Frequently Asked Questions
The arc energy normally consumed in excessive melting of the base material can be exploited to melt additional filler material in the form of metal powder. In this way significant improvements in metal deposition rates can be accomplished without raising the arc energy. The metal powder dispensing equipment is relatively inexpensive and is simple to operate. It can easily be attached to both fixed and portable equipment such as a tractor mounted SAW head.
The two most common methods of adding metal powders are illustrated in Figs.1 and 2. That shown in Fig.1 is the forward feed method, in which metal powder supplied from a metering device, usually a bucket wheel dispenser, is delivered to the joint ahead of the flux. The second method, shown in Fig.2, is by wire feed, where two or three streams of metal powder, usually metered through a controlling orifice, are directed onto the electrode wire. The powder becomes magnetically attached to the electrode wire and is carried into the molten pool through the flux layer. For applications in which high toughness is a requirement, different powder compositions are needed for the two feeding methods. This is due to the different elemental recovery rates in each case.
In the forward feed method powder is laid ahead of the weld. Because of the reduction in penetration which results, the technique is not normally used to make single pass welds where the root must be penetrated. In thick section welds, when powder is used, it is normally added during the fill process. Excessive powder additions can lead to lack of fusion defects but with modest addition rates, typically 5kg/hr for single wire welding and 7.5hg/hr for tandem wire welding, the reduction in penetration does not significantly increase the risk of defects. This increase in metal deposition rate is normally exploited to achieve a reduction in the number of weld passes to complete the joint, typically by a factor of 30-50%. A further benefit is a reduced flux consumption, for which the cost savings more than offset the extra cost of the powder addition.
In the wire feed process, the metal powder is fed directly onto the slightly extended electrode wire to which it adheres. The powder becomes molten by the excess heat in the weld pool. The resulting weld bead size is increased which suggests that the melting efficiency is improved in comparison with the forward feed method, allowing higher addition rates to be used. With a single wire, a typical addition rate would be 9kg/hr. Typical applications for the wire feed technique include fillet welds and circumferential seams on small diameter work pieces (which would not be practical for the forward feed technique).