Jeff Ding, Aerospace Welding Engineer at NASA Marshall Space Flight Center
- Sunday, 01 November 2009
When using conventional FSW to weld .500-in thick commercially pure (CP) titanium, the FSW pin tool must rotate between 700 – 900 RPM to generate the frictional energy required to plasticize the material. With thermal stir welding, I first heat the part with a specially designed induction coil; it heats very quickly through the thickness. Once you’re up to some temperature – it could be 1400 degrees, it could be 1600 degrees – whatever temperature it is that you want to stir, you move the part into your stir rod and all that stir rod does is it stirs the material together just like the little pin on a friction stir welding pin tool. The stir rod protrudes through the middle of two containment plates that contain the material as it is being stirred. Containment plates are stationary – they do not rotate. The containment plates also supply the compressive load to the stirred material for microstructure consolidation. The force they compress with is also controlled independently. Yes, I’m getting what we call adiabatic heating from the friction given off by the stir rod, but the primary source of heating is the induction coil. I have done welds rotating at only 200 RPM in ½-inch thick titanium.
Now, what’s the benefit of independent control of heating, stirring and forging pressures? It certainly increases life of the stir rod. Since the material is already at temperature when the stirring begins, the stir rod is primarily just a mixing tool that moves plastic material within the weld zone. In FSW, the shoulder/pin assembly must provide both heat and stir functions. This reduces life of the shoulder/pin assembly at the temperatures required to join CP Ti. Other benefits of independent control I am not sure of yet. TSW is very new. I’m just now looking at the .500-in thick CP Ti weld microstructures to compare the microstructures that were stirred using 100 RPM, 200 RPM, and 300 RPM, to see if I can see any differences. What I suspect, is that the strain rate being induced into the microstructure with the TSW process is much less than using FSW. I have data that shows the FSW pin tool rotational speed to heat and stir .500-in thick CP Ti must be between 700-900 RPM. Since commercially pure titanium has no alloying elements, the microstructure – the grains – are free to grow very large very quickly. The high strain rate induced into the microstructure creates tears and results in wormholes. With thermal stir welding, you can rotate very slowly and put a lot less strain into the microstructure. I believe that’s one reason why we’re getting good welds on .500-in thick titanium. I just got done completing an eight (8) foot long weld in .500-in thick CP Ti. Visually, it looks great! Radiography will tell the real story. By the way, the work I’m doing with the .500-in thick CP Ti is for the Office of Naval Research (ONR) – not NASA programs. The technology used to support the Constellation Program must be at a high TRL. TSW isn’t there yet, but I’m getting there. I am going to do a study to compare welding Haynes 230 with FSW and TSW. This will be my first work using TSW to support the Constellation Program.
Ding: So right now, TSW development is being done for ONR. ONR funds the TSW work through an SBIR with Keystone Synergistic Enterprises, Inc., Port St. Lucie, Fl. Keystone, in turn, funds MSFC through a reimbursable Space Act Agreement. I develop the process, provide Keystone with all the data that is confidential to them, and then they present it to the Navy. The Navy is very interested in this process, because the Navy is looking at this low-cost titanium product that’s processed a different way than your usual titanium. Instead of costing $60 a pound, it’ll cost $5 a pound. The reduced cost comes from the processing of the titanium. When the titanium is processed, there are a lot of tramp elements left in the metal such as chlorine. This presents problems when welding the low cost titanium. When it is welded with fusion weld processes, like TIG, MIG, or electron beam, a lot of oxides form, resulting in inferior weld properties. It has to be welded without melting, meaning, a solid state process such as FSW or TSW must be used. So far, ONR is quite impressed with TSW.
NTB: What is ultrasonic stir welding?
Ding: First of all I’m happy to say that the U.S. patent was allowed for the ultrasonic stir welding (USW) process just a few weeks ago. USW is also a solid state welding process. I heat the weld piece using high powered ultrasonic energy. Ultrasonic stir welding is an idea I got after looking at video of ultrasonic assisted drilling. The video shows the effects of drilling through, say, a half-inch thick steel plate with a quarter-inch drill bit, with and without ultrasonic energy applied. Without ultrasonic energy, the drill takes considerable time and force to drill through the steel plate. A load cell records the amount of force it takes to push the drill clear through the plate and is represented in a strip chart. During the drilling with no ultrasonics, the needle on the strip chart goes very high on the scale and then drops to zero when the drill pops through the steel plate. When the ultrasonic energy is applied, the drill cuts through the steel plate very quickly with significantly less force. The strip chart needle barely rises from the bottom of the scale. Without ultrasonic energy, the typical metal chips fly off the drill bit while drilling. When the ultrasonic energy is turned on, one long metallic “apple peel” is discharged, or, peels away, from the drilling process – no chips. This is because the ultrasonic energy plasticizes the steel at the interface between the drill tip and the workpiece. It’s really pretty amazing! So this video showed the heating of the metal by the drill bit and the reduction of forces when drilling. And what is USW? It’s primarily a very, very small drill bit (to stir the plastic material) with a non rotating containment plate to contain the plastic material.
Last summer I set up a little high-powered ultrasonic test bed to generate data. The primary data showed that I can heat metals into a plastic temperature state with ultrasonic energy and I can significantly reduce plunging forces. So ultrasonic stir welding, I believe, will be a way that we can take a solid state weld process and integrate it with an off-the-shelf robot for welding. Right now a huge robust robot is required to absorb the loads for friction stir welding, but with ultrasonics, I think, an off-the-shelf robot will be able to perform USW