Page 79 - MSAM-1-1

Materials Science in Additive Manufacturing A ML model for AM PSP of Ti64

a linear regression analysis was performed to calculate the Where, τ CRSS is the crystal critical resolved shear stress,
principal stress of each specimen. dW is the rate of work done, and d is the incremental
ε
strain.
2.2.3. EBSD data extraction
In a given transformed state of strain in a crystal, Demir
Grain orientation during machining of polycrystalline metals (2008) pointed out that the Taylor factor shows that the
influences the machining response, that is, cutting force and minimum work is done against the slip resistances for each
surface finish quality. Demir and Mercan (2018) have shown five of 12 slip system combinations . Since this study
[38]
that the effect of elastic and plastic anisotropy of the material
on cutting force cannot be neglected . Hence, several aims to predict the machining behavior from the material
[35]
models have been developed for deformation mechanisms structure, general shear stress and strain conditions were
and stress in polycrystalline alloys like Ti-6Al-4V to predict applied to the SF and Taylor factor calculation. In this
and simulate slip, twinning, and detwinning in hexagonal study, the SF value and Taylor factor value calculated on
(HCP) unit cell in the past decades. three replicates of EBSD data for every material surface
and were treated as independent input variables in the S-P
The Schmid factor (SF) was selected in this study linkage workflow.
to capture the local slip and twinning activity in the
microstructure which have direct effects on related 2.3. Machining experiment and specific cutting
machining behavior at a macroscale. SF is defined as a ratio energy
of shear stress on the system  and system applied stress
s
 , and can also be represented as a product of cosine of 2.3.1. Machine set up
n
the angles between the applied stress axis and slip plane The machining experiment was performed on a Haas
normal ∅, and applied stress axis and slip direction λ. VF2SS 3 axis vertical CNC machining center. Peripheral
σ milling applied on the specimen blocks mounted on
SF = s cos∅ = cosλ (III) a custom-built vise. Cutting tools selected for the
σ n experiments were 6.35 mm (1/4 inch) nominal diameter

Huang et al. (2019) indicated that the Schmid-based six flute carbide end mills with KC635M TiAlN coating
model assumes that the macroscopic stress is used in the (Model HPFT250S6075), which is recommended for
calculation of the local solved shear stress in a specific machining titanium alloys. The tools have flat square
slip and twinning system, and the evolution of critical end geometry, with zero radial rake angle and 45 axial
resolved shear stress modeling depends on the local stress rake angle, which made the modeling of tool geometry
and plastic strain, that is, applied slip system and current for specific cutting energy calculation easier. Vise and
grain orientation , which is directly related to machining workpiece were mounted on a Kistler 9257A dynamometer,
[36]
behavior of the material. which connected to a Kistler Type 5010 Charge Amplifier,
In Ti-6Al-4V, the dominant α phase is an HCP crystal as shown in Figure 2.
structure which has the following slip systems: Basal slip The cutting force data signals were collected and
{0 0 0 1}(1 1 2 0), prismatic slip {1 0 1 0}(1 1 2 0), restored by Data Physics Quattro Dynamic Signal Analyzer
pyramidal slip {1 0 1 1} (1 1 2 0), first-order pyramidal using a sampling frequency of 2560 Hz. After each tool set
up, a dial indicator with a resolution of 0.00127 mm was
slip {1 0 1 1}(1 1 2 3), and second-order pyramidal slip used to measure the tool runout to ensure each machining
{1 1 2 2}(1 1 2 3). Hémery and Villechaise (2018) have path has an acceptable tolerance.
shown that the dominant slip systems in Ti-6Al-4V are
basal, prismatic, and first-order pyramidal . These are the Based on the Kennametal tool manufacturer’s
[37]
three slip systems considered in this study. recommendation for Ti-6Al-4V alloys milling, three levels
of machining parameters were selected for the machining
While the SF is based on the isostress assumption, experiments, as shown in Table 1.
another descriptor, the Taylor factor (M) is based on an
isostrain assumption, which measures the work done on As described above, for EB-PBF, as-AM L-PBF, and
the crystal for a given orientation and deformation, which heat-treated L-PBF specimens, two feed directions, XY
can be represented as: and XZ, as shown in Figure 3, that are perpendicular and
dW parallel to the build orientation respectively, were utilized
M � (IV) in the experiment. The radial immersion was 50% for the
CRSS CRSS d experiment.

Volume 1 Issue 1 (2022) 7 https://doi.org/10.18063/msam.v1i1.6