Manufacturing Processes
Manufacturing Processes tests casting, joining, forming, and powder metallurgy. The exam favors process selection, defect diagnosis, and simple solidification or load calculations.
Core rules
- Chvorinov's rule: t=B(V/A)^2. The riser must have a larger modulus V/A than the casting and freeze last; a riser also needs sufficient volume.
- Casting defects: shrinkage, gas porosity, hot tearing, misrun, cold shut; directional solidification and chills reduce shrinkage porosity.
- Welding: heat input H=VI/v; dilution is parent metal in weld metal; hydrogen can cause cold cracking in steels.
- Forming: rolling load depends on flow stress and contact length; friction and redundant work increase load. Recrystallization anneal restores ductility after cold work.
- Powder metallurgy: particle size distribution controls packing; compaction gives green strength; sintering densifies by diffusion, often with shrinkage.
Worked example
Compare solidification times of a 100×100×10 mm plate and a 20 mm cube using t=B(V/A)^2. Plate: V=100,000 mm3; A=2(100×100)+4(100×10)=24,000 mm2; V/A=4.167 mm. Cube: V=8,000 mm3; A=6(20×20)=2,400 mm2; V/A=3.333 mm. Time ratio=(4.167/3.333)^2=1.56. The plate freezes about 56% longer. A riser placed on the plate should have a still larger modulus so it remains liquid.
Traps
- Using only volume to size a riser; modulus and feeding distance matter.
- Confusing welding voltage and current in heat-input calculations.
- Ignoring dilution when specifying weld composition.
- Assuming sintering always shrinks uniformly; it can distort.
- Confusing green strength with sintered strength.