Turning of Additively Manufactured Ti6Al4V: Effect of the Highly Oriented Microstructure on the Surface Integrity.

Lucia Lizzul,Rachele Bertolini,Andrea Ghiotti,Stefania Bruschi

doi:10.3390/ma14112842

Abstract

Additive manufacturing processes induce a high orientation in the microstructure of the printed part due to the strong thermal gradients developed during the process caused by the highly concentrated heat source that is used to melt the metal powder layer-by-layer. The resulting microstructural anisotropy may have an effect on the post-processing operations such as machining ones. This paper investigates the influence of the anisotropy in turning operations carried out on laser powder bed fused Ti6Al4V parts manufactured with different scanning strategies. The machinability under both transverse and cylindrical turning operations was assessed in terms of surface integrity, considering both surface and sub-surface aspects. The effect of the different cooling conditions, that is flood and cryogenic ones, was studied as well. The outcomes showed that the microstructural anisotropy had a remarkable effect on the machining operations and that the cryogenic cooling enhanced the effect of the anisotropy in determining the surface integrity.

Highlights

At the industrial level, the Additive Manufacturing (AM) of metals has been developing to meet the demands of process automation and for the reduction of material wastage [1,2]
The microstructure is composed of three different features: (i) elongated prior β grains developed along the Build-up Direction (BD), (ii) continuous lines of αGB phase layers that decorate the β grains boundaries, and (iii) α + β lamellae within β grains organized in the Widmanstätten morphology
The impact on the machined surface integrity of the anisotropic microstructure of the AM Ti6Al4V alloy was investigated in this study

Summary

Introduction

The Additive Manufacturing (AM) of metals has been developing to meet the demands of process automation and for the reduction of material wastage [1,2]. AM parts are, manufactured by deposing and subsequently melting material layer-by-layer, starting from a digital model that helps in accommodating complex design together with the ease of model and data manipulation. Strong thermal gradients develop during the deposition process due to the concentrated heat source used to melt the metal powder, such as laser or electron beams, inducing a highly oriented microstructure along the. Compared to the conventionally manufactured parts, the use of AM allows for the reduction of the mass of aircraft components, leading to lighter structures together with reduced costs, material wastage, and lead times [10]. For the medical sector AM allows the production

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