Influence of vibration on grain size and degree of grain refinement in mild steel weldments

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Mild steel plates were welded under stationary and dynamic (longitudinal and transverse vibration) conditions. The frequency and amplitude ranges selected were 0 Hz to 300 Hz and 0 /xm to 30 jxm, respectively. It has been observed on the basis of experimental findings that vibration, in general, reduces the grain size of the weldment, and this reduction is appreciable at a lower amplitude of vibration. This may be due to the lower energy required for nucleation of the solid phase. Fast removal of latent heat of solidification from the solid liquid interface plays a significant part in the grain refinement under vibration.

I. INTRODUCTION The application of vibration to solidifying metals was first attempted by Boyle and Taylor in 1926. They found that ultrasonic vibrations could effectively degas light metals. Both refinement of grain structure and degassing of the melt can be obtained in most metals if a vibration of sufficient intensity is introduced during solidification. Vibrations also greatly influence liquid and solid solubilities and size and distribution of various phases in the metallographic structure of solidifying metals. Ganaha et al.1 studied the grain structures in twodimensional GTA welds of a number of alloys to predict the mechanisms of grain refinement in the weldments and found columnar and equiaxed grains in different ranges of intensity of vibration. Fisher2 studied the effects of vibrational energy on the solidification of Al alloys and concluded that vibration reduces or eliminates pipe formation and shrinkage cavities. Vibration induces improved nucleation which leads to grain refinement and the multiplication of eutectic cells. Shukla et al?-A observed porosity reduction and grain refinement in solid-solution-type aluminum alloys. Pearce and Kerr5 found grain refinement in magnetic GTA welds completely penetrating thin sheets of several aluminum alloys. In some alloys magnetic stirring was shown to extend the range of welding conditions that produce a partially equiaxed structure and to widen the equiaxed fraction of equiaxed welds. This was attributed to magnetic stirring lowering the temperature gradient, allowing nucleation and growth of Al-rich grains further ahead of the columnar with low titanium levels. Magnetic stirring might be causing refinement by sweeping grains from the molten zone ahead of the advancing interface. Kou and Le 6 had investigated the effect of low frequency transverse arc oscillation on the structure and properties of 2014 aluminum welds. They 2228 http://journals.cambridge.org

J. Mater. Res.,Downloaded: Vol. 8, No. 12 9, JanSep 2015 1993

observed alternating columnar grains and a significant increase in ductility and strength of the welds. Kou and Le7 also studied arc oscillation effects on grain structure and solidification cracking in GTA welds of 5052 aluminum alloy. They achieved grain refining in welds made with all three types of arc oscillation (longitudinal, transverse, and circular). Scarbrough and Burgan8 have established from their experimental f