Week 9 - Cast Defects & Forging

Four methods to compensate for shrinkage

1. Deliberately oversize pattern

2. Add risers to supply extra liquid

3. Ensure adequate feeding pathways

4. Eliminate hot spots - Add nucleus

5 Feeding mechanisms

1. Liquid feeding - All liquid, super-heated, so flows very easily

2. Mass feeding - Free floating crystals in liquid, difficulty starts to feed.

3. Interdendritic feeding - Dendrites form, meaning feeding struggles.

4. Burst feeding - Pressure from flow breaks open dendrites and liquid rushes

5. Solid feeding - Now solid, isolated liquid pocket provides to surrounding

Gas Porosity VS Shrinkage Porosity

Gas porosity: Al typically, dissolves hydrogen very well. Nucleation forms gas bubbles which don’t escape forming a “gas pore”. A nice round bubble in structure.

Shrinkage Porosity: Form late due to dendrites, a very jagged random structure.

Hot spot

Last region to solidify as geometrically thicker then other regions. Shrinks and causes residual stress. Can fix through reducing section thickness, placing riser or adding nucleus.

Hot tearing mechanism & Prevention

Cracking of the grain when it is in mushy zone along grain (inter granular), happens when the cast is geometrically restrained.

Prevent:

• Grain refinement

• Reduce superheat

• Collapsible mold

• Fillet

Micro & Macrosegregation

Microsegragtion - Dendrite arm scale, when solute rejected at solidification front. Can be fixed by working/treatment.

Macrosegregation - Entire structure material varies, due to large movements of solid in liquid. Cannot be fixed.

Open Die Forging

No restrictions, simple compression forces to shape material. Two major types:

• Upsetting: Axially compressing cylinder to reduce cross section, barrelling happens which is curvature of sides.

• Cogging: Cross section reduced by successive operations.

Closed die forging

Die completely encloses structure, steps are as follows:

1. Blank - Crop bar to get desired shape

2. Edging - Redistribute material so can fit in die.

3. Blocking - Rough shapes in dies

4. Finishing - Fully presses in

5. Trimming - Trim of flash

Flashes are little areas left free so the metal has somewhere to go.

Forging defects

1. Laps - When die is overfilled, and oscillates then pressed down causing cracks, brittleness and weakness

2. Internal cracks - When die is underfille, metal cracks to fill

Die design rules

1. Parting line at the largest cross section to enable rejection into flash

2. Generous fillets.

3. Metal always flows in easiest region - Keep in mind when geometry.

4. Give a draft angle of around 3 degrees.

Economics of forging

1. Most expensive initial setup cost, very expensive captial investment

2. At maximum production run, only reusable die forging is cheaper

3. Mechanical properties are the best in forging - Even contain flow lines, but are stronger and more ductile with lines