Most hot upsetting applications at are AC. AC is relatively inexpensive compared with MFDC. If you have AC equipment and power is not an issue it would be logical to hot upset with AC.
Hot upsetting can require a lot of power. This could be a large requirement on the plant electrical system. If the plant does not have the power without a large investment in new power, MFDC controls would reduce this power requirement substantially and could become economically advantageous over AC. MFDC also offers very fine control of the actual weld cycle which also could be helpful. Additionally if you already have MFDC equipment then why not consider adapting it for hot upsetting.
There are several variables that must be considered during flash welding.
Joint Design: The part must provide for even heat on both sides of the weld. Adequate length of material must be provided to allow for flash. There must be suitable area to clamp the part. There must be room for the flash product to escape from the weld area.
Heat Balance: Two pieces of the same size and material will heat the same but if the materials differ they will not. One part will heat more and in turn flash more. The clamp location must be located to allow for this added flash material.
Surface Preparation: Normally is not an issue with flash welding.
Flash and butt welds are tested much like other resistance welds. Quality criteria is established at the start of a production run. This criteria is used for subsequent evaluation during the manufacturing process. Tensile or destructive testing is the normal method of testing random samples during a production run.
There are several variables in the machine set up that can affect the weld. These are secondary voltage, flashing rate, flash time, current and upset distance and force. The effects of these variables are related to how they cause the parts to be over heated or to cool or forged too little or too much. If the material is too plastic or not enough different observations and results will occur.
Flash and butt welding both use the work piece as the electrode. They both use a clamp to hold the parts and apply force. This clamp carries current so it must conduct current and dissipate heat. This makes copper alloys a first choice. In many cases the Group A copper alloys will be used for this application. If mechanical wear is an issue Class 3 might replace Class 2.