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Materials

Relatore: Prof. Maurizio Vedani

Tutor: Prof. Stefano Beretta

Università di Provenienza: Politecnico di Milano - Ingegneria dei Materiali e delle Nanotecnologie

Titolo della Tesi: Surface Features and Their Effects on Parts Produced by Laser Powder Bed Fusion (LPBF)

Surface Features and Their Effects on Parts Produced by Laser Powder Bed Fusion (LPBF)

Introduction

Despite the disrupt benefits of Laser Powder Bed Fusion (LPBF), the full potential of the technology has a long journey to go. For instance, the quality of LPBF surfaces and near surface regions, the governing physical phenomena and their effects on the mechanical behaviour of the parts are not fully comprehended. In this regard, the current work starts with a particular attention to surface and sub-surface regions of parts produced by LPBF in term of their morphology, microstructure, chemistry and mechanical behaviour to improve the general understanding of defect genesis. Consequently, the effects of different surface finishing post manufacturing treatments on the fatigue behaviour of the parts in terms of residual surface and sub-surface pores are examined. Lastly, a comprehensive metrological multi-scale analysis is performed to evaluate the effects of surface features and volumetric defects typical of additively manufactured materials. Fatigue tests and finite element analysis were conducted and a fracture mechanics model was proposed. Moreover, a statistical model describing the competition between volumetric defects and surface irregularities was developed and validated.

Objectives

  • Sound understanding of the morphological features of LPBF surfaces.
  • An investigation into spatter generation and the corresponding post generation effect on the quality of the final part.
  • Thorough grasp on the surface and sub-surface defect driven phenomena.
  • Realization of a suitable metrological method and the appropriate parametrization of LPBF surfaces.
  • Investigation on the effect of varieties of surface conditions on the fatigue behaviour of LPBF parts.
  • Understanding the combined effect of surface and sub-surface defects on the fatigue behaviour of LPBF parts.

Results

Fatigue behaviour of AlSi10Mg samples subjected to different surface finishing process i.e. sandblasting, vibro- finishing and, machining and polishing was investigated in terms of residual surface anomalies and sub-surface defects. Regardless of the presence of sub-surface pores, classical metrological approach was unable to identify such features. Therefore, the study calls for a measurement methodology of defects, and a model which can employ these measurements. Machining and polishing at sufficient depth exhibited the best results due to the complete elimination of surface protrusions and sub- surface porosity.
Most of the commonly used metrological methods for surface characterization were proven to be unsuitable for determining important features such as undercuts and sub-surfaces pores. Hence, a comprehensive coupled investigation of metrological methods and cross- sectional analysis is performed which successfully evaluated the effects of surface features and volumetric defects typical of additively manufactured materials. Fatigue tests and fractographic analyses were conducted to support the finite element simulations and proposed fracture mechanics model. The results demonstrate that the standard metrological methods cannot provide all of the data needed to model the fatigue behaviour of additively manufactured materials robustly. Moreover, a statistical model describing the competition between volumetric defects and surface irregularities was developed and validated.

Conclusions

  • Three different surface features have been identified on the surface of SLM processed AlSi7Mg alloy: ballings, spatter particles and partially melted metal powders.
  • Spatters were shown to be the most detrimental LPBF by-product leading to formation of sub-surface and bulk lack of fusion defects.
  • A multi-scale metrological analysis was proposed and proved to correlate with the fatigue behaviour of distinct surfaces.
  • Post manufacturing surface finishing processes has to be carried out after detailed investigation of the surface anomalies and sub-surface porosity to ensure the elimination of such features.

References

M. Hamidi Nasab, D. Gastaldi, N.F. Lecis, M. Vedani, On morphological surface features of the parts printed by selective laser melting (SLM), Addit. Manuf. 24 (2018) 373–377.
M. Hamidi Nasab, M. Vedani, E. Boillat, R.E. Logé, A Study on the Surface Features of the SLM Processed Parts and Their Journey to Become Bulk Defects, in: Eur. Powder Metall. Assoc., 2019.
M. Hamidi Nasab, A. Giussani, D. Gastaldi, V. Tirelli, M. Vedani, Effect of Surface and Subsurface Defects on Fatigue Behavior of AlSi10Mg Alloy Processed by Laser Powder Bed Fusion (L-PBF), Metals (Basel), 9 (2019) 1063.
M. Hamidi Nasab, S. Romano, D. Gastaldi, S. Beretta, M. Vedani, Combined effect of surface anomalies and volumetric defects on fatigue assessment of AlSi7Mg fabricated via laser powder bed fusion, Addit. Manuf. (2019) 100918.