The Strange Color on Brass Surface

What alloy of brass are you using? What method of welding are you using? TIG, arc, gas? Some types of brass will hot forge well but will not weld very well (or at all). Also, it is important to have the correct filler metal to match the base metal. The surface of parts must always be clean and free of all oxides for good weld job. You mention "heating parts to 300 degree" is this C or F ? The "purple" color could be caused by thin layer of oxide that forms between 300º-600º F. -GSG

CW613N/CuZn39Pb2Sn, by laser welding. We heating the parts 300 C.

I don't know very much about laser welding. Using other methods, the inclusion of Pb in brass alloys usually makes welding very difficult or impossible. Negative results include hot cracking and porosity. So you are pre-heating to 300º to help prevent this? I will ask our laser expert what he thinks.

this link may be of use

http://www.weldingtipsandtricks.com/