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HPAM - Moataz - Superplastic and Thixotropic Forming - Coggle Diagram
HPAM - Moataz - Superplastic and Thixotropic Forming
Thixoforming
Thixotropic state - structure can be achieved in material in the mushy state is subjected to high shearing action (e.g. by stirring)
Formation of nearly spheroidal degenerate dendritic particles and the viscosity drops markedly - large spheroidal liquid pockets surrounded by a solid matrix
Occurs in lower part of temperature range of solid plus liquid region of phase diagram
Thixoforming, thixomolding and thixocasting are all interchangeable terms
Each alloy has its own thixoforming temperature range
Can be used as a near net shape or a net shape manufacturing route - very useful, as no machining required, reduces complexity and cost, and is more energy efficient than casting
Process characteristics versus conventional casting
Lower molding temperatures, material is in semi-solid state
The temperature control at the exit end is +/- 2 degrees
Elimination of most foundry operations (handling of molten metal, use of fluxes, waste management etc.)
Production of complex components directly from particulate material.
Potential to produce dispersion strengthened/ hardened materials
Applications
Large scale production of complex products from low temperature alloys (e.g. Mg, Al)
Significant increase in die life versus die casting (lower temperatures, less ablation)
Low porosity and enhanced properties of the material versus casting - lower inclusions + oxidation
Superplastic forming
Definition - "the ability of a material to exhibit, in a generally isotropic manner, very high tensile elongations to failure". Can withstand extreme amounts of plastic deformation before final fracture - resistant to necking
Requirements for superplastic behaviour
Fine grain size (less than 10 microns)
Equiaxed and stable grain structure, resistant to coarsening - needs intermetallic particles to pin grain boundaries and help promote maximum grain rotation and grain boundary sliding
High processing temperatures, more than 0.5 Tm (needs to be deformable at low temperatures)
Low deformation (strain rate range 10^-2 to 10^-5 s^-1)
High strain rate sensitivity (m>0.3)
Grain coarsening resistance - minimal grain elongation
Not only does a superplastic material need to be able to withstand high levels of strain, but also be resistant to necking when the applied strain rate is very high.
Techniques to create fine grain structure
Rolling based (to enable recrystallisation)
Extensive plastic deformation (comporessive)
In a log stress versus log strain rate, superplasticity occurs in region ii, where steady state (linear) creep is occuring
Strain rate sensitivity is the same as the resistance to necking - needs to be greater than 0.3 for superplastic behaviour
Flow stress (sigma) = K (material and structure constant) * strain rate (epsilon)^ m (strain rate sensitivity)
m value - strain rate sensitivity (resistance to necking)
The m value increases with the increase in grain size and temperature.
It varies at a given strain rate and temperature at different strain levels
m is an exponent, so can change due to stimuli,
Increasing temperature also increases superplasticity (increases the elongation to failure)
Minimising the strain rate maximises the superplastic behaviour, as this reduces the likelihood of necking occuring
Can be used in a number of different sectors (automotive, aerospace, marine and railway)