Semisolid Metal SSM Processing A New Approach to Metal Casting

1
Semi-solid Metal (SSM) Processing A New
Approach to Metal Casting

• By
• Dr. Muhammad Kamran

2
Introduction

• Discovered at MIT
• in 1971
• During study of hot tearing of steel during
  solidification
• Modeled by Sn-15Pb alloy using Coeutte Rheometer
• Semi-solid Metal Processing
• Non-dendritic structure
• Solid-liquid region
• Suitable fraction solid
• Property exploited is
• Rheology
• Steady state rheology (Rheology)
• Transient state rheology (Pseudoplasticity)
• Time dependent rheology (Thixotropy)

3
Evolution to Non-dendritic Microstructure

• Under Forced Convection
• Fragmentation of primary dendrites by
• Root melting of secondary dendritic arms
• Bending and shearing of secondary denritic arms
• Bending and root melting of secondary arms
• Fragments of primary dendrites grow into globular
  grains under forced convection

4
Evolution to Non-dendritic Microstructure

• Partial melting and Iso-thermal holding
• Low temperature eutectic phase melts in
  semi-solid range
• Highly stressed grain boundaries favours wetting
  of grains with liquid
• Grain become globular to reduce curvature and
  liquid-solid interface energy

5
Introduction

• Characteristics of Slurry
• Sheared semi-solid slurry shows apparent
  viscosity similar to machine oil
• SSM Processing exploits
• Solidification mechanism in the solid-liquid
  range i.e. nucleation and growth mechanisms

6
Alloy Characteristics

• Solidification range
• Low melting point phase/eutectic phase
• Temperature Sensitivity of solid fraction
• Castability
• Potential for aging etc.

7
Introduction

• Classification suggested by Flemings et.Al.
• Rheocasting
• Shearing the liquid while cooling from liquid to
  liquid solid range and casting a net-shape
  product
• Thixocasting
• Shearing a partially melted suitable precursor in
  semi-solid range
• Compocasting
• Shearing a miscible or immiscible mixture of
  composite in the solid-liquid range

8
Evolution to Non-dendritic Microstructure

• Other Mechanisms
• Grain refinement
• Spray forming and sintering
• Ultrasonic vibrations etc.

9
Processing Techniques

• Techniques for feed stock
• Techniques for Finished Product

10
Semi-solid Metal Processing Techniques

• Techniques for non-dendritic Feedstock
• Mechanical Stirring
• Megnatohydrodynamic (MHD) Stirring
• Stress Induced and melt Activated (SIMA) Process
• Spray Forming
• Liquidus Casting
• Ultrasonic treatment
• Grain refienment etc.

11
Semi-solid Metal Processing Techniques

• Mechanical Stirring
• Developed at MIT
• Stirring of liquid alloy while controlled cooling
  in the semi-solid range using auger, impeller or
  paddle
• Batch type or continuous process
• For Rheocasting and Thixocasting

12
Semi-solid Metal Processing Techniques

• Magnetohydrodynamic (MHD) Stirring
• Patented by Alumax Inc.
• Vigorous stirring by rotating magnetic field
  within the mould
• Normally for producing feedstock for thixocasting
• Disintegrated fine dendrites which convert to
  globular grains on partial melting
• Most widely used process

13
Semi-solid Metal Processing Techniques

• Stress Induced and Melt Activated (SIMA) Process
• Severe deformation of precursor results in
  residual stresses
• On heating recrystallisation occurs
• On partial melting low melting point phase
  becomes liquid
• High residual stress favours wetting of grain
  boundaries
• Higher curvature and liquid-solid interface
  energy favours globular grain morphology
• Longer holding time favours coarsening and
  agglomeration which deteriorate
  semi-solidprocessability

14
Semi-solid Metal Processing Techniques

• Liquidus Casting
• Pouring of melt at near-liqudus temperature
• Higher rate of Nucleation favouring fine grain
  size
• On partial melting fine grains transform to
  globular grains
• Grain Refinement
• Promote heterogeneous nucleation
• Finer grain size
• Grains transform to globular on partial melting

15
Semi-solid Metal Processing Techniques

• Techniques for Net-shape Product
• Rheocasting
• Thixoforming
• Thixomoulding
• Rheomoulding etc.

16
Semi-solid Metal Processing Techniques

• Rheocasting
• Non-dendritic ally slurry obtained from shearing
  while cooling from liquid state
• Mechanical stirring
• MHD Stirring
• New Rheocasting (NRC)

17
Semi-solid Metal Processing Techniques

• Thixocasting
• Partial melting of suitable feedstock
• Mechanical stirring
• MHD stirring
• SIMA process
• Spray forming
• Thixocasting
• Thixoforging

18
Semi-solid Metal Processing Techniques

• Thixomoulding
• Simultaneous heating and shearing of solid
  feedstock in a chamber by rotating barrel
• Designed for Mg alloy
• Controlled atmosphere
• Similar to injection moulding

19
Semi-solid Metal Processing Techniques

• Rheomoulding
• Vigorous Shearing of liquid alloy using screw
  type drive while cooling
• Similar to Injection moulding
• Suitable even for immiscible alloy formation and
  processing
• Twin Screw Rheomoulding (TSRM) process is an
  upgradation

20
Commercial features

• Advantages
• Energy efficient
• Production rate compete that for pressure die
  casting
• Smooth die filling with no air entrapment
  favouring product soundness
• Lower processing temperature
• Lower Impact to the die
• Reduced solidification shrinkage

21
Commercial features

• Disadvantages
• Relatively higher feedstock material cost
• Precise control of operating condition is
  required.
• Liquid segregation may occur as a result of
  non-uniform heating

22
Commercial features

• Potential Applications
• Replacement of permanent mould parts to eliminate
  machining and finishing
• Pressure tight parts such as master brake
  cylinders, fuel rails, air conditioner compressor
  housing etc.
• High strength parts such as engine mounts, tie
  rods etc.
• Wear resistant parts made from hypereutectic
  alloys such as compressor piston, brake drums,
  gear shift levers etc.
• Forged parts requiring excessive tooling

23
Commercial features

• Current Applications/Products

Hydraulic Brake Valve
Automobile Wheels
Brake master Cylinder
24
Commercial features
Door Pillars for Audi
Control arms for Steering
Fiat Engine bracket
25
Future Developments

• Process development
• Mechanism of formation of non-dendritic structure
• Alloy development
• Simulation studies of process
• Viable process for steel and Cast Iron

26

• Thank You for Your Patience