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Welding trials of new Aubert & Duval nickel alloy, AD730®

Aubert & Duval designed the new nickel alloy AD730® in response to the need for metals that can function at high temperatures in fields such as aerospace, energy, and the automotive industry.

The industrial environment

The need to enhance the efficiency of aero engines has been the driving force for the development of new materials that combine high tensile strength, resistance to fatigue and creep with the capability to operate in the 700°C/750°C (1292°F/1382°F) temperature range.

In the same way, new power generation concepts, aiming at higher efficiency and reduced CO2emission, require novel alloys able to cope with adverse conditions associated with A-USC (Advanced Ultra Supercritical) operations. There again, advanced creep and fatigue properties, as well as microstructural stability at elevated temperatures are key.

Last but not least, the alloy’s hot working ability is a significant aspect influencing the final cost-efficiency of the product.

AD730® has been developed to address these market challenges.

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^(-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.

Figure 1: 2.5 mm deep EB welded plates
Figure 1: 2.5 mm deep EB welded plates

A previous study showed that AD730® can be joined using Linear Friction Welding (LFW) without causing liquation cracking. This early test was expanded upon at TWI.

Coupons of 10x40mm (400mm2) faying area and 127.5mm final length were welded to allow us to apply a pressure up to 600MPa at the highest force capability of the machine (250kN). Different friction pressure, amplitude, and frequencies were applied to the specimens before they were assessed to determine the optimum parameters for the quality of the weld.

The tests found that tensile properties seemed to slightly improve with post weld heat treatment. More mechanical tests are needed alongside research to assess other sets of pressure, amplitude, and frequency, as well as EDX inspection of the flash.

However, overall, it was found that LFW provides a fast rate of production and unique microstructure, which is best with a friction pressure of 350MPa and oscillations at 35Hz with an amplitude of 1.5mm.

This is just the first step in understanding the behaviour of AD730® under welding and further tests are required to gain solid conclusions. While these tests offer an idea of the general trends of AD730® under welding, an in-depth study on more specimens would allow for a better understanding of the impact of welding on the microstructure and consequently on the mechanical properties and metallurgical effect of the weldment. It would also be worthwhile to further examine the hardness results as they relate to weld impact on ductility and undertake further tensile tests on larger specimens.

That said, these first welding trials on AD730® proved that it is fairly weldable due to the tested mechanical properties and microstructure not being altered to a critical extent. TIG welds showed no obvious brittle phases that would systematically lead to cracking and a reasonable range of parameters could be used to weld either autogenously or with 625 consumable thin plates. Meanwhile, EB welding offered a narrow heat affected zone and the ability to weld very thick parts. LFW was found to be a fast and effective process and does not melt the material.

With the processes investigated, no drastic decrease in mechanical properties was observed, creating a base from which further research can be conducted, according to application, to find the best processes for use by Aubert & Duval’s clients.

TWI would like to thank Solene Coeuret for all her valuable work on this project.

For more information, please email

Figure 2: Linear friction welded specimen
Figure 2: Linear friction welded specimen