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Validation of process models for additive manufacturing

TWI in collaboration with the SIMULIA brand of Dassault Systèmes is validating Finite Element Modelling (FEM) techniques capable of accurately simulating additive manufacturing (AM) processes, allowing improved part design and process setup before any physical manufacturing takes place.

Additive manufacturing techniques are increasingly being chosen over other manufacturing processes in industries such as aerospace, medical devices and automotive. While the technique has been around for decades, the past few years have seen an impressive number of advances by both the academic and industrial communities towards printing functional parts for end-use as well as tooling applications. While progress is being made, the technique can be prone to manufacturing defects which often can be mitigated only by costly experimentation.

To support the development of AM technologies, TWI has initiated Core Research Programme research focusing on validating FEM techniques to simulate AM processes. In particular, recent work has focussed on the Selective Laser Melting (SLM) process.

Experimental work

TWI produced double cantilever parts (as shown in Figure 1) using a Renishaw AM250 machine. All parts were produced using Ti-6Al-4V Grade 23 metal powder with a powder particle size range between 15 and 45μm. A 90° alternating scan strategy was used, comprising a series of parallel hatch lines and four boundary contours, rotated by 90° every layer.

After manufacturing the double cantilevers, TWI used wire electrical discharge machining to cut the support structures just below the solid beam surfaces. Upon cutting, the presence of residual stresses generated deflections of the remaining double-sided cantilever beam structure. The out-of-plane deflections were measured using a FaroArm precision measuring tool.

Modelling approach

The simulations used new physics-based FEM formulations available in Abaqus 2017 from the SIMULIA brand of Dassault Systèmes. The new features of Abaqus 2017 enable the exact machine information about the powder recoating sequence, laser scan path, and process parameters to be directly input in the FE model. These state-of-the-art capabilities enable progressive element activation, progressive heating computations and cooling of the evolving solid surface of the part as the build progresses.

The simulation model employed temperature-dependent material properties for the heat transfer and stress analysis simulations. An image of the residual stresses after the wire cutting is shown in Figure 2.

Validation

The measured deflections from the test pieces were compared with the FEM predictions as shown in Figure 3. There is strong agreement between the predictions and measurements, leading to confidence in the use of this new modelling approach.

Figure 1. SLM double cantilevers after wire cutting
Figure 1. SLM double cantilevers after wire cutting
Figure 2. Image of residual stress after cutting
Figure 2. Image of residual stress after cutting
Figure 3. Comparison between experimental measurements and predictions of deflection
Figure 3. Comparison between experimental measurements and predictions of deflection

Conclusions

Through this research, TWI has validated new Finite Element Modelling and analysis techniques in collaboration with the SIMULIA brand of Dassault Systèmes to simulate AM processes. The validation activities undertaken to-date provide high confidence in this approach.

1 Tripathy S, Chin C, London T, Anakalkhope U and Oancea V (2017): ‘Process modelling and validation of powder bed metal additive manufacturing’, NAFEMS World Congress 2017, 11-14 June 2017, Stockholm, Sweden.

For further information please see Integrity Management or email contactus@twi.co.uk

Avatar Tyler London Regional Team Manager – Numerical Modelling and Optimisation

Tyler joined the Numerical Modelling and Optimisation section of TWI in 2011 after obtaining an MSc in Mathematical Modelling & Scientific Computing from the University of Oxford. He specialises in engineering analysis: applying mathematical modelling and finite element analysis to support research, product development and failure investigations related to structural integrity and materials performance. Tyler is a Chartered Engineer, Chartered Mathematician, Chartered Scientist and a NAFEMS-certified, advanced-level Professional Simulation Engineer. He has published over 20 peer-reviewed publications and conference papers, and actively participates in many national and international codes and standards committees including BS 7910 residual stress, R6 weld modelling guidelines, and the EN 13094 FEA sub-group.

For more information please email:


contactus@twi.co.uk