JNTU B.Tech IV LIGHT METALS AND ALLOYS Previous Question Paper (March - 2021 - R16)

Two important alloys in the Al-Mg system are the 5052 and 5083 series. The 5052 alloy contains approximately 2.5% Mg and 0.25% Cr; it exhibits excellent corrosion resistance, good weldability, and moderate strength, making it ideal for marine equipment, fuel tanks, and sheet metal work. The 5083 alloy contains approximately 4.5% Mg, 0.7% Mn, and 0.15% Cr; it provides high strength in the welded condition and exceptional resistance to seawater, commonly used in shipbuilding, pressure vessels, and cryogenic applications.

The microstructure of an Al-4% Cu alloy in the solution-treated and quenched condition consists of a supersaturated solid solution of copper in aluminum (α-phase). Upon ageing, fine, coherent precipitates of CuAl₂ (θ' phase) form within the α-matrix, which act as obstacles to dislocation motion, significantly increasing the alloy's hardness and strength.

The ageing behavior follows these stages:

1. Natural Ageing: Occurs at room temperature, where GP (Guinier-Preston) zones form, leading to a gradual increase in hardness.
2. Artificial Ageing: Occurs at elevated temperatures (e.g., 150°C–200°C), where the transformation sequence is GP zones → θ'' → θ' → θ (equilibrium CuAl₂).
3. Peak Ageing: The point where the distribution of coherent θ' precipitates provides maximum resistance to dislocation movement, resulting in peak hardness.
4. Over-ageing: Prolonged exposure at high temperatures causes the precipitates to coarsen and lose coherency, transforming into the stable, incoherent θ phase, which results in a reduction of mechanical strength.