TWI Industrial Member Report Summary 980/2011
By Geoff Melton
Health and safety in welding is always an important topic and with an increasing litigious and informed society, interest will continue to accelerate in the future. Welding is regarded as being a hazardous activity, requiring measures to be in place to minimise the risk.
Public awareness of possible ill health arising from inhalation of fume from arc welding processes has increased significantly recently. However, although a vast amount of data is available, there is no clear understanding of the mechanisms involved in fume generation. Measurements have shown that fume emission rates vary significantly with welding parameters, but a detailed examination of the mechanisms involved in fume formation and the relationship between metal transfer mode and fume formation rate has not been undertaken.
Welding fume consists of metal oxide particles that remain suspended in the air and thus may be inhaled by welders. Previous work on mechanisms of fume formation suggest vaporisation, condensation and oxidation of the welding wire and droplets in the arc zone to be the main source of fume. Fume increases with an increased droplet temperature, longer time in the arc zone, lower metal transfer stability and higher arc energy.
In dip transfer a small amount of fume originates from the wire, but explosive metal transfer creates spatter which is ejected from the arc zone, is oxidised and evaporates to form fume. In globular transfer there is more fume evaporated from the molten droplet and some fume occurs from spatter as the droplet necks and breaks free from the wire tip. Welding conditions affect the fume formation rate and welding current is considered to be the most important parameter. Generally, increasing current increases the fume, but the relationship is not entirely clear. A decrease in fume at the transition point between globular and spray transfer has been observed with higher levels of fume if the voltage is not optimised.
- Establish the relationship between welding fume formation and metal transfer modes in MIG/MAG welding
- Propose techniques to minimise emission of fume, based on control of metal transfer mode.