Binary phase diagram

The phase diagram shows you which microstructural phases an alloy forms at certain temperatures and mixing ratios. When welding, you use it to estimate how the melt and the heat-affected zone will behave during cooling. If you know the phase boundaries, you know whether you can run into problems with a material like cracks or embrittlement. Make sure you choose the filler metal and parameters so that you land in the desired microstructural range and don't run into critical phases.