am welding aluminium using mig at work , i have done it before at other places of work , but am having trouble with the contact tips, as soon as wire touches work it "cracks" and fuses back to tip and is very frustrating as i managed ok before. i have set rolls so wire slips if there is too much resistance, i have liner as straight as poss , am using 1mm dia wire with a 1.2mm tip, and alu-shield gas. thanks