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Joining – Friction stir welding Table of contents 3 Friction stir welding .................................................................................................................. 2
3.1 Process technology and principles ................................................................................... 2 3.2 General introduction ......................................................................................................... 3 3.3 Materials and weldability .................................................................................................. 4 3.4 FSW process window ....................................................................................................... 5 3.5 Properties of joints ............................................................................................................ 6 3.6 Static and fatigue strength ............................................................................................... 8 3.7 Inspection, qualification and specification procedures ..................................................... 9 3.8 Joint types and tolerances ............................................................................................. 11 3.9 Pro-s and Con-s ............................................................................................................. 12 3.10 Welding equipment – Standard and special FSW machines ....................................... 13 3.11 Welding equipment – Backing and clamping ............................................................... 15
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3 Friction stir welding
3.1 Process technology and principles Process Technology FSW is a solid state welding process performed at temperatures lower than the melting point of the alloy. The work pieces are rigidly clamped in a fixed position and a specially profiled rotating tool traversed through the joint line produces the friction heating. The tool is crushing the joint line, breaking up the oxide film by a mechanical stirring and forging of the hot and plastic material. The resulting joint exhibits a finer grain structure than the base metal. The FSW tools can permit over 1500 m of weld to be produced in 4 mm thick aluminium extrusions without changing the tool.
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3.2 General introduction See also:
AAM – Applications – 3 Car body > Seats > Rear seats > Extrusions design – Backrest of Volvo V70
History Friction Stir Welding (FSW) is a new joining process invented by The Welding Institute (TWI) in England and patented in 1991. Licenses may be obtained from TWI. General FSW is a solid state joining method based on friction heating and local plastic flow in the joint region by stirring with a rotating tool pin. Practically all wrought and cast aluminium alloys can be joined by FSW. Applications Butt and lap joints in e.g.:
Back seat frames Engine cradles Suspension arms Wheels Fuel tanks Wide and long panels Tailor welded blanks
Large figure: FSW machine and handling robot as a part of a fully automated line for producing back seat frames. Small Figure: The back seat frame after welding.
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3.3 Materials and weldability Alloys Practically all alloys are weldable by FSW. The weld speed is affected by the materials' strength at elevated temperatures. Metallurgy FSW is a form of local hot working, and material is heated to temperatures where melting does not occur. Microstructure of the weld zone The weld has a fine grained structure with typically 3 to 6 mm equiaxed subgrains (similar to extrusion welds). Formability The fine-grained microstructure in the weld/HAZ provides good ductility. Figures: Microstructure (upper) and flow pattern (lower) in the joint cross section.
Microstructure of FSW weld
Source: Hydro Aluminium
Flow pattern of FSW weld
Source: SAPA
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3.4 FSW process window The figure shows a general process window which illustrates the applied FSW process power versus actual weld speed and profile/sheet thickness for softer and harder aluminium alloys. Along the y-axis the process power is a function of the friction coefficient between the steel tool and aluminium, the downward tool force (N) and the tool shoulder peripheral velocity (m/s). Along the x-axis is the selected weld speed and actual thickness of the welded component. The diagram shows that harder alloys need relative higher power, welded at relative lower speeds to generate sound welds, compared to softer alloys at the same thickness. If an alloy is welded at too low power, or at too high speed, then a brittle cold lap weld will be the result. On the other hand, if an alloy is welded by too high power at a too low speed, the material will soften too much and tool shoulder will penetrate the profile surface.
Process window shows power as f(friction coef., tool force [N], tool shoulder velocity [m/s]) vs weld speed, w, [mm/s] and thickness, t,[mm] for chemical composition/hot strength.
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3.5 Properties of joints
Heat input Even though the temperature history of FSW welds shows that the heat input is more favourable than in case of fusion welding, there will be a heat affected zone (HAZ). Materials in age-hardened or cold worked tempers have a softer weld zone.
Post-weld heat treatment Hardness profiles for EN AW-6082-T6 (above) and T4 (below), as-welded and after post-weld heat treatment, show that the base metal hardness can be almost fully restored (s. table below). FSW involves no use of filler material.
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Good corrosion resistance Consequently, the base metal composition is unchanged in the joint and there is no segregation of alloying elements. Together with low residual stresses and absence of melting FSW joints have good corrosion resistance.
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3.6 Static and fatigue strength Static properties are generally better than for fusion welds. In T6 temper alloys tensile strength is >70% of base metal strength. Welding in the T4-temper condition followed by a post weld ageing could give >90% of BM tensile strength in the weld.
Due to its fine grained microstructure and smooth nature of the weld surface the fatigue properties in sound weld are close to those of the base alloy.
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3.7 Inspection, qualification and specification procedures Surface appearance The surface is smooth and flush with the part's surface. Top surface has wave-marks from the tool (see figure below). Visual inspection Procedure is to visually inspect top and root. Dye penetrant is used to detect root flaws in suspected areas. NDT Radiographic NDT is used for spot checks. Acceptance criteria are the same as for fusion welds.
WPQ Reported parameters are tool size, tool tilt angle, tool forces, rotation speed and weld speed. Weld Procedure Specification Rp0,2, Rm, Elongation are reported together with bend tests, visual inspection and NDT according to classification societies' rules for fusion welding.
Higher speed
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3.8 Joint types and tolerances Product tolerances For extrusions +0.3, -0.1 mm thickness tolerances are specified within 20 mm from the joint line. Profile straightness tolerance is 1 mm/m. Process can tolerate max gap of 10% of thickness. Joint types Butt welds and overlap welds are most applied. Sufficient backing must be ensured prior to welding.
Double FSW-joint for hollow extrusion
Source: SAPA
Geometries: a) square butt welds, b) combined butt/lap welds, c) single lap weld, d) multiple lap weld, e) 3 piece T-butt weld, f) 2 piece T-butt weld, g) edge butt weld, h) corner fillet weld.
Different joint geometries
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3.9 Pro-s and Con-s Pro-s + excellent repeatability + low tool cost + no consumables + few weld variables + low heat distortion + improved mech. prop. + no fumes Con-s - high investment - extensive clamping - need backing support - critical tolerances
Welding without melting
Improved overall product tolerances
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3.10 Welding equipment – Standard and special FSW machines
Even if Friction Stir welds of proper quality can be successfully produced in a rigid milling machine the special machine offers many advantages regarding handling, positioning, clamping and operation control equipment to assure a correct weld quality. At present there are only a few manufacturers of welding equipment specially designed for FSW on the market. FSW by robots has been reported but is still limited to small welds due to stability challenges.
Panel welding equipment with three welding heads possible for simultaneous operation
Double spindle welding machine in a fully automated line producing automotive parts
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A standard 5-axis milling machine slightly modified for all-round welding purpose
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3.11 Welding equipment – Backing and clamping The fixturing and clamping is one of the most important parts of the process. It must support the welding forces, keeping the parts in place, offers a fast and effective handling not only before welding but also of the joined component. Finally the welding spindle must have access to all the joints to be welded. Hollow profiles can be designed with internal backing by locating material or supporting legs in proper positions.
Example of joint design of hollow profiles (top and right)
Typical weld design of FSW a plate cover to a cavity (left)
