Laves Phase in Alloy 718 Fusion Zone

D. Sivakumar,S.G.K. Manikandan,M. Kamaraj

doi:10.5772/intechopen.70325

Abstract

The gas tungsten arc (GTA) welded fusion zone of alloy 718 has been well investigated on the formation of interdendritic Laves phase. This article deals with the various process control methodologies for minimizing the microsegregation and Laves phase such as the enhanced weld cooling rate in GTA welding (GTAW) process and modification in weld metal chemistry. Even though the high energy density welding processes such as electron beam (EB) and laser beam (LB) welding techniques are proven in minimizing the microsegregation and Laves phase, the requirement of conventional GTA welding process still exists in the aerospace industry due to the complex shapes of the components and the inaccessible conditions for executing the welding process. The enhancement in the weld cooling rate and modified weld metal chemistry resulted in the refined fusion zone microstructure and reduced microsegregation. Enhanced weld efficiency on ultimate tensile strength (UTS) and 0.2% YS at 25°C was observed to the tune of 85 to 93% by employing cryogenic cooling in GTA welding process. Similar improvement in weld efficiency at 650°C was observed. However, the evaluated weld efficiencies with cryogenically cooled weld metal were marginally lower than the previous works in EB and LB welds only by 2–3%.

Highlights

Alloy 718 is extensively used in the high-temperature systems of cryogenic rocket engines in Indian launch vehicles
Use of Nb as age hardener in place of Al or Ti in alloy 718 eliminated the problem of strain age cracking, but resulted in the formation of brittle interconnected Laves phase during welding [1] which are the favorable sites for easy crack
Filler metals with significantly different compositions from that of the base material composition were used to minimize Laves phase in some of the welding applications [8]. This was the driving force for studying the solidification behavior of alloy 718 fusion zone welded with high melting point solute (Mo) (16 wt%) filler metal, even though it was well established by various researchers [9, 10]

Summary

Introduction

Alloy 718 is extensively used in the high-temperature systems of cryogenic rocket engines in Indian launch vehicles. Filler metals with significantly different compositions from that of the base material composition were used to minimize Laves phase in some of the welding applications [8] This was the driving force for studying the solidification behavior of alloy 718 fusion zone welded with high Mo (16 wt%) filler metal, even though it was well established by various researchers [9, 10]. In view of the above, an attempt has been made in the present study to evaluate the effectiveness of cooling techniques in controlling the formation of Laves phase and the high-temperature mechanical properties in alloy 718 GTA welds. This chapter deals with the control of laves phase formation in the fusion zones of alloy 718 weldments by enhanced weld cooling rate through modified pulse current waveform and shielding gases (Argon and Helium) in GTA welding process and change of weld metal chemistry using solid solution (FM1) and matching composition/age-hardenable (FM2) filler metal. The mechanical properties at room temperature and elevated temperatures have been correlated to the Laves phase in the weld fusion zone of alloy 718

Materials and methods

Microstructural characterization of fusion zone

Mechanical property characterization of weldments

Findings

Inferences