Development of AD730® alloy
AD730® is a fully-innovative cast & wrought nickel-based superalloy that withstands high temperatures (750°C/1382°F) while preserving strength, creep and fatigue resistance at a competitive cost.
The breakthrough mainly comes from the unique properties-versus-cost balance of AD730®, due to its recycling and forging ability.
While AD730® has high functioning tensile, creep, and fatigue properties, the large amount of alloying elements mean that nickel alloys can be difficult to weld.
TWI were called to investigate how the new alloy held up under three different welding processes – Mechanised TIG Welding, Electron Beam Welding, and Linear Friction Welding. All three processes were chosen due to their use in the aerospace industry.
In a project part-funded by Aubert & Duval, each process was tested with simple geometries to find a range of parameters for the trials, while microscopic inspections, hardness, and tensile tests were performed on the acceptable samples to assess the impact of welding and post-weld heat treatment on the mechanical properties of AD730®.
Mechanised TIG Welding tests
The mechanised TIG welding was carried out in two phases on different plate thicknesses, 2.5mm plates with a single pass weld and 8mm plates with multiple runs. The tests used Argon gas at a flow rate of 10L.min−1 and a 1.5% lanthanated EWLa-1.5/WL15 tungsten electrode. The current, voltage, and travel speed were adjusted manually.
Once the welds were completed, the plates and the macrosections were cut and prepared before the examination began.
As the AD730® was expected to be difficult to weld, it was decided to begin the TIG welding with a highly ductile wire. This was to test that the alloy could be welded in the first instance but, with that being successful, we would recommend using a filler that fits the composition, or is close to that of AD730® to allow for higher performance applications.
Reducing the heat input and the travel speed is known to reduce crack sensitivity when welding nickel alloys so, as a result, the heat input should be kept low. A range of parameters give acceptable weld shape, no cracks and fair properties - around 70A, 10V, with a travel speed of 2.0mm.s^(-1) - leading to a heat input of about 0.35kJ.mm^(-1).
The TIG welding trials also successfully joined the AD730® autogenously (without the use of filler addition).
TWI would also like to thank Welding Alloy, who provided a spool of wire and offered some additional support for the TIG Welding process.
Electron Beam Welding tests
Since Electron Beam (EB) welding is intended to weld thick pieces, tests were carried out on 2.5mm, 30mm, and 60mm samples with different preheating to assess the influence of the thickness and preheat at high temperatures.
The sample sizes were chosen to provide a result on the depth that an EB weld could reach, with a belief that, with powerful enough EB welding equipment, test trials could see 100mm deep work pieces being easily weldable.