Effect of Process Parameters on the Crack Formation in Laser Metal Powder Deposition of Alloy 718
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INTRODUCTION
NICKEL (Ni)- and Nickel-iron (Ni-Fe)-based superalloys are widely used in the aero, nuclear, and petrochemical industries due to their capabilities to retain their mechanical properties when used in elevated temperature applications. However, due to the complex chemical composition in these alloys, several secondary phases form in the austenitic c matrix, which have proven to increase the hot cracking susceptibility of the material.[1] Due to the rapid precipitation of the strengthening phase c’ in Ni- and Ni-Fe-based superalloys, leading to strain age cracking, many Ni-based superalloys are considered to have low weldability. The Ni-Fe-based superalloy Alloy 718, on the other hand, is mainly strengthened by the c’’ phase, which has a slower precipitation kinetics compared with the c’ phase. Alloy 718 is therefore considered to have better weldability compared with most of the other Ni-based superalloys. However, in Alloy 718, Laves phase forms through the eutectic reaction L fi c + Laves at
ANDREAS SEGERSTARK, JOEL ANDERSSON and LARSERIK SVENSSON are with the Department of Engineering Science, University West, Trollha¨ttan, 461 86, Sweden. Contact e-mail: [email protected] OLANREWAJU OJO is with the Department of Mechanical Engineering, University of Manitoba, Winnipeg, R3T 5V6, Canada. Manuscript submitted December 12, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
approximately 1170 C, which aids hot cracking susceptibility.[2] During the deposition of successive layers in laser metal powder deposition (LMPD), the deposited material is reheated multiple times, and much of the deposited material beneath the last layer of deposit constitutes the heat-affected-zone (HAZ) several times, although at different peak temperatures. In the mushy zone of the HAZ, low melting point constituents, such as the Laves phase and MC-carbides are locally melted through eutectic and constitutional liquation reactions, respectively. If the strain, at this point, exceeds a critical value, the liquid separates, and a crack will initiate.[3] HAZ liquation cracks have previously been reported in LMPD of Alloy 718 by Chen et al.[4] where they found that a high scanning speed and a high heat input (HI) resulted in a longer total crack length (TCL). In addition, they found that most of the cracks resided at high-angle grain boundaries. Their experiment was conducted with a HI in the range of 300 to 600 J/mm, while the line mass was kept constant throughout the experiment. Understanding the cracking mechanisms and how to avoid cracking are essential when the size and complexity of the builds increases, as this will lead to an increased complexity of the thermal history which in turn will yield larger thermal stresses acting on the build. In this paper, cracked Alloy 718 samples, produced using a HI of 30 to 50 J/mm, have been microstructurally characterized using optical and electron microscopy. Total crack length (TCL) has been measured
from OM images and correlated to processing parameters. , the influences of the
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