MANUFACTURE OF FUNCTIONALLY GRADIENT OBJECTS THROUGH WELD-DEPOSITION

M A, Somashekara and S, Suryakumar (2016) MANUFACTURE OF FUNCTIONALLY GRADIENT OBJECTS THROUGH WELD-DEPOSITION. PhD thesis, Indian institute of technology Hyderabad.

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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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IITH Creators:
IITH CreatorsORCiD
S, Suryakumarhttp://orcid.org/0000-0001-6015-7593
Item Type: Thesis (PhD)
Subjects: Others > Mechanics
Divisions: Department of Mechanical & Aerospace Engineering
Depositing User: Team Library
Date Deposited: 16 May 2019 11:50
Last Modified: 16 May 2019 11:50
URI: http://raiith.iith.ac.in/id/eprint/5204
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