|About this Abstract
||2020 AWS Professional Program
||2020 AWS Professional Program
||New Approaches in Friction Welding Advanced PM Nickel Base Superalloys
||Tracy W. Nelson, Caleb Brown, Scott Taysom, Carl Sorensen, Martin Morra, Nikole Kucza
|On-Site Speaker (Planned)
||Tracy W. Nelson
Next generation powder metallurgy (PM) nickel-base superalloy for aerospace applications offer new challenges in joining. For decades friction welding processes, e.g. rotary, inertia and linear friction welding, have been the processes of choice for joining nickel-base superalloy gas turbine components. Successful application of these processes has relied on the ideology that the materials being joined have sufficient elevated temperature plasticity to undergo the large plastic upsets traditionally applied in these processes. Newer PM nickel-base superalloys are specifically designed to exhibit high resistance to plastic deformation even at elevated temperatures, making them difficult to join using traditional practices. New understanding and methodologies must be developed to join the next generation of high temperature strength-plasticity limited nickel-base superalloys.
A highly instrumented continuous drive friction welding (FW) machine was used to perform continuous drive FW in alloy 718 and a PM high temperature disc alloy. FW samples had an outside diameter of 25.4 mm and an inside diameter 20.3 mm. Marker studies were performed at 1000 rpm at feed rates of 1, 2 and 4 mm/sec. Process data including rotational speed, process power, forces, sample torque, sample thermal cycles and spindle position were recorded and used for analysis. Tungsten tracers were used to better understand material flow across the diameter of samples under different weld parameters in alloy 718. Tungsten wire and powder were imbedded in the FW samples at varying diameters prior to welding. MicroCT was used to evaluate the location of markers post-FW. Process strains and strain rates were estimated from the results of these marker studies. Knowledge gained from markers were used to develop and investigate new process approaches to FW of PM high temperature disc alloys. Both parameters and sample geometries were explored individually and in combination.
Results and Discussion
Results from the marker studies indicated a bifurcation of material flow through the wall thickness. The bulk of the material undergoes a simple ridged body rotation transferring motion from longitudinal to radial flow. This material forms the flash commonly observed in FW in ductile metals. The deformation observed is similar to what is reported in equal channel angular pressing and orthogonal machining. Strains and strain rates are dependent upon the federate, not rotational speed. The strain in this region is between 1.2 and 1.8, and the mean strain rate between 100 and 102.2 s-1. The levels of strain and strain rate decrease as process time increases.
A much smaller volume of material located along the centerline of wall thickness experiences primarily azimuthal flow at the interface between the two weld samples. The degree of azimuthal flow is dependent on both rotational speed and feed rate. The feed rate contributes to the rate at which this material moves radially. The strain rates in this volume of material were between 103.5 and 103.7. Strain was sufficiently high to create continuous dynamic recrystallization resulting in grain sizes at the interface on the order of 10-9 m.
Friction welds in the PM high temperature disc alloy were successful when heating and deformation were kept localized. Sample geometries that create quasi hydrostatic conditions were successful. Parameters that minimized bulk deformation were also beneficial. Strengths up to 80% of the base metal tensile strength were achieve in this initial study.
Marker studies of FW in alloy 718 have elucidated new understanding in material flow and deformation in FW of tube samples. Results show two primary regions of flow which undergo significantly different strain paths, as well as levels of strain and strain rate. Understanding the deformation paths and their contribution to the welding processes has enable successful FW in PM high temperature disc alloys. The authors will present the results and analysis of the marker studies and their importance to the fundamental principles of FW processes. They will also present two novel approaches that have been used to successfully join PM high temperature disc alloys.
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