Response of Weldment Heat Affected Zone Hardness to the Operational Welding Current Resistance

Abstract

This paper presents the welding of selected engineering materials: aluminium, mild steel and cast iron using the shielded metal arc technique and evaluation of the heat affected zone (HAZ) hardness of their respective weldments similarly cooled in palm oil. The weldment HAZ hardness for aluminium, mild steel and cast iron were 407, 503 and 870 VPN respectively. Three models were derived, validated and used for evaluating the dependency of HAZ hardness of the materials on the welding current resistance. The results of the analyses clearly revealed that the HAZ hardness of these weldments are significantly affected by the resistance to the welding current flow. It was discovered that on welding aluminium, cast iron and mild steel, and similarly cooling their respective weldments in palm oil, the model; ea = 1000 Ϧ9.6831 (ec em)-1 predicts aluminium weldment HAZ hardness by multiplying a thousand of the evaluated Welding Current Resistance Product Rule (WCRPR); (Ra RcRm)9.6831 or Welding Current Resistance Equivalent (WCRE); Ϧ9.6831 with reciprocal of the product of cast iron and mild steel weldment HAZ hardness (ec em)-1. Other derived models reveal that the HAZ hardness of weldments of each of cast iron and mild steel relative to the others is also significantly dependent on the WCRPR or WCRE which is the resultant operational input of their respective welding current resistance. The validity of the model was found to be rooted in the core model expression; 0.001(ea ec em) = (Ra Rc Rm)9.6831 where both sides of the core model expression are correspondingly equal. Computational analysis of experimental and model predicted results indicate that aluminium, cast iron and mild steel weldment HAZ hardness per unit welding current resistance as evaluated from experiment and derived model are 174.431 & 174.433, 711.831 & 711.839 and 411.553 & 411.557 (VHN) Ω-1 respectively. Deviational analysis shows that the maximum deviation of model-predicted HAZ hardness from the experimental results is less than 0.0012%. This invariably implies over 99.99 % confidence level for the derived models as well as over 0.99 reliability response coefficients of aluminum, cast iron and mild steel weldment HAZ hardness to the operational influence of welding current resistance.