So far, you havn't received any answer, so I'll try to give you some help. I don't know either what Electric Fusion Welding and Flash Welding are. I've looked into my books but they don't say a word about them.
So, I'll suggest you to do the following: being from Germany, it shouldn't be difficult for you to get in contact with Van Leeuweren from Holland. Van Leeuweren is probably the largest welded pipe manufacturer in Europe. If they don't know, nobody in the world will. Give them a phone call, which in Europe is cheap, and ask what those terms mean.
Once you know it, I'd like to know it also, so send me an e-mail to the address indicated below.
Thank you and good luck.
Giovanni S. Crisi
Sao Paulo - Brazil
gscrisi@mackenzie.com.br

I believe they are talking about "Electric Arc Welding" when they refer to "Electric Fusion Welding", which is defined by AWS as "a group of welding processes which produce coalescence of metals by heating them with an arc, with or without the application of pressure, and with or without filler metal", i.e. any arc welding process, such as SAW, SMAW, GMAW, GTAW, etc., whereas, "Flash Welding" on the other hand uses an electric arc to heat the base metals. It combines resistance welding, arc welding and pressure welding. "Flash Welding" provides a strong, clean weld joint. The two pieces to be joined are brought together under light pressure. A low voltage, high amperage current travels between the base materials. As soon as the current is established, the pieces are drawn apart very slightly. This causes an arc between the two pieces. The arc heats the two surfaces and when they are heated to the proper temperature, they are forced together under very high pressure. This is done without the use of any filler metal. The weld joint is a single straight line. It usually distorts the material at the joint as well, but not enough to harm it. This is usually referred to as the "flash" or "enlargement". One good example of "Flash Welding" would be in the Penthouse of some oil burning power plants. The Boiler tubes up there are dissimilar metal welds (T-22 C/S to Stainless). They usually exhibit the enlargement called the "Flash". I'm not sure, but I believe some rails on railroad tracks are also welded by this method. As far as using "Flash Welding" on longseams, I have not seen one to date, but I'm sure it can be done. A good reference is a book called "Modern Welding" by Althouse, Turnquist, Bowditch and Bowditch. It is available on www.amazon.com.