PSI - Issue 13

Ali Waqas et al. / Procedia Structural Integrity 13 (2018) 2065–2070 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Additive manufacturing (AM) is a process to create parts by addition of material layer by layer attaining near-net shape. AM is suitable for manufacturing of complex parts by utilizing lesser material as compared to the subtractive manufacturing resulting in a cost-efficient way (Frazier, 2014). Near -net shape is obtained by depositing successive layers of material melting them in powdered or wire form using heat source from an electron beam, laser beam, or plasma or electric arc (Antonysamy, 2012; Buckner & Lonnie, 8 – 9 October 2012). Wire and arc additive manufacturing (WAAM) is the method of creating 3D metal parts in a layer by layer fashion using electric arc as the energy source while combining it with a wire feed mechanism. The mechanical properties of the resulting material mostly vary with the parameters involved including current, voltage and travel speed. Generally, the mechanical properties are better as compared to the casted material which can be beneficial in aerospace industry applications (Brandl, Baufeld, & Leyens, 2010). WAAM cells can be based on either robotic systems or computer numerical controlled gantries. With the latter being typically retrofitted machine tools, and with robotic machining becoming more common, both systems will be able to provide integrated machining (Pandremenos, Doukas, Stavropoulos, & Chryssolouris, September 2011,). GMAW for additive manufacturing has some problems associated with the process. The primary problem being the difference in height at arc striking and arc extinguishing region. Different approaches have been adopted to control the parameters to finally attain the desired forming appearance (Xiong, Yin, & Zhang, 2016). But during this process, material undergoes high temperature cycles with different cooling rates and preheat and post heat effects leading to a unique microstructure (Zhao, Zhang, Yin, & Wu, 2011). Toughness is the ability of the material to absorb energy which depends on the strength of material along with its ductile or brittle behavior (Cao, Zhang, & Huang, 2017). Factors affecting the toughness include composition, microstructure and the temperature. For carbon steels, it generally varies from 10 to 200 J depending on the percentage of carbon and testing temperature. For layered structure, toughness anisotropy can occur according to the extent of change in microstructure and grain size (Rojas, Martinez, & Vera, 2014). Salimi A et al. have worked on the influence of direction on impact properties of banded steel structures studying the anisotropy in horizontal and vertical direction (Salimi, Zadeh, & Toroghinejad, 2013). E, Lucon et al. have studied the possibility for testing at room temperature instead of lower temperatures statistically showing lesser scatter for room temperature testing (Lucon, Mccowan, & Santoyo, 2015). The results of absorbed impact energy must be supported by shear fracture area showing the same behavior of material (Dean, Manahan, & Mccowan, 2008). So, the study should include the metallography and fractography to validate the results from Charpy impact testing. Fractography is the science that can examine fracture surfaces of the failed material after the actual breakage identifying the origin of fracture, crack propagation, the mechanism of material failure and nature of stress. A broad categorization of fractures includes ductile and brittle fractures. Ductile fractures are identified by tearing of material along with plastic deformation while this phenomenon is not true for brittle material which exhibit almost no plastic deformation. Microscopically, ductile fracture in metals displays a dimpled surface appearance created by micro-void coalescence. Brittle fracture is evident from cleavage facets, intergranular facets and striations (Parrington, 2003). Charpy impact tests have been performed in this study at room temperature to study the impact toughness of components made by robotic assisted GMAW in direction parallel and perpendicular to the deposition direction. Next section gives a brief description of method to create the thin wall and extraction of samples according to ASTM standard. In the results and discussion part, Brinell hardness and Charpy impact results along with metallography and fractography of the specimen are given. 2. Method and Experiment The construction of specimen has been done in layer by layer fashion. Robotic assisted GMAW has been used to create a thin wall using welding electrode ER70S-6. The start of the weld bead, here referred as arc striking area, exhibits increased height due to rapid dissipation of heat as compared to the rest of the weld bead (steady stage) while the end of the weld bead, here referred as arc extinguishing area, has a decreasing slope as the weld bead ends due to the sudden termination of current source. This behavior creates a significant difference in height at the arc striking