JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III MANUFACTURING PROCESS AND MACHINE TOOLS Previous Question Paper (September - 2021 - R16)

Investment casting, also known as lost-wax casting, offers high dimensional accuracy and excellent surface finish. Its advantages include the ability to cast complex shapes with intricate details, compatibility with a wide range of ferrous and non-ferrous alloys, and minimal requirement for secondary machining due to near-net-shape production. Limitations include high production costs per unit, long lead times due to pattern preparation, and size limitations for very large components.

Typical product applications include turbine blades for jet engines, medical implants like hip replacements, firearm components, jewelry, and complex parts for the aerospace and automotive industries where precision is critical.

The Heat Affected Zone (HAZ) is the region of the base metal surrounding the weld that has not been melted but has had its microstructure and properties altered by the heat of welding. Failures occur here primarily because this zone undergoes significant thermal cycling, leading to several detrimental effects:

1. Microstructural Changes: The rapid heating and cooling cycles can cause grain growth, phase transformations, or the formation of brittle phases like martensite in steels, which reduces ductility and toughness.
2. Residual Stresses: Non-uniform expansion and contraction during the welding process create high localized tensile residual stresses, which act as precursors to crack initiation.
3. Precipitate Dissolution or Coarsening: In precipitation-hardened alloys, the heat can cause strengthening phases to dissolve or coarsen, leading to a significant loss of mechanical strength in the zone immediately adjacent to the fusion line.
4. Hydrogen Embrittlement: The thermal cycle can promote the diffusion of hydrogen into the HAZ, which, combined with residual stresses and a brittle microstructure, makes the region highly susceptible to hydrogen-induced cold cracking.

Because the HAZ often exhibits a gradient of mechanical properties that differ from both the parent metal and the weld metal, it frequently acts as the weakest link in the weldment under mechanical loading.