Purpose Twin-wire welding-based additive manufacturing (TWAM) is a unique process which uses gas metal arc welding (GMAW)-based twin-wire weld-deposition to create functionally gradient materials (FGMs). Presented study aims to focus on creating metallic objects with a hardness gradient using GMAW of twin-wire weld deposition setup. Design/methodology/approach By using dissimilar filler wires in twin-wire weld-deposition, it is possible to create metallic objects with varying hardness. This is made possible by individually controlling the proportion of each filler wire used. ER70S-6 and ER110S-G are the two filler wires used for the study; the former has lower hardness than the latter. In the current study, methodology and 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. Subsequently, sample parts with gradient in various directions have been fabricated. Findings For dissimilar twin-wire weld-deposition used here, it is observed that the resultant hardness is in the volumetric proportion of the hardness of the individual filler wires. This aids the fabrication of FGMs using arc based weld-deposition with localized control of hardness, achieved through the control of the ratio of wire speeds of the individual filler wires. Four sample parts were fabricated to demonstrate the concept of realizing FGMs through TWAM. The fabricated parts showed good match with the desired hardness variation. Research limitations/implications This paper successfully presents the capability of TWAM for creating gradient metallic objects with varying hardness. Although developed using ER70S-6 and ER110S-G filler wire combination, the methodology can be extended for other filler wire combinations too for creating FGMs Originality/value GMAW-based twin-wire welding for additive manufacturing is a novel process which uses dissimilar filler wires for creating FGMs. This paper describes methodology of the same followed by illustration of parts created with bi-directional hardness gradient.

Various energy sources are available for sintering and/or depositing the material in additive manufacturing for metallic objects. These can be mainly categorized as laser based, electron beam based and arc based. While laser and electron offer better surface finish, it is possible to achieve high deposition rates in arc based weld-deposition. The inferior surface finish can be compensated by going for a hybrid system, combining deposition and machining. Twin-wire based weld-deposition, used in the present work, makes it possible to even realize functionally gradient material matrix; the use of two different filler materials into a single weld-pool makes this possible. Wire speed, torch speed and filler material are important factors that effect the composition of the deposited volume. Determination of the operating range and effect of these process parameters therefore is important to control the properties of the weld-deposited gradient objects. The current work presents the material composition of two filler materials ER70S6 and ER110SG with different wire speed and torch speed. Deposited material elemental compositions were analyzed using ED-XRF machine.

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