The change in chemical composition of a welding filler metal caused by the admixture of the base metal or previous weld metal in the weld bead. It is measured by the percentage of base metal or previous weld metal in the weld bead. See Figure D-4, which shows a cross-section of a weld deposit.
The percentage dilution can be determined by measuring areas labeled A and B. Percentage dilution is then calculated as:
B _
A+B * 100
Dilution is usually considered as a percentage of the base metal which has entered into the weld metal.
When two pieces of base metal are welded together, the final composition of the weld deposit consists of a mixture of base metal and filler metal. The portion of the base metal that has been melted in with the filler material and has diluted it may be expressed in percent dilution. This is determined by the following formula:
Dilution = Weight of base metal in weld *100
Total weight of weld
Typical values of dilution for various processes are shown in Table D-2.
Many factors affect dilution. The greatest dilution occurs when no filler metal is added. In this instance, all of the weld deposit is self-generated by the base metal. Similarly, a single-pass weld will have a higher percentage of dilution than a multi-pass weld. There is always considerable dilution in the root pass. The greater the amount of weaving, the greater the dilution. Dilution as low as 2% has been achieved with the plasma arc hot wire cladding operation utilizing two hot wires connected in series. The application was welding copper rotating bands on artillery shells.