when processed through SLM and subsequent heat treatments. From this, it was evident that alloys that can achieve appropriate mechanical properties are susceptible to cracking during the SLM process. AA6061 is an alloy within this category and it was selected for investigation due to its suitable mechanical properties, general use and availability in powder form. The potential sources of cracking are addressed in the literature review and a solution was proposed based on welding practices of AA6061. A powder blend of AA6061 powder and AlSi10Mg was made and processed by SLM. This imitates the practice of welding AA6061 with an Al-Si eutectic filler material. The blend ratio was selected to effectively adjust the silicon content of AA6061 by 1% and reflects the cracking susceptibility of binary Al-Mg2Si and Al-Si alloys. Optimisation of SLM processing parameters was performed on a Realizer SLM100 for AA6061, AlSi10Mg and the blend of the two materials. The process parameters, the density measurements methods and the design of experiments used for this study were scrutinised to contribute to the best practice for obtaining optimum densities. A high level of cracking was observed for every AA6061 sample that was produced. An unsuccessful attempt was made to influence the cracks through process strategies, namely reducing layer thickness and rescanning of each layer. SLM process parameter studies of the blended material demonstrated no cracking in any sample. The causes of cracking were investigated, with a view to gain deeper understanding for alloy design and development for the SLM process. The microstructure of SLM built AA6061, AlSi10Mg and the blended material was studied using electron channelling contrast imaging, electron backscatter diffraction and chemical etching. This revealed that AA6061 and the blended material samples exhibited very similar grain structures, in contrast to the AlSi10Mg samples. The difference between AA6061 and the blended material was that pores could be found within the grain boundaries of the AA6061, which are likely the initiation points of cracks. The effect of the increased silicon content in the blended material was to provide enough silicon so i that no gaps were seen in grain boundaries. The cracks propagate through grain boundaries and this dictates the direction of crack growth. Unidirectional samples were produced in an attempt to understand how the accumulation of stresses affects the direction and occurrence of cracks. These revealed no relation between cracks and principal stresses; rather cracks occurred at even spacings along scan directions irrespective of the part shape. The distance between cracks was ten times greater than the hatch spacing and is likely to relate to the weakest position within the microstructure due to grain misalignment. Fractography was performed on the crack surfaces to examine if inclusions could be found that would weaken the material. High levels of oxygen were found on crack surfaces, but this formed after the crack surfaces open. Similar thick oxides are found on the top surfaces of samples and these were examined through energy dispersive X-ray spectroscopy. A reduction in surface oxides was observed with when the layer was rescanned. This reduction of surface oxide was not met with a reduction in oxygen content measured within the body of the samples. This study demonstrated that the surface appearance of the samples related to the level of oxides on the surface, which discolour the metal samples. A similar difference in discolouration was observed with samples produced with different laser beam diameters, but measurement of the oxide content was inconclusive due to the high amount of spatter on the surface of samples. The content and influence of oxides remains a concern for aluminium samples produced through SLM and deep etching of samples was used to examine the characteristics of the network of oxides within AA6061, AlSi10Mg and the blended material. This study has therefore contributed to the understanding of aluminium alloys in SLM by; furthering understanding of the causes of cracking of an aluminium alloy in SLM, demonstrating a novel alloy with improved processability, and contributions to the knowledge of oxides within SLM aluminium specimen. ii Acknowledgements The author would like to acknowledge the contributions of Dr. Peter Fox, Dr. Robert Deffley and LPW staff, Dr. Matt Fulton and the centre for global eco-innovation, Dr. Karl Dawson and staff at imaging Centre at Liverpool, Mr. Dave Atkinson, Mr. Jack Carter-Hallam and staff in the engineering school office, Ms. Rebecca Garrard and family and friends for their support, without which this thesis wold not have been possible. John 11:35 iii iv Contents Abstract .......................................................................................................................... i Acknowledgements ...................................................................................................... iii Contents ........................................................................................................................ v Nomenclature .............................................................................................................. ix Background ............................................................................................................ 1 Development of Selective Laser Melting .................................................................. 1 Applications ............................................................................................................... 3 Materials Used in SLM .............................................................................................. 7 Aims and Objectives .................................................................................................. 9 Literature Review ................................................................................................. 11 Introduction ............................................................................................................ 11 Aluminium Alloys in SLM ......................................................................................... 12 Cast Alloys in SLM ........................................................................................... 13 Wrought Alloys in SLM .................................................................................... 15 Powders .................................................................................................................. 19 Powder Production Techniques ...................................................................... 19 Powder Characterisation and Specification .................................................... 21 Powder Reuse ................................................................................................. 23 Gas .......................................................................................................................... 24 Process Atmosphere ....................................................................................... 25 Hydrogen in SLM Aluminium .......................................................................... 25 Oxides in SLM Aluminium ............................................................................... 27 Melting .................................................................................................................... 30 Laser Interactions during SLM......................................................................... 31 Spot Size and Shape ........................................................................................ 32 Balling .............................................................................................................. 34 Process Parameter Development Strategy ..................................................... 35 Solidification and Strengthening Mechanisms ....................................................... 36 Strengthening Mechanisms ............................................................................ 40 Microstructure of SLM Aluminium.................................................................. 43 Thermal Stresses ..................................................................................................... 57 v Mechanisms of Thermal Stress ....................................................................... 57 Accumulation of Stress with SLM .................................................................... 58 Cracking under thermal stresses in SLM ........................................................
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