Improving the fatigue behaviour of a selectively laser melted aluminium alloy: Influence of heat treatment and surface quality

Significance Statement

Additive manufacturing plays an important role in fabrication and also has the potential to fulfill demands for reducing the cost and time spent on designing-to-manufacturing via saving on raw materials and introducing a single step production process.

Selective laser melting is widely used in fabricating complex structures in various industries such as in aerospace and orthopedics applications. Nesma Aboulkhair and colleagues from the Centre for Additive Manufacturing at the University of Nottingham in the United Kingdom looked at the effect of surface quality as a factor influencing the fatigue behavior of selectively laser melted parts made from Al alloys and proposed heat treatment as an alternative to post process machining that would improve the fatigue performance. The recent work is now published in journal, Materials and Design.

To expand to the load bearing applications, the researchers suggested validating the selectively laser melted parts, focusing on both static and cyclic loading. Among their numerous studies in this field, they investigated the fatigue performance of selectively laser melted AlSi10Mg considering the effects of surface quality and heat treatment – independently and simultaneously – in this most recent study.

A Renishaw AM250 selectively laser melted machine was used to fabricate two batches of standard fatigue test specimens with reduced gauge sections and continuous radius of curvature between the grip ends. The team ensured that the process parameters used were optimized to produce parts with minimal porosity and a relatively small layer thickness was used to minimize surface defects and irregularities. They used optical microscopy to inspect the machine samples to ensure no cracks or undercuts, the fracture surfaces were also cross-sectioned along the normal plane, polished, etched, and microscopically imaged. The specimens were also cleaned with ethanol before testing to remove any surface dirt or oils. When they ran uniaxial fatigue tests, specimens were loaded until failure or until nominal fatigue life of 3 × 107 was reached.

The authors observed that solely machining the samples could not improve the fatigue life at higher stress levels but an enhancement was achieved at the lower stress levels. Moreover, the heat treatment improved the fatigue performance of the material and failure under cyclic loading was found to originate at surface or sub-surface defects and propagated along the melt pool boundary. According to the authors, it was demonstrated that heat treatment only without surface machining yields better fatigue performance and doubles the fatigue life.

The method of heat treating and machining of the selectively laser melted AlSi10Mg actualized the authors’ aim of improving fatigue performance of selectively laser melted parts. The results of this study will be of interest for industrial sectors producing highly intricate, and functionally graded parts.  

Improving the fatigue behaviour of a selectively laser melted aluminium alloy: Influence of heat treatment and surface quality. Advances in Engineering

About the author

Nesma T. Aboulkhair

Nesma has a Bachelor of Science (BSc) degree in Mechanical Engineering from Cairo University (Egypt). She attended the American University in Cairo (Egypt), earning a Master of Science (MSc) degree in Mechanical Engineering with a specialisation in Materials and Manufacturing in 2012. For her Master’s degree, Nesma has conducted research on developing nanocrystalline aluminium using severe plastic deformation. Nesma has worked as a research assistant at the Youssef Jameel Science and Technology Research Centre (YJ-STRC), conducting several research on aluminium processing. In 2015, she has been awarded a Doctor of Philosophy (PhD) degree in Materials Engineering and Materials Design at the University of Nottingham (UK). Her PhD focused on the various aspects of additive manufacture (AM) of Al alloys using selective laser melting.

Her experience in metal AM varied between the process, such as parametric studies for materials qualification, and the investigations on the microstructural and mechanical properties. Over the years of her studies, Nesma has received several awards and scholarships. During her time at the American University in Cairo, Nesma was awarded a laboratory fellowship, a university fellowship, and an award for outstanding academic performance. At the University of Nottingham, she was awarded the prestigious Dean of Engineering Scholarship for International Research Excellence, the postgraduate endowed award in a tri-campus competition, and best student manuscript in the Industrial Laser Applications symposium 2015, among other awards.

Nesma is currently a postdoctoral research fellow in the additive manufacturing and 3D printing research group at the University of Nottingham (UK). Her current research projects span across several metal additive manufacture processes, such as powder-based systems and droplet-on-demand technologies for high melting temperature precious metals. 

About the author

Ian Maskery

Ian Maskery attended the University of York (UK) for four years until 2009, obtaining a Master of Physics (MPhys) degree with first class honours. He then attended the University of Warwick (UK), where he conducted research into novel magnetic systems and ‘spintronic’ candidate materials. This research employed a wide range of experimental and theoretical methods, most prominently magnetic Compton scattering, a high-energy X-ray scattering technique performed at synchrotron facilities in Japan and France. Ian obtained his PhD in Physics from the University of Warwick in 2013.

Since 2013, Ian has been a postdoctoral researcher with the Additive Manufacturing and 3D Printing Research Group (AM3DPRG) at the University of Nottingham (UK). His research employs selective laser melting (SLM), a process for manufacturing metal components from powder feedstock. He has investigated the mechanical and microstructural properties of materials made by SLM, with a focus on the high strength aluminium alloy Al-Si10-Mg. Ian also investigates the use of lightweight cellular or lattice structures in additive manufacturing, examining relationships between their design, manufacture and mechanical performance.  

About the author

Nicola M. Everitt

Dr. Nicola Everitt’s field of expertise is in structure/property relationships, focusing on mechanical property evaluation of small samples, from plant tissues (e.g. roots, seeds) and biomedical materials (e.g. degradable polymer scaffolds and bone), to thin hard films and additive manufactured microstructures. She obtained her first degree, B.Sc. (Hons) in Materials Science, with one year

research placement at the University of Bath. For her doctorate studies she went to the University of Oxford and obtained her D.Phil. for Research on Indentation creep in Magnesium Oxide and Germanium. Straight after this she went to the University of Bristol as a Lecturer in materials in the Aerospace Engineering Department (becoming a Senior lecturer in 1997). She moved to University of Nottingham as Associate Professor in the Department of Mechanical, Materials and Manufacturing Engineering in 2001. Her most recent award is the International Association of Advanced Materials Scientist Medal (IAAM Scientist medal) for the year 2016 due to her “notable and outstanding contribution in the field of Advanced Materials Science and Technology”.

Nicola has particular interest and expertise in micro- and nano-indentation testing, and dynamic materials analysis i.e. the behaviour of materials at different rates of loading or strain at different temperatures. This work is very relevant to understanding the behaviour of 3D printed structures.  

About the author

Ian Ashcroft

Ian Ashcroft is Professor of Solid Mechanics at the University of Nottingham and a member of the Additive Manufacturing Research Group. His expertise is in both experimental and computational mechanics with particular interest in the effects of complex loading and environment on materials and structures and in the development of multi-scale and multi-physics modelling techniques. He has had sustained funding for this work, including a personal DSTL Fellowship from 2000-2003, and funding from industry, MOD and EPSRC. Since 2005 he has focused his research into the application of solid mechanics to additive manufacturing, particularly in developing multi-physics modelling techniques for AM processes and the post processing performance of parts and the development of design and optimisation techniques to exploit the design freedoms of various AM technologies.

He has published his work extensively, with over 150 peer reviewed journal publications, approximately 3000 citations and a scholar h-index of 33 (27 since 2011). Current funding is in excess of £8M, including EPSRC, INNOVATE UK and industry funded projects. He is a Co-Investigator on the EPSRC Centre for Innovative Manufacturing in Additive Manufacturing (CIMAM) – where he leads research in multi-physics modelling and computational design and optimisation – as well as being Programme Director of the associated Centre for Doctoral Training (CDT) in AM.

 

About the author

Chris Tuck

Prof Chris Tuck is an expert in 3D Printing and Additive Manufacturing with over 13 years working in Research and Development of the processes, materials and applications of this disruptive technology. Chris has a BEng (Hons) in Materials Science and Engineering from Brunel University and an EngD from Cranfield University. Chris joined the Additive Manufacturing (AM) Research Group at Loughborough University in 2003 as a Research Associate becoming a Lecturer in Innovative Design and Manufacturing in 2008.

In 2012 Chris moved to The University of Nottingham’s Faculty of Engineering and is Deputy Director of the EPSRC Centre of Innovative Manufacturing in Additive Manufacturing and currently running a number of projects based around the manufacture of multi-material and multifunctional inkjet printing, nano-scale additive manufacturing systems, and the development of metallic AM systems for use in industry.

In 2014 Chris launched and became Director of the EPSRC Centre for Doctoral Training in AM and was made Full Professor in 2016. Chris was an Executive Member of the ASTM F42 AM standards committee, BSi AM008 and ISO TC261. In October 2014 Chris became a Co-Founder of the University of Nottingham’s spin out company Added Scientific.  

Journal Reference

Nesma T. Aboulkhair1,2, Ian Maskery1, Chris Tuck1, Ian Ashcroft1, Nicola M. Everitt2, Improving the Fatigue Behaviour of a Selectively Laser Melted Aluminium Alloy: Influence of Heat Treatment and Surface Quality, Materials and Design 104 (2016) 174 –182.

Show Affiliations
  1. dditive Manufacturing and 3D Printing Research Group, Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, United Kingdom.
  2. Bioengineering Research Group, Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, United Kingdom.

 

 

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