Application of Stir Friction Welding Technology

Friction Stir Welding of Typical Materials

(1) Welding of Aluminum Alloys

Initially, friction stir welding was primarily developed to address the welding of thin aluminum alloy sheets.

With the development of friction stir welding tools and process technology, currently, friction stir welding can weld all series of aluminum alloy materials, including those high-strength aluminum alloy materials that are difficult to connect by fusion welding methods, such as the 2xxx (Al-Cu) series and 7xxx (Al-Zn) series of aluminum alloys.

It can also be used to connect different types of aluminum alloy materials, such as the welding of 5xxx (Al-Mg) and 6xxx (Al-Mg-Si) series aluminum alloys. In terms of workpiece thickness, single-pass friction stir welding can achieve the welding of aluminum alloy materials with a thickness of 0.4~100mm; double-pass welding can weld butt plates up to 180mm thick.

Figure 4-28 shows a friction stir welded joint of a 70mm thick aluminum alloy.Using friction stir welding to weld aluminum alloys results in a joint with higher mechanical properties than one obtained through fusion welding. Table 4-18 lists the welding parameters for friction stir welding of aluminum alloys.

Figure 4-28 70mm thick aluminum alloy friction stir welded joint

By studying the effects of welding speed, stir welding head rotation speed, and other parameters on joint performance, and performing parameter optimization, the optimal welding parameter matching range can be found.

Welding with parameters within this optimal range, coupled with appropriate axial pressure and stir welding head structural parameters, can easily obtain a friction stir welded joint with the best performance.

Table 4-18 Welding Parameters for Friction Stir Welding of Aluminum Alloys

MaterialsPlate Thickness
mm
Rotational Speed
(r/min)
Welding Speed
(mm/min)
10505560 ~1840155
2024-T66. 5400 ~ 120060
2024-T36.4215 ~36077~267
20951. 61000246
21955. 8200 ~ 2501.59
5052-O2200040
5083850070 ~200
6061-T66. 3800120
6. 5400 ~ 120060
AA6081-T4 (USA)5. 81000350
6061 Aluminum Composite Material41500500
608242200 ~ 2500700 ~1400
7075-T641000300
20244200037. 5

(2) Welding of Magnesium Alloys

Currently, the magnesium alloys welded using the friction stir welding method mainly include AM50, AM60, AZ91, AZ31, and MB8, among others. Experimental results show that when the welding parameters are appropriately selected, a dense weld structure can be achieved, and the joint strength can reach 90% to 98% of the base material strength.

Table 4-19 lists the mechanical properties of the MB8 magnesium alloy friction stir welds, while Table 4-20 presents the results of the bending test for the AZ31 magnesium alloy friction stir welds.

Table 4-19 Mechanical Properties of MB8 Magnesium Alloy Friction Stir Welds

Welding Speed/(mm/min)306095118235300
Tensile Strength/MPaSpecimen 1143141146134159172
Specimen 2130132138135151167
Ratio of Joint Strength to Base Material Strength (%)Specimen 1646365607176
Specimen 2585761606774

Table 4-20 Results of the Bending Test for AZ31 Magnesium Alloy Friction Stir Welded Joints

Specimen NumberWelding ParametersBending Angle / (°)Span / mmBend Resistance / MPa
Friction Stir Welding Head Speed / (r/min)Welding Speed / (mm/min)
160011830° Reverse Bend70233.2
275075085° Reverse Bend70279.2
3150030080° Forward Bend70303.2

(3) Welding of Copper Alloys

Due to copper’s high melting point and excellent thermal conductivity, it’s difficult to melt the parent metal during fusion welding. The filler metal and the parent metal are hard to fuse well, which can lead to welding defects. After welding, the grains grow significantly, greatly reducing the strength and plasticity of the joint.

Copper’s linear expansion coefficient and shrinkage rate are relatively large, leading to severe post-welding deformation, poor appearance, and substantial residual stress. The joint is also prone to hot cracking. Copper’s excellent thermal conductivity results in rapid cooling of the weld seam, making porosity another defect in copper and its alloy fusion welding.

In summary, fusion welding of copper alloys not only consumes a lot of energy, but also requires very strict process conditions, and it is difficult to obtain weld seams with excellent comprehensive properties.

Using friction stir welding for copper alloys avoids many defects and shortcomings of fusion welding. The weld seam has a uniform and smooth appearance with no defects, and the deformation compared to fusion welding is minimal, as shown in Figure 4-29.

The welding operation is simple; before welding, it’s only necessary to remove grease from the joint surface with organic solvents such as acetone. There’s no need for beveling or removing the oxide film, nor for removing the excess height after welding, which improves productivity. The welding process consumes less energy, doesn’t require filler material, and has a low welding cost.

(4) Welding of Titanium Alloys

Traditionally, titanium alloy materials are mainly welded by methods such as argon arc welding, plasma arc welding, and electron beam welding. However, due to the stringent conditions and complex process of fusion welding, and the tendency to produce defects, the joint strength is lower.

Therefore, the research and application of friction stir welding in titanium alloy welding are becoming increasingly widespread. Using friction stir welding to weld the titanium alloy Ti-6 Al-4V can yield high-quality weld seams, with fast welding speed, low cost, good efficiency, and simple operation.

The weldability of titanium alloys by friction stir welding is similar to that of copper alloys, more difficult than aluminum alloys, but less than steel.

(5) Welding of Steel

In recent years, research into the weldability of steel by friction stir welding has increased. Similar to aluminum alloys, steel friction stir welds also have a weld nugget area, a thermo-mechanically affected area, and a heat-affected area. The weld nugget area has an equiaxed grain structure, with smaller grains than the parent material area.

The thermo-mechanically affected area outside the weld nugget area has a sub-grain structure, with grain size similar to the weld nugget area, about half that of the parent material area. The structures of the weld nugget area and the thermo-mechanically affected area have undergone recovery and recrystallization, similar to aluminum alloy friction stir welding.

Industrial Applications of Friction Stir Welding

Since its invention, friction stir welding technology has been successfully applied in many industries, demonstrating its superior technical performance.

Friction stir welding technology has moved from experimental research and engineering development stages to large-scale industrial application. So far, it has been incredibly successful in manufacturing sectors such as aerospace, transportation, and defense equipment. The applications of friction stir welding in the industrial field can be seen in Table 4-21.

Table 4-21 Application of Friction Stir Welding in the Industrial Field

Application FieldsApplication Examples
AerospaceAerospace: Fuel storage tanks, engine load-bearing frames, manned return capsules, connections between frames, installation of preformed parts for aircraft, welding of aircraft walls, floors, and structural parts, and skin, etc.
Land TransportationLand Transportation: High-speed trains, freight trains, subway cars, trams, container bodies, automobile engines, chassis, body brackets, wheel hubs, hydraulic forming pipe fittings, preformed parts of car doors, vehicle space frames, truck bodies, lifting platforms of load-carrying vehicles, car cranes, armored vehicle protective plates, etc.
Weapon IndustryWeapon Industry: Tanks, main body structures, and protective armor plates of armored vehicles.
Home Appliance IndustryHome Appliance Industry: Refrigerator cooling plates, kitchen appliances and equipment, light alloy containers, home decorations, magnesium alloy products, etc.
Civil and Construction IndustryCivil and Construction Industry: Aluminum alloy reactors, heat exchangers, central air conditioning, natural gas and liquefied gas storage tanks, aluminum alloy bridges, decorative plates, door and window frames, pipelines, etc.
Figure 4-30: Delta II Rocket’s Space Fuel Storage Tank Welded by Friction Stir Welding
Figure 4-31: Delta IV Rocket’s Space Fuel Storage Tank Welded by Friction Stir Welding
Figure 4-32: Ariane 5 Booster’s Main Load-Bearing Frame Manufactured Using Friction Stir Welding (FSW)
Figure 4-33: Schematic Diagram of Ariane 5 Engine’s Main Load-Bearing Frame Structure

(1) The Application of Friction Stir Welding in the Aerospace Field

Friction stir welding, with its unique technical advantages, is receiving increasing attention and widespread application in the aerospace field.

In the manufacturing process of spacecraft, the selection of materials for structural parts such as fuel storage tanks, engine load-bearing frames, and manned return capsules is often the 2000 and 7000 series aluminum alloy materials with poor fusion welding weldability.

Using friction stir welding technology can significantly improve the quality of the weld seam, reduce welding stress and deformation, and enhance the reliability of the structure.

Boeing partnered with the British Welding Institute at the inception of friction stir welding and utilized the technology for connecting the booster segments of the Delta II and Delta IV rockets. With friction stir welding, the strength of the booster segment weld joint increased by 30% to 50%, manufacturing costs were reduced by 60%, and the manufacturing cycle was reduced from 23 days to 6 days.

As of April 2002, the friction stir weld seams applied to the Delta II rocket reached 2100 meters, and those applied to the Delta IV rocket reached 1200 meters, with no defects. Figure 4-30 shows the Delta II rocket’s space fuel storage tank welded by friction stir welding. Figure 4-31 shows the Delta IV rocket’s space fuel storage tank welded by friction stir welding.

Europe’s Fokker Aerospace Company used friction stir welding technology to manufacture the main load-bearing frame of the Ariane 5 booster, as shown in Figure 4-32. This frame is assembled from 12 integrally machined flat plates with wing-shaped reinforcements, as shown in Figure 4-33, and the material is 7075-T7351.

Due to the poor fusion welding performance of this material, the original product manufacturing used riveting technology, but the company now uses a friction stir welded lap joint structure to replace the original riveted structure. Practice has shown that the friction stir welded lap joint fully meets the usage requirements, reduces the structure’s weight, and improves productivity.

(2) Application of Friction Stir Welding in Aircraft Manufacturing

Currently, friction stir welding technology is utilized in both military and commercial aircraft manufacturing. In 1997, the American Eclipse Aviation Corporation significantly invested in the development of friction stir welding in aircraft manufacturing, aiming to create cost-effective business jets and enhance their core competitiveness through the use of FSW technology.

Figure 4-34 illustrates the Eclipse N500 model commercial jet manufactured by Eclipse Aviation Corporation. Figure 4-35 showcases the fuselage of the Eclipse N500, which has been welded using friction stir welding. The skin, wing ribs, chord supports, aircraft flooring, and assembly of structural components, previously riveted, have been replaced by friction stir welding.

With 263 friction stir weld seams replacing over 7,000 bolt fasteners, manufacturing efficiency has been significantly boosted, costs reduced, and aircraft weight decreased. Jetliners made using friction stir welding have entered mass production. NASA views these purely friction stir welded aircraft as “air taxis”, with plans for their widespread use across 3,000 small airports in the United States.

(3) Application of Friction Stir Welding in Shipbuilding Industry

The growing use of aluminum alloy has become a new trend in shipbuilding. As early as 1996, the Norwegian company Marine applied friction stir welding technology to the manufacturing of ship structural components.

The company provides shipyards with standard-sized, friction stir welded aluminum alloy prefabricated profiles, reducing manufacturing cycles, making hull assembly more precise and simpler, and reducing the total production costs of the ship by at least 5%.

Figure 4-36 shows the ship profiles manufactured using friction stir welding, while Figure 4-37 exhibits a catamaran built using friction stir welded prefabricated plates.

The demands for lighter trains, higher joint strength, and structural safety are increasing. Aluminum alloy, due to its high strength-to-weight ratio and good corrosion resistance, is finding increasingly broad application in various train manufacturing. For example, train carriages, wall panels, and base plates can all be made from aluminum alloy materials.

However, welding aluminum alloy using fusion welding methods can easily lead to porosity and cracks, especially the presence of surface oxide film makes the fusion welding connection of aluminum alloy quite challenging.

Therefore, friction stir welding technology has become the preferred method to replace fusion welding in aluminum alloy train manufacturing, now becoming a main trend in train manufacturing.

In 2011, the Changchun Railway Vehicles Company of the China North Railway Group first applied friction stir welding technology to successfully complete the welding of the aluminum alloy body, and has now applied friction stir welding technology to the welding of some structures of the CRH3 train, as shown in Figure 4-38.

Figure 4-34: Eclipse N500 Model Commercial Jet
Figure 4-35: Fuselage of the Eclipse N500 Commercial Jet, Welded Using Friction Stir Welding Technology
Figure 4-36: Marine Profiles Manufactured Using Friction Stir Welding
Figure 4-37: Catamaran Manufactured with Hulls Made from Prefabricated Plates Using Friction Stir Welding

Shipbuilding and marine industries were the first two sectors where friction stir welding was commercially applied, with applications including:

1) Welding of decks, side panels, bows, hulls, waterproof cabin walls, and floors.

2) Welding of aluminum alloy profiles, hull shells, and main structural components.

3) Welding of helicopter landing platforms.

4) Welding of offshore observation stations.

5) Welding of marine transportation structural components.

6) Welding of hollow flat plates in marine refrigerators.

Figure 4-38: Friction stir welding is also widely used in the rail transit industry. As train speeds continue to increase, so too does the demand for lighter trains, stronger joints, and safer structures. Aluminum alloys are increasingly used in various train manufacturing processes due to their high strength-to-weight ratio and corrosion resistance.

For example, train carriages, wall panels, and base plates can all be made from aluminum alloy materials. However, using fusion welding to join aluminum alloys can lead to porosity and cracking, particularly due to the presence of surface oxide films. As a result, friction stir welding has become the preferred alternative to fusion welding in aluminum alloy train manufacturing, now a mainstream trend.

In 2011, the Changchun Railway Vehicle Company of the China North Vehicle Group Corporation first used friction stir welding to successfully complete the welding of an aluminum alloy body, and has since applied the technology to the welding of some structures in the CRH3 multiple unit trains.

Figure 4-38: CRH3 Multiple Unit Trains Produced by Changchun Railway Vehicle Company

Advantages of using friction stir welding to weld aluminum alloy train bodies include:

1) Minimal deformation and shrinkage, no discoloration of the metal after welding, allowing for precise body manufacturing.

2) Friction stir welded joints are stronger than MIG welded joints, and deformation is 1/12 of that of MIG welding.

3) Hollow aluminum alloy extruded profiles reduce the number of parts in body manufacturing and enable the installation of large-size inner wall templates.

4) Low cost, low maintenance expenses, low operating requirements, and low energy consumption.

Current applications of friction stir welding related to rail vehicles include: high-speed train bodies, railway freight cars, subway carriages and trams, railway oil tankers and cargo holds, and railway container bodies, etc.

5) Friction stir welding is also used in automobile manufacturing. Using aluminum alloys for car body material is an effective way to reduce vehicle weight, improving fuel efficiency and vehicle safety.

Automotive design experts hope to replace current steel body structures with aluminum alloys, but how to connect aluminum alloys has always been a problem. The advent of friction stir welding technology has successfully solved the aluminum alloy welding problem.

In the past, the connection of car body plates used resistance spot welding technology, which required large special equipment to provide continuous high current. The only energy consumption in friction stir welding is the electric energy consumed by driving the stir welding head to rotate and applying forging pressure.

The entire welding process does not require the large current and compressed air required by traditional resistance spot welding. Compared with resistance spot welding, welding aluminum alloy with friction stir welding saves 99% of energy consumption. And because there is no need for large power supply equipment and special spot welding equipment, the equipment cost is reduced by 40%.

The welding process is splash-free and dust-free, making the welding process more environmentally friendly and the welding environment safer.

Mazda Motor Corporation was the first automaker to apply friction stir welding to car body manufacturing. Figure 4-39 shows the 2004 Mazda RX-8 aluminum alloy rear door and engine hood manufactured using this technology.

At present, the application of friction stir welding in the automotive manufacturing industry mainly includes: automobile wheel hubs, hydraulic forming pipe accessories, automobile door preformed parts, body space frames of cars, station wagons, trucks, motorcycles, etc., tail lifting platforms of trucks, automobile cranes, automobile fuel tanks, aluminum alloy car repairs, etc.

Figure 4-39: RX-8 Aluminum Alloy Rear Door and Engine Hood Manufactured Using Friction Stir Welding Technology

6)Friction stir welding has successfully solved the problem of connecting light alloy metals and is being increasingly used in various industries such as weaponry, construction, electricity, energy, and home appliances.

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