HPAM Moataz - Friction Welding
Fundamentals
Solid state welding process - no liquid melt formed
Minimal defects, environmentally friendly and reduced thermal distortions
Applications: - marine, railway, and aerospace
Linear friction welding
solid state joining process where frictional heat generated is used to join two rubbing components - one piece is fixed whilst the other oscillates. Pressed together under axial pressure
Parameters - forging force, amplitude and frequency
4 stages
Contact, conditioning (smoothing of asperities), burn-off (flash formation), forge
Blisks
Weight saving, as no dovetail arrangement required to hold the two components together
Blades can be individually replaced
Aerodynamics of discs are also improved
Works well for Ti, as this is not well suited to conventional welding
Blades are attached directly onto disk using linear friction welding
Eliminates need for weighty root fixing configurations
Vs machining
In favour of LFW
Cost effective for larger blisks
Enables optimised blade/disc materials
Enables hollow blisk
Blade replacement
In favour of machining
Cost effective for small blisks
Low material usage
Long manufacture time
Electrochemical machining uses HF - dangerous working conditions, environmentally undesirable
Useful for Ti, as more efficient material use (reduces expense)
No melting, less oxides compared to fusion welding
Inertia friction welding
Rotating flywheel - converted into frictional energy - pressed together - one stationary, one attached to rotating flywheel
No melt, so suitable for difficult to weld materials
Marine, railway and aerospace applications
Friction stir welding
Heat is generated by friction
A tool is passed between two workpieces
Plastic deformation occurs and the materials are literally stirred together
Advantages are - good weld quality, no fumes, reduced work distortion
Ti friction welding
Advantages
Solid state welding - no melting
Self-cleaning: oxides ejected into flash
Better material optimisation for design and manufacturing (do not have to use weldable material that may not have as good mechanical properties)
Concerns
High residual stresses: steep thermal gradient, combined with severe plastic deformation
Localised, yet severe, microstructural anisotropies e.g. texture
Tool wear and price
Quantification of residual stresses
Stresses build up in a body when it returns to equilibrium, due to thermal or mechanical effects
Lattice strain gauges: strain = deformed lattice parameter - standard lattice parameter/ standard lattice parameter
XRD - used for thin walled structures (lower energy) - good spatial resolution
Neutron diffraction - useful for thick walled samples (higher energy particles) - poor spatial resolution - Co content reduces its effectiveness
Forging pressure was found to have most significant effect on controlling the residual stress development - other factors were found to be unimportant
The stresses were generally higher than that due to linear friction welding and were spread over a larger region, with a maximum occurring away from the weld line. Considerable reduction in stresses can be achieved using post weld heat treatmeant
Macrotexture
Preferred orientation affects relative intensity of the peaks.
Texture has a significant influence on the properties of the material, especially Ti alloys due to the HCP structure.
The severe plastic deformation during friction welding typically results in strong stresses.
Modulus of Ti greatly decreases (by up to 33%) if not aligned in favourable orientation - effect of declination angle
The LFW parameters can control the level of texturing
Microstructure
Weld line - alpha to beta transformation occurs, followed by alpha decomposition on cooling, completely altering the parent structure.
Thermo mechanically affected zone - the material experiences thermomechanical deformation, reorientation or phase fraction alterations.
HAZ is not detected in Ti due to poor themal conductivity - heat energy is not transferred much to surrounding metal
Increasing forging pressure decreases the grain size and also decreases to weld line and TMAZ thickness
Increasing frequency and amplitude slightly increases weld line and TMAZ thickness, but decreases the grain size
Weld line experiences temperatures of approx. 950 C - in beta region, so causes growth of beta grains.
The beta grain size and the weld line and TMAZ width are all inversely proportional to the power input
The beta grain growth is controlled by grain growth following dynamic recrystallisation.
Weld line in Ti 6246
Narrow region - 0.5 um.
A mixture of a'' martensite and beta are present in weld line, suggesting that rapid flash ejection occurred (leading to rapid cooling)
Following PWHT, fine needle like ppts in the WL, replacing the original microsturcture
A TMAZ can be observed in the welds produced with considerably high forging pressures - shows deformed microstructure yet unrecrystallised. The use of a high forging pressure creates and considerably narrow weld region.
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PWHT creates a homogeneous modulus distribution
PWHT creates a high modulus weld region. Microhardness can be increased if fine a particles formed from metastable beta grains
Weld nugget cross section
Unaffected material (base material)
Heat affected zone (not displayed in Ti welds)
Thermomechanically affected zone
Weld nugget - stirred zone - dynamically recrystallised
Tool materials
Tungsten
Strong at elevated temperatures, relatively cheap
Poor toughness, wears rapidly when used on high hardness alloys
Tungsten Rhenium
Improved strength, hardness and ductility over W tool
Expensive (due to rhenium)
Ceramic (e.g. reaction bonded Boron Nitride)
High strength, hardness and wear resistance at elevated temperatures
Reactive with Ti workpiece, expensive, restrictions in tool design
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W-La
Good machinability, high strength
Reports of pin flattening during plunge (does not do well under high strain rates), expensive