Investigations on Similar, Dissimilar and Functionally Graded Materials Depositions using Double Wire Arc Additive Manufacturing

In this thesis, investigations on depositions of similar, dissimilar and functionally graded
materials using gas metal arc welding (GMAW)-based wire arc additive manufacturing
(WAAM) employing single and double wire feed techniques were carried out. Initially
single bead geometry model was developed utilizing response surface methodology and
experiments conducted utilizing Box-Behenken design of experiments for deposition of
austenitic stainless steel (ASS) 304L by WAAM. Bead geometry parameters i.e., bead width,
bead height and bead cross-sectional area etc. were expressed in terms of the process
parameters like voltage, wire feed rate, torch speed and gas flow rate. A multi-bead
deposition model was proposed to determine optimal process parameters for near-net shape
deposition in WAAM utilizing a genetic algorithm (GA) and single bead geometry model.
The proposed approach was validated through deposition of three shapes, i.e., slice, wall and
block. It was revealed that optimal bead sizes and degree of overlapping were different for
fabricating different shaped parts. Mechanical testing and metallurgical characterization
were carried out to compare the deposited ASS304L through single wire feed (SWF) and
double wire arc additive manufacturing (DWAAM) techniques. DWAAM deposition was
seen to deliver higher deposition rate and superior mechanical properties compared to that of
the single wire feed deposition for ASS 304L. The effects of process parameters on bead
geometry and microstructures in deposition of dissimilar austenitic-ferritic stainless steels
(ASS304L/FSS430L) by DWAAM were also investigated. Bead size and deposition rate
was increased in dissimilar ASS304L/FSS430L depositions by DWAAM with voltage, wire
feed rate and gas flow rate. The microstructure of dissimilar ASS304L/FSS430L contained
with a large number of dendrites having growth direction along the temperature gradient.
Moreover, contents of major allowing elements in the deposited ASS304L/FSS430L were
seen to vary with the wire feed rate, and mass fraction of element Ni got enhanced with
increase of ASS 304L wire feed rate. Bimetallic structures (BMS) comprising of Inconel 625
and ASS 304L with and without a functionally gradient intermediate layer (IL) were
deposited through DWAAM. The functionally gradient IL was made by gradually changing
the compositions from pure Inconel 625 to pure 304L stainless steel (SS) by varying the feed
rates of two wires. Scanning electron microscopy and energy dispersive spectroscopy tests
were conducted to investigate microstructures and chemical composition variations at the
interface of the BMS. It was found that the chemical composition altered gradually through
the addition of IL compared to the sudden change in case of BMS without IL. Microstructure
of pure ASS304L, ASS304L/IN625 (50%/50%) BMS, and pure IN 625 were found as
columnar dendritic structure with some amount of secondary dendrite, columnar dendrites
with secondary dendrite, and columnar dendrites, respectively. Tensile tests, microhardness
study and fracture morphology of the BMS revealed better mechanical properties of the
BMS with IL (BMS-IL) compared to that without IL. The DWAAM method exhibited
capability of fabrication of high-quality similar, dissimilar, bimetallic and functionally
gradient materials structures.