Friction Stir welding of Aluminum
Outlines TYPES OF WELDING INTRODUCTION TO FSW ALUMINUM WELDABILILTY THE PROCESS PRINCIPLE THE TOOL PROCESS PARAMETERS WELD ZONES MICROSTRUCTURE OF WELD ZONES MECHANICAL PROPERTIES ADVANTAGES OF FSW FSW DEFECTS DISADVANTAGES OF FSW APPLICATIONS FSW IN AEROSPACE BOBBING
Types of welding
Welding can be categorized to fusion and plastic welding. Fusion Heated to melting temperature. No pressure is required. e.g. Gas welding, Arc welding. Plastic (solid state) Heated to plastic state (below melting temperature). Pressure is required. Welds created by (frictional heating & mechanical deformation). No filler metal , joint strength is better, low distortion and residual stresses. eg: friction welding, forge welding.
Introduction FSW is a plastic (solid state) welding process , it was invented by Wayne Thomas at TWI (The Welding Institute) in 1991. FSW is capable of joining materials which has low melting temperature & high conductivity, that are difficult to be welded by fusion welding. It is primarily used on aluminum, and most often on extruded aluminum (non-heat treatable alloys), and on structures which need superior weld strength without a post weld heat treatment.
AL is hot short unlike other metal, as Al gets brittle at a certain elevated temperature. Al is difficult to be welded by traditional methods (Gas welding , Arc welding), due to high thermal conductivity, resulting in defects like porosity, cracks etc. Methods used to weld AL TIG _Ac_. Friction Stir welding. Laser welding. MIG also can be used. Al also can be joined by (adhesives, soldering, brazing).
The process principle 1. 2.
4. 5. 6.
Plates are butted together and clamped securely. A rotating non-consumable tool penetrates the material and generates frictional heat , that soften the material (without reaching melting temperature). Then mechanically intermixes the two pieces of metal at the place of the joint (stirring). Shoulder hence act as a die and forge the solid state region. Solid state continues joint created when traversing takes place. After finishing the tool is drawn out.
ďƒ H13 (Hot work steel tools) are the most used in welding AL. ďƒ Tool must be: * Strong * Tough * Has hard wearing at welding temperature * Has good oxidation resistance * Has thermal conductivity.
The strength of FSW depends on: Spindle speed High speed high quality and strength of weld & for hard materials Low speed large welds sizes Feed rate Low better welded joint & good surface finish Depth of penetration Deep shoulder of tool plunges into the WP & concave weld produced
Weld zones Friction stir weld in its cross-section consists of three main zones: Nugget, stir zone (dynamically recrystallized zone). Thermo-mechanically affected zone (TMAZ). heat affected zone (HAZ).
Weld zones Nugget zone (dynamic recrystallized zone): Region of heavily deformed material corresponds to the location of the probe during welding. The grain in this zoon are equiaxed and smaller than them in the parent material. Unique feature of this zone is occurrence of concentric rings as “onion rings” The flow arm zone On the upper surface of the weld and consists of material that is dragged by the shoulder from the retreating side of the weld, around the rear of the tool, and deposited on the advancing side.
Weld zones The thermo-mechanically affected zone (TMAZ) This region is unique for FSW. Occurs on both sides of the stir zone. The effect of welding on the microstructure is correspondingly smaller. Grains in this zone is plastically deformed due to the shear force from tool rotation and traverse. The degree of plastic deformation is increased when it get closer to the nugget The heat affected zone (HAZ) Region not plastically deformed ,but it`s affected by thermal energy. The temperatures are lower than TMAZ but may still have a significant effect if the microstructure is thermally unstable. In age-hardened aluminum alloys this region commonly exhibits the poorest mechanical properties.
Weld zones microstructure
a: base material b: heat affected zone (HAZ) c: Thermo-mechanically affected zone (TMAZ)
Weld zones microstructure
a: Nugget position (a) b: Nugget position (b) c: Nugget position (c)
FSW Advantages Metallurgical • Fine microstructure (high strength). • Absence of cracking. • Good dimensions stability and repeatability. • No loss of alloying elements. • Finer microstructure than parent metal Environmental • Safety due to the absence of toxic flames & gases. • Low environment impact. Economical • Only requires 2.5% from energy needed in laser weld. • No consumption (low cost). • Can be easily automated on simple milling machine. • No surface cleaning or finishing required.
Source : FSW processing
Controlled by RPM & IPM Related to LOP & LOF • Chip LOF • Excessive indentation • Wormhole • Lack of penetration (LOP) • Scalloping • Lack of fusion (LOF) • ribbon flash • Root flow • Faying surface Notice • Surface galling LOP = Lack of penetration • Collapsed nugget LOF = Lack of fusion • Surface LOF
Cont. Welding defects Excessive Flow Arm Formation and injection of material into Advancing Side
Collapse of Nugget
Cause * Excessive flow arm formation * Excessive material flow into Zone I advancing side * Excessively hot weld * Too high weld pitch
Surface Lack of Fill
Cause * Insufficient flow arm formation across top surface * Insufficient forge pressure * Improper backside support * Insufficient plunge depth. * Separation of the plates
FSW Disadvantages Work pieces must be rigidly clamped. Slower traverse rate than fusion welding. The entire thickness of plate doesn`t get welded as the tool does not penetrate the whole thickness. ((and to avoid this issue we weld both sides or by using Bobbing)).
FSW Applications Fields AEROSPACE SHIP BUILDING & OFFSORE AUTOMOTIVE FABRICATIONS RAILWAYS.
Products Eclipse 5000 jet Hybrid Rims
liquid oxygen and Hydrogen Tanks car Inner doorframes
space shuttle tanks Rigid Structure Ford suspension link
FSW VS rivets in aerospace
FSW used primarily in the main airframe, it provides a continuous bond along the aluminum surfaces of the frame. FSW product 3 times stronger than traditional aluminum bond FSW is also Lighter No increase in weight because rivets are no longer used to adhere the frame pieces together. It saves about 1600 work hours, as it`s computer controlled. Less expensive due to the total time of production. Saves fuel and makes every flight more efficient. No gases or smokes (ecofriendly). Source : Eclipse
Bobbing has been invented also by (TWI) to solve the root flaws problems in single sided welds and to weld thicker plates. Advantages • No backing bar is needed. • Less complex fixtures. • Eliminates weld roots, and root defects. • Net axial forces almost zero. • Simple control • Tolerance to thickness variation. Source : TWI
Aman Allah Ahmed Abu-zeid
Arsany Samy Goda
Ahmed Taha Abdel maged
Mostafa Ashour Abd-Alwahed