At present, there is no consensus on the maximum allowable limits for impurity elements in aluminum bronze castings which could adversely affect the ductility and promote cracking during welding and heat treatment. This work was carried out to evaluate the effect of some impurity elements on the mechanical properties and heat treatment of two most popular aluminum bronze alloys namely, C95800 and C95400. In addition, the cracking in the heat affected zone (HAZ) during welding of alloy C95800 was investigated.
Selected impurity elements (Pb, Zn, Sn, Bi, Se, Cr, Si and Be) were added as one, two or three element combinations in alloy C95800. Some of the samples were given the corrosion inhibition heat treatment of annealing at 700C for six hours followed by air cooling. The mechanical properties were evaluated under as-cast and heat treated conditions. The as-cast strengths (UTS and YS) were always above the ASTM specified limits. However, the ductility (% elongation) was reduced to just above and in some cases below the minimum specified value. The heat treatment marginally increases strength but reduces the ductility. Only Pb, Sn , Si and Bi appear to be the elements to have any major impact on the ductility (%elongation). Of this Si should be controlled below 0.1% to have good ductility. In case of other three elements, namely Pb, Sn and Bi the effect is more significant when they elements are present in combination. The maximum content of lead (preferably 0.01%) and bismuth (maximum of 0.05%) should be less than 0.06% to achieve good ductility. Tin content should be controlled below 0.1% when present together with lead or bismuth.
The weldability of alloy C95800 was assessed by conducting full bend test. However, it was found out that this test could not be used to predict the cracking in heat affected zone (HAZ). The simulation of the HAZ, using a Gleeble 2000 weld simulator, was found to be more representative than the bend test. Of all the impurity elements tested only lead and bismuth were found to promote cracking in the HAZ. The other impurity elements (Zn, Sn, Se, Cr, Si and Be) did not have any significant effect on HAZ cracking. Based on the results for various combinations of the impurity elements and microstructural observations, the limits to prevent HAZ cracking in C95800 can be specified as follows: Lead - 0.05%, Zinc - 1.2%, Tin - 0.3%, Bismuth - 0.03%, Selenium - 0.035%,
Only five elements, Pb, Sn, Zn, Bi and Se were added to alloy C95400. The heat treatment for this alloy consists of solutionizing and annealing treatment. The solutionizing was carried out at 900C for one hour followed by water quench. The castings were then annealed at 400C for one hour. Alloy C95400 exhibited high yield strength but very poor ductility in the as-cast and heat treated conditions, probably because of the high aluminum and nickel contents used in the investigation. Aluminum was found to be more important than impurity elements when mechanical properties are concerned. Only when the aluminum content was below 10.6% the ductility specified by ASTM could be achieved. Further work to assess the effect of aluminum on mechanical properties is needed. The effect of impurity elements on the as-cast properties was minimum. However, after heat-treatment, an alloy with either lead or bismuth at very low levels (0.02%) failed in a complete brittle manner at very low stresses indicating the adverse effect of these elements. It is suggested that these two elements should be controlled below 0.02%.
Materials Technology Laboratory
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