M A, Somashekara and S, Suryakumar
(2016)
MANUFACTURE OF FUNCTIONALLY GRADIENT
OBJECTS THROUGH WELD-DEPOSITION.
PhD thesis, Indian institute of technology Hyderabad.
Abstract
Functionally Gradient Material (FGM) may have a controlled variation of the
material matrix so as to obtain the desired distribution of the properties such as
color, density, porosity, hardness, toughness etc. There is a growing interest in FGMs
due to their ability to offer high toughness, high strength, machinability, better
resistance to corrosion and oxidation effects, and facilitating bonding of metals
without severe internal thermal stresses. However, actual realization of FGMs still
remains a challenge. Most naturally occurring objects are gradient in nature; examples
are bamboo, bone, stone etc. Most man-made objects on the other hand are uniform.
This is mainly due to the complexity involved in their design and subsequent
manufacturing.
The objects built through Additive Manufacturing techniques are inhomogeneous or
non-uniform, i.e., they are inherently anisotropic. When this inherent nature is
carefully exploited, the anisotropy transforms into the desired distribution of the
properties. Weld-deposition based Additive Manufacturing techniques offer unique
advantages on that front due to their ability to control the properties of the
deposited matrix by controlling the process parameters like current, layer thickness
etc. Preliminary experiments carried out this subject have shown that the hardness
of the material is dependent on the weld-deposition current. Hence, online control
of the same will help in manufacturing a metal matrix with variable hardness value.
The variation possible through this method however will be limited in nature. A wider
control of material properties can be obtained with the help of tandem weld-deposition
setup like twin-wire. In twin-wire weld-deposition, two filler wires (electrodes) are
guided separately and it is possible to control each filler wire individually. The present
work focuses on obtaining a wide range of material properties by selecting filler wires
with complementary properties and controlling the deposition rate of each of them
separately. The experimental setup of Twin-wire Weld-deposition based Additive Manufacturing
(TWAM) are discussed in detail. Working principle of twin-wire weld-deposition
process along with the individual attachments viz. welding torch, wire feeder and
power source are also presented. ER70S-6 and ER110S-G are the two filler wires used
for the study; the former has lower hardness than the latter.
The range of process parameter for different combinations of these filler wires was
determined and the operating range of the same was identified. A second order
regression equation for predicting weld bead geometry of width and height as a
function of wire speed and torch speed was generated based on a series of experiments
and subsequently validated. Subsequently, the criterion for adapting the twin-wire
welding from joining to weld-deposition of a complete layer like thermal steady-state
condition, effect of torch direction and effect of overlapping beads have also been
studied.
Having established the primary process parameters and the secondary operating
condition for the TWAM process, various experiments carried out to identify the
suitable process parameters at a given location for a desired variation of hardness have
been presented. A predictive model for obtaining the wire speed of the filler wires
required for a desired value of hardness was also created. The following four sample
layers were fabricated to demonstrate the concept of realizing FGMs through TWAM
(1) gradient in stepover direction (2) gradient in weld-deposition direction and (3)
gradient in both the directions (4) gradient in three dimensions. The latter two as the
hardness variation is occurring in every weld-bead, a given weld-bead has to be
divided into multiple sub-programs and each sub-program representing the particular
set of process parameters has to be called from the robot controller. The fabricated
parts showed good match with the desired hardness values for a given location.
Furthermore, to demonstrate the possible applications of TWAM, two illustrative
examples were fabricated.
Once the methodology for fabrication of FGMs has been established, characterization
of objects fabricated through TWAM have been undertaken. Specimen made with five
different combinations of filler wires {100:0, 75:25, 50:50, 25:75, 0:100} were used
for the analysis. These specimen were examined further by subjecting them to micro
hardness, microstructural, X-Ray Fluorescence (XRF), Energy Dispersive X-ray
analysis (EDAX) and X-Ray diffraction (XRD) analysis. Further on, the width of the
transition region while switching over from one set of parameters to another was also
investigated. That will help in assessing the best possible resolution of the gradient
matrix possible.
Modelling of the welding process is felt necessary to understand the evolution of the
material properties and to better control the thermal and structural characteristics like
residual stresses resulting from the process. With the help of Finite Element Analysis
(FEA) and experimental methods, the effect of area filling paths on the residual
stresses developed during weld-deposition have been investigated. Three area-filling
patterns viz. raster, spiral-in and spiral-out were chosen. FEA for these three patterns
was done using ANSYS Mechanical APDL. The twin-wire arc weld-deposition was
modeled as a set of two moving heat sources separated at a fixed distance. The
deposited material was activated by element birth method once the arc passes over a
location, simulating the weld material deposition. The temperature gradient induced
residual stresses produced during and post material deposition were predicted using
passively coupled thermo-mechanical simulations. For validation, the residual stresses
in the weld-deposition specimen were measured using an X-ray diffraction (XRD)
system. Temperature distribution plays a critical role in the evolution of the residual
stresses during weld-deposition. Hence, two metrics viz., thermal mismatch profile
and secant-temperature rate were introduced to quantify preheat and conduction. It
was observed that raster patterns had the lowest thermal mismatch and secant-rates
resulting in lowest residual stresses of the three area-fill patterns. Residual stresses
from experiments are of the same order as those obtained from elastic-FE simulations,
however, with a low accuracy of the prediction. Hence, these cannot be directly used
for investigating the residual stresses developed. Nevertheless, for comparing the
various area-fill patterns, these simulations can provide preliminary insights.
With a combination of (1) process parameter study of twin wire deposition, (2)
manufacturing of gradient objects, (3) characterization of gradient layers, (4)
modelling of twin wire deposition process, this research attempt tries to establish twinwire weld-deposition based additive manufacturing as the viable method for the
manufacture of functionally gradient materials.
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