Friction Stir Welding of Similar and Dissimilar Alloys

Department: Mining and Nuclear Engineering

Submitted by: Mitchell Smith

Advisor: Dr. Carlos H. Castaño

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Abstract

This project studies the effects of welding similar and dissimilar alloys together and comparing those welds to the original metals. Nano-reinforced alloys are also researched to see if they could be welded with the FSW and what would need done to do so. Trial welds were made to understand the programming and mechanics of the friction-stir-weld machine. Welds were made on a single aluminum sample and then butt joints were created by welding two samples of aluminum together. For this project, one welding pin was used and the metal welded was kept to a thickness of 0.25 inches. The largest problem encountered was the separation of the metal samples when attempting a butt-joint weld. When the spinning bit was forced into the metal, the two pieces would separate if not clamped well enough. Welds were visually inspected to verify that they were properly made. Further tests, both destructive and non-destructive, are needed to quantify what is happening to the metal in the weld.

Introduction

Friction Stir Welding (FSW) is a modern welding process with unknown possibilities. Welding processes before FSW needed to melt the metal so two pieces could be combined while in its liquid state; sometimes a filler metal would be added to help strengthen the weld joint. In traditional welding, the heat source used to melt metal comes from burning of a gas from an electrical current. The ability to join similar alloy metals together allows the manufacturing of much stronger structures when compared to riveting or bolting. When welding, the molten metal can become very reactive with the oxygen in the air around it. This is why many methods of welding have a shielding process of some kind. Two types of shielding processes are used with flux and inert gases. The flux is usually built into the filler metal that is added to the weld. It melts with the metal and remains on the outside of the weld helping to keep the molten metal free from contaminants. When gas is used to shield a weld, it may contain one or a combination of gasses, including, but not limited to Carbon Dioxide (CO2) and Argon. These gasses do not react with the weld; they displace the air around the molten weld long enough for it to cool and become solid without any imperfections.

Friction Stir Welding does not melt the metal and therefore does not need any type of shielding. Since no flux is used, the weld requires no cleaning or grinding after it is finished. The heat is generated from friction coming from a rotating pin that has been forced into the material. The amount of heat generated is determined by how fast the pin spins (revolutions per minute, RPM) and how fast the pin is forced into new material (inches per minute, IPM).

Since the needed heat is generated from friction, there are no harmful gasses present or bright light from the arcing of electricity. These are just a few reasons how FSW is safer and cleaner than conventional welding.

Smith 2Figure 1: Gap Formed from Under Clamping (Reverse Side of Weld)

The most difficult part of friction stir welding is in the set up. In general, friction stir welders must maintain large amounts of down force. During the welding of the aluminum, the welder was left set at the 20,000 pounds of down force. This was more than needed but it did not alter the quality of the welds. When friction stir welding stainless steel, 40 Kilo Newton’s is required, which is just under 9000 pounds of force (Friction Stir Welding. 2015, January 1). When joining two pieces of metal, the large down force can push the two samples apart when the welder begins if they are not clamped down well enough (see Figure 1). This causes a problem because FSW does not use a filler metal, so any gap that is formed will cause an imperfection in the weld. The needed clamping or fixturing for the metal may cause limits on what can be welded since the welder and the metal must be attached to each other somehow.

Discussion

Friction Stir Welding (FSW), invented by Wayne Thomas at TWI Ltd in 1991, overcomes many of the problems associated with traditional joining techniques. FSW is a solid-state process which produces welds of high quality in difficult-to-weld materials such as aluminum, and is fast becoming the process of choice for manufacturing lightweight transport structures such as

boats, trains and aeroplanes [].FSW has been widely used on all aluminum alloys, especially on 2000, 6000, and 7000 series alloys. One of the aluminum alloys used for this project was 5083. The 5000 series alloy has an increased concentration of magnesium in the aluminum. The 5083 was available from the supplier in a size that would require minimum preparation to friction stir weld.

The first step taken was learning how to operate the FSW machine located in Fulton Hall. This was a trial and error learning process. After reading through the previously produced manual and making

some trial passes, the correct settings were established to complete a successful weld. Learning how to measure and set the angle of the weld pin was important so that the leading side of the pin was higher than the trailing side. This was a major component of making sure the bit would not plunge in the weld sample too far (see Figure 3) or completely through and damage the weld bed that the samples were clamped to. This adjustment, once set, remained at that setting for all the welds that were done. With the angle of the pin not perpendicular to the metal, the plunge depth had to be reduced. This measurement was taken with magnetic calipers and a flat metal surface (see Figure 2). Since the same pin was used for all the welds, this offset also remained constant for the project. When trying to replicate a process, the more variables that can be held constant will lead to more consistent results. After

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Figure 2: Plunge Depth to Deep Figure 3: Measuring Pin

and Shoulder Height

research, proper spindle speeds and bed speeds were established to reduce any chance of damage to the machine, pin, or weld sample. The welds were made inside this range with some variation and showed no signs of imperfection.

With similar alloy welds completed, the next phase was the joining of dissimilar alloys. The two dissimilar alloys were aluminum 5083 and aluminum 1100. Problems with welding dissimilar alloys is that sometimes they do not want to combine with each other during the stir process and voids and tunnels are formed (figure 4). These voids can be affected by the spin speed of the pin and how fast the pin is traveling down the weld. Both speeds affect the temperature of the metal during the weld. With variation of the speeds the tunneling can be reduced and eliminated, in some cases. Further research is being done in which aluminum is being welded to copper and magnesium (Mubiayi, M., & Akinlabi, E. 2013).

Other welds that still need completed are to alloys that have been reinforced with ceramic nano-particles. Ways to weld these materials are not well known yet. The particles do not remain homogenized, evenly mixed, in the metal if it is melted. Homogenization is needed to keep the nano particles distributed evenly throughout the metal. These metals are being researched for use in many fields of engineering. Oak Ridge National Lab has been researching how to join oxide dispersion strengthened (ODS) steels, nanostructured ferritic alloys (NFAs), reduced-activation ferritic/martensitic (RAFM) steels, and dissimilar metal joining between ODS/NFAs and RAFM steels through friction stir welding technology for use in possible future fusion reactors (Sokolov, M., & Tan, L. 2103, January 1). Fusion reactors are not the only application where the nuclear field could benefit from these nano reinforced metals. When building current fission reactors, and any power plant in general, higher burn temperatures lead to high efficiencies. Current burn temperatures are limited to what the materials that contain the burning material can withstand. Materials in the nuclear engineering also have to stand up to radiation damage along with thermal fatigue. If nano-particle strengthened materials can maintain their material properties better after irradiation than what is currently available, then that is another reason nuclear engineers should be researching them.

With all completed welds, tests are needed to verify what the material properties of the metals are after it has been welded. The highest achievement of any weld process is to have the welded material to maintain the original properties as it did before the weld. There are many non-destructive ways to check welds for defects and to see how the weld has formed.

When examining welds the first steps are by visual inspection. This is done to check for penetration of the weld, possible expansion or separation of weld joint, void formation, and any other obvious defect. The next step would be to use a microscope to look at cross sections of the weld. Small samples are cut from the weld; care must be taken to limit heat input so not to

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Figure 4: Tunneling in Dissimilar Weld Joint

alter the metal from extra heat input. These samples are prepared through a sanding and polishing process to ensure that the different parts of the weld can be seen. This figure is a general representation of what would be seen from a cross section of the weld under five to ten times magnification. The four main areas of the weld are represented here (see Figure 5). Increasing the magnification to twenty and up to one hundred times magnification along with the use of scanning electron microscopes (SEM), grain boundaries can be identified in the metal. These are important because the grain boundaries are what form between the crystallites of the metal as it cools. This type of inspection would be the best way to see how dissimilar alloys mix together after being friction stir welded. The images below are grain boundaries from aluminum 5083 (left) and copper (right). The structures can be difficult to identify but they are different. This microstructure/nanostructure that develops can be used to see how the material was formed or altered. The grain boundaries form when the crystallites are not aligned properly; the larger the areas between boundaries, the more molecules are arranged uniformly. Some metals after forging and welding processes, go through various heat treatments to try to relieve internal stresses and to strengthen the metal.

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Figure 5: Animated Crossection of a Friction Stir Weld

Figure 7: Grain Structure of Pure Copper

Welds are often checked with X-ray (Radiographic) and Ultrasonic inspection. Both of these methods allow visualization of the interior of the weld. Microscopic cracks, holes, and other voids become visible with these tests. A good weld will have consistent characteristics throughout the weld; the variations in the images are the imperfections. This non-destructive test can save money and time by testing materials before any destructive testing is done. Even the smallest imperfection inside the weld can drastically affect how it will perform in a stress test. If a weld is found to have a defect then it can be discarded and a new one produced before any more testing is completed.

A tensile test is a stress test that is used to see how far a material will stretch before fracturing. The material goes through plastic deformation which is when the bonds between atoms that make up the material are stretched past their rebound point and are unable to return to their previous configuration(Elastic/Plastic Deformation, n.d.). The difference in lengths is the amount of plastic deformation that the

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250 Microns

Figure 6: Grain Structure of Aluminum 5083

Figure 8: Tensile Test

material can withstand before fracturing (see Figure 8). A fracture is a crack the forms when the material is stretched and propagates until the material splits apart (Kopeliovich, D. 2014, March 5). With the friction stir welder not melting the metal it maintains more of its tensile property. A simple test would be to see if a friction-stir-welded sample and an unwelded sample would stretch the same amount. If they had similar results the next step would be to examine the crystal structure and the grain boundaries at the surface of the fracture.

Another test is the Charpy test, which is an impact test to see how much energy is absorbed by the test sample when it fractures (Friction Stir Welding. 2015, January 1). This test helps to determine the toughness of a material. Toughness is the ability to absorb energy and plastically deform without fracturing. Materials that break into two pieces have a low toughness and material that bends without completely breaking has a high toughness. Testing both parent materials and comparing them to the friction-stir-welded sample would show if any toughness was gained or lost from the weld.

As material research continues to grow in all fields of study, metal production and metal joining processes will remain an important field so that technology may continue to grow. Friction stir welding is a proven and needed way to join the metals of today and the future. Friction stir welding allows for materials that were once very challenging to weld to be welded faster and with higher quality than before. Materials that were thought to be unweldable are beginning to be joined with friction-stir-welding methods. The more scientists and engineers are able to understand and use different and new materials, the further and faster technology will be able to grow.

Acknowledgements

I would like to give credit and thank all the individuals who helped me through this project. Although my progress was not what I had originally hoped and planned for, I have learned a lot about the experimental and research process.

Dr. Castaño:

Thank you for allowing me to step into this OURE project that was already established. This opportunity has given me some real life experience on conducting research outside the classroom. You have provided me with this opportunity and the lessons I learned from it will be

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Figure 9: Charpy Test

invaluable as I further my education to become a Nuclear Engineer. Your vast interests in material science have opened my eyes to some of what is currently being researched and developed.

Gretchen Smith:

To my wife, thank you for encouraging me to take on this project and to continue to try new things. You have done your part to keep me on track at school and have been willing to help when I needed you too. Thank you for the support you have given me since I started my engineering studies.

Michael Hall and Charlie Moore:

To the both of you, thanks for helping me do the dirty work of cleaning and preparing a place to work and for assisting me with the welding itself. I am glad I was able to meet you guys and work with you. I have always been able to work much better in a group than by myself and your presence allowed me to do my best.

Dedie Wilson:

Thank you for allowing me to step into this project and to continue where it left off. I appreciate that you take part in providing undergraduate students a chance to do experiments and research outside the confinements of the classroom. Thank you for always having such quick responses to emailed questions that I have had throughout this project.

References

Casati, R., & Vedani, M. (2014). Metal Matrix Composites Reinforced by Nano-Particles—A Review. Metals, (4), 65-83.

Elastic/Plastic Deformation. (n.d.). Retrieved April 1, 2015, from https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Structure/deformation.htm

Feng, Z., Yu, Z., Hoelzer, D., Sokolov, M., & Tan, L. (2103, January 1). Friction Stir Welding of ODS Steels and Advanced Ferritic Structural Steels. Retrieved April 1, 2015, from

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http://web.ornl.gov/sci/physical_sciences_directorate/mst/fusionreactor/pdf/Vol.54/2.5_Feng.pdf

Friction Stir Welding. (2015, January 1). Retrieved April 1, 2015, from http://www.twi-global.com/capabilities/joining-technologies/friction-processes/friction-stir-welding/

Kopeliovich, D. (2014, March 5). Materials Engineering. Retrieved April 1, 2015, from http://www.substech.com/dokuwiki/doku.php?id=fracture_toughness

Miao, P., Odette, G., et al (2007). The microstructure and strength properties of MA957 nanostructured ferritic alloy joints produced by friction stir and electro-spark deposition welding. Journal of Nuclear Materials, 1197-1202. Retrieved April 1, 2015, from http://www.ncnr.nist.gov/programs/sans/pdf/publications/0598.pdf

Morishige, T., Kawaguchi, A., Tsujikawa, M., Hino, M., Hirata, T., & Higashi, K. (2008). Dissimilar Welding of Al and Mg Alloys by FSW. Materials Transactions, 49(5), 1129-1131.

Mubiayi, M., & Akinlabi, E. (2013). Friction Stir Welding of Dissimilar Materials between Aluminium Alloys and Copper - An Overview. Proceedings of the World Congress on Engineering, 3(WCE 2013).

Figure 5: Kallee, S., & Nicholas, D. (2003, January 1). Friction Stir Welding at TWI. Retrieved April 1, 2015, from http://materialteknologi.hig.no/Lettvektdesign/joining methods/joining-welding-friction stir weld.htm

Figure 6: AluMATTER Grain Size. (2010, January 1). Retrieved April 1, 2015, from http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=208&pageid=2144416605

Figure 7: Resources: Standards & Properties - Copper & Copper Alloy Microstructures: Coppers. (2015, January 1). Retrieved April 1, 2015, from http://www.copper.org/resources/properties/microstructure/coppers.html

Figure 8 and 9: Friction Stir Welding. (2015, January 1). Retrieved April 1, 2015, from http://www.twi-global.com/capabilities/joining-technologies/friction-processes/friction-stir-welding/

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