Page 98 - MSAM-2-3

Materials Science in Additive Manufacturing Validation of a novel ML model for AM-PSP

1.4. Strain hardening and temperature effects on the alloys. The Taylor model has been used to represent the
Ti-6Al-4V AM parts strain rate and strain information statistically.
Strain hardening is one of the most important 1.5. Characterization and machinability of
considerations in the evaluation of the machining of AM-processed titanium
metallic materials. The strength, ductility, toughness, and
deformability of materials are intimately related to strain- Titanium alloys are widely used in aerospace, bio-medical,
hardening characteristics [63-67] . For Ti-6Al-4V, the dominant and defense applications. Titanium alloys are often
HCP metals present a more complex case due to their fabricated by conventional costly forging and casting
low symmetry that restricts the number of slip systems, processes, which have a lower material utilization ratio
which leads to a strong plastic anisotropy. Different strain and lower production rate. Ti-6Al-4V alloy is an α-β dual-
hardening behaviors have been reported in many Ti alloys, phase moderate-strength alloy with high comprehensive
such as compression testing at high temperatures ; stage mechanical and chemical properties. Due to its excellent
[64]
III strain hardening happens only at elevated temperatures strength-to-weight ratio, Ti-6Al-4V alloy is widely used
on tensile flow behavior testing . Hardening capacity in the aerospace industry for manufacturing blades, discs,
[65]
and strain hardening exponent decreased with increasing and cooling parts in compressors, as well as biomedical
strain rate or decreasing temperature on electron beam implants.
welding of Ti-6Al-4V. Stage III hardening happened after In critical aircraft structures, Ti-6Al-4V has been
yielding in the heat affect zone (HAZ). Furthermore, the processed through forging. Ti-6Al-4V accounts for half
hardening rate was strongly dependent on the strain rate of the total titanium alloy application . AM technologies
[68]
and temperature. provide a new opportunity to produce large and complex
However, the heterogeneity in AM Ti-6Al-4V parts Ti-6Al-4V products. Since there is an ever-growing
might lead to varied material properties. According to research interest and application of this alloy, this research
the previous research, the mechanical properties are will focus on Ti-6Al-4V alloys, which have been fabricated
more heterogeneous along the AM build direction than through PBF and DED AM technologies.
the parallel HAZ. Mechanical properties such as strength Several investigators have focused on the material
and microhardness are microstructure-dependent. The and mechanical properties of different AM-processed
mechanical heterogeneity along the AM path is dependent Ti-6Al-4V alloys. Rafi et al. compared Ti-6Al-4V
on the changing trend of the size of β columnar grain and alloys fabricated through the SLM process and EB-PBF
martensite. The strain and strain rate heterogeneity also process and found that martensitic α’ microstructure
depend on the microstructure heterogeneity . dominates the SLM parts while α phase with β separating
[62]
According to the previous research, for Ti-6Al-4V the α lamellae structure observed in EB-PBF sample.
alloys, the flow stress increases when the strain rate SLM Ti-6Al-4V samples have higher tensile strength,
increases and temperature decreases. Furthermore, the lower ductility, and higher fatigue limits than EB-PBF-
[69]
fracture strain increases with an increasing strain rate at a produced samples . Liu et al. investigated the mechanical
certain temperature. For a certain strain rate, the smallest properties of Ti-6Al-4V fabricated using laser melting
fracture strain was observed at 700°C. Furthermore, the deposition-SLM (LMD-SLM) hybrid AM process and
strain rate sensitivity increases with increasing strain and found that the density of hybrid-forming can reach 99.5%
strain rate. Moreover, a direct correlation is found between with the existence of pores with diameter <20 μm, and
the depth and density of the dimples and the magnitude of tensile strength and elongation of the hybrid AM process
[70]
the fracture strain . To better understand the Ti-6Al-4V can reach 918 MPa and 11%, respectively . Shi et al.
[67]
machinability, cryogenic machining was applied to investigated Ti-6Al-4V alloys fabricated through SLM and
titanium alloys, and people found that the cryogenic wire arc AM (WAAM) processes. They found that short
coolant extended the tool life. Moreover, titanium chips β columnar grains consisting of primarily martensite
show less deformation and heat was generated during chip α’ in SLM parts. In contrast, coarse β columnar grains
formation with low feed and high depth of cut combination. awash with α lamellae lead to epitaxial growth observed
At present, the machinability of titanium alloys is focused in WAAM parts. Compared to the WAAM sample, SLM
on the wrought parts. However, AM titanium shows a parts showed higher tensile strength and lower elongation,
dramatic difference among mechanical properties, such as especially in samples built in a vertical direction .
[71]
heterogeneity, tensile stress, microhardness, and so on. Due to inherently different thermal cycling conditions
During the machining of titanium, the strain magnitude and process parameters across AM processes, this will lead
and strain rate involved directly affect the machinability of to diverse microstructure representation and mechanical

Volume 2 Issue 3 (2023) 6 https://doi.org/10.36922/msam.0999