Question

In regards to the metal extrusion manufacturing process:

1. How would you ensure that you obtained a fine-grained microstructure in a metal casting? Carefully explain the mechanism/s that lead to the fine grain size.

Answer 1

There are three ways to control grain size:

(i)Thermal

(ii)Chemical

(iii)Mechanical

In a standard casting process, thermal control is the easiest to perform. A low pouring temperature is prolonged so that the alloy can solidify and this controls the grain size. In the chemical approach, innoculants are used. Innoculants are compounds that have lower softening point than the alloy. A disadvantage is that it introduces oxides which become nucleation site for wear crack commencement.  

The mechanical approach relies on the frequent turbulence of the liquid metal as it solidifies. The turbulence breaks up the dendrites as they appear to form, thus supplying more nucleation sites and hindering grain advancement. Although, because solidification is confined, microporosity arises in the middle of the grains, and pore closure by a hot isostatic processing (HIP) action is required.

One technique for restraining particle dimensions in aluminum alloys is by initiating particles to assist as nucleants. Grains will propagate via heterogeneous nucleation. For a particular grade of undercooling below the solidifying temperature, aluminum grains in the melt will nucleate on the exterior of the attached particles. Grains will grow in the configuration of dendrites developing radially off from the exterior of the nucleant. Solute grains can then be connected which bound the progress of dendrites, leading to grain purification. One typical approach is to originate a extremely minor fragment of the melt to solidify at a considerable excessive temperature than the rest. This will produce seed crystals that function as a pattern when the remaining part of the substance drops to its melting temperature and begins to solidify. As a large amount of microscopic seed crystals are available, a roughly equal amount of crystallites develop, and the dimension of any one grain is restricted.