Hybrid Additive Manufacturing of Al-Ti6Al4V with SLM e CS

Title: Hybrid additive manufacturing of Al-Ti6Al4V functionally graded materials with selective laser melting and cold spraying

Authors: Rocco Lupoi, ShuoYin, XingchenYan, ChaoyueChen, RichardJenkins, MinLiu

Keywords: Selective laser melting (SLM), Cold spraying (CS), Functionally graded material (FGM), Additive manufacturing (AM), XRD, Grain microstructure .

Abstract: A hybrid additive manufacturing technology for fabricating functionally graded materials (FGMs) is proposed in this paper. The new process represents a combination of two existing additive manufacturing processes, selective laser melting (SLM) and cold spraying (CS). The targeted experiment of Al and Al + Al2O3 deposited onto SLM Ti6Al4V via CS reveals that the hybrid additive manufacturing process can produce thick, dense and machinable FGMs composed of non-weldable metals without intermetallic phase formation at the multi-materials interface. The SLM Ti6Al4V part exhibited fully acicular martensitic microstructure in contrast with α + β microstructure in the Ti6Al4V feedstock, while the grain structure of the CS Al part had no significant change as compared with the Al feedstock. Due to the phase transformation of the SLM part and work hardening of the CS part, the overall hardness of the FMGs was higher than that of the feedstock.

Purpose: A hybrid AM technology combining SLM and CS is proposed in this work to fabricate metal-metal and metal matrix composite (MMC)-metal FGMs.

Methodology: For proving the feasibility of this hybrid AM process, a targeted experiment using CS to deposit pure Al and Al + Al2O3 MMC onto SLM Ti6Al4V part was carried out. The reason for choosing Ti6Al4V and Al as the feedstock is: firstly, Ti is non-weldable with Al due to the formation of brittle intermetallic phase (Al3Ti, Al2Ti) and the substantial difference in melting temperature and thermal expansion ratio (Tomashchuk et al., 2015); secondly, dense Ti and Ti6Al4V are difficult to produce with CS due to the high strength-to-weight ratio limiting the plastic deformation of Ti6Al4V particles. In addition, dense Al and Al alloys are hard to produce with SLM due to its high reflectivity (Vo et al., 2013), thereby it is almost impossible to produce an FGM composted of Al and Ti6Al4V with solo CS or SLM technology. In terms of the potential applications of the Al-Ti6Al4V FGM, it could be applied as a structural material in the fields of aerospace and automotive

Limitations: Despite gaining dense structure, the fabricated FGMs also had some defects. The defect in the SLM Ti6Al4V part was represented by large pores as a result of the accumulation of unmelted particles and surface roughness, while the CS part’s defect was mainly in the form of small pores caused by the insufficient particle plastic deformation. In addition, grain structure study reveals that the SLM Ti6Al4V part exhibited fully acicular martensitic microstructure, in contrast, to α + β microstructure in the feedstock. Therefore, the SLM Ti6Al4V part was slightly harder than the Ti6Al4V feedstock. The grain structure of the CS Al part had no significant change as compared with the Al feedstock, but the hardness of the CS Al part was much higher than that of the Al feedstock due to the work hardening effect. Furthermore, the analysis on the fracture surfaces indicates a high-quality adhesive and cohesive bonding of the FGMs.

Findings: The hybrid additive manufacturing process effectively prevented the brittle intermetallic phase formation at the connecting interface, producing thick, dense and machinable FGMs composited of non-weldable metals.

Practical implications: Al alloys and Ti alloys are both widely used as structural materials in aircraft, vehicle and luxury-bike manufacturing. Ti alloys have high strength, while Al alloys are light and low-cost. Therefore, it would be promising if Ti alloys are applied as the core material surrounded by thick Al alloy layer which provides a strengthening effect. Such structural material can provide sufficiently high strength and reduce total weight and material’ cost at the same time. The addition of Al2O3 reinforcements in the outside Al alloy layer will allow a further improvement of the wear-resistance performance. It is worthy to note that the proposed CS-SLM hybrid AM process can be not only used for producing Al-Ti6Al4V FGMs but also suitable for other material combinations and applications. Particularly, based on this hybrid AM process, the CS deposit can be used to modify the original structure of an SLM component by adding new features and also to restore a damaged SLM component.

Grafical Abstract:

Full reference: Yin, S., Yan, X., Chen, C., Jenkins, R., Liu, M., & Lupoi, R. (2018). Produzione di additivi ibridi di materiali funzionalmente classificati Al-Ti6Al4V con fusione laser selettiva e spruzzatura a freddo. Journal of Materials Processing Technology , 255 , 650-655.

Link: https://www.sciencedirect.com/science/article/pii/S0924013618300165#sec0010