place of an integrally formed hard metal block 1 as used in the above described example a hard metal block 1 formed by inserting a wedge shaped plate 15 in between a pair of hard metal plates 14 as shown in fig1 may be used the transition insert material m obtained as shown in fig9 may be used for example as illustrated in fig1 in which an upper skirt wall 30 of an aluminum alloy is welded to a lower skirt wall 31 of steel through the transition insert material m since the upper and lower parts 7 and 1 of the material m are made of an aluminum alloy and a stainless steel respectively the welding between the part 7 and the wall 30 and between the part 1 and the wall 31 can be carried out easily the metallic structures of these soft metal and hard metal blocks in their states prior to bonding are shown in fig6 a and those after bonding are shown in fig6 b a number of combinations of a stainless steel sus 304 jis and an aluminum alloy a 5083 jis were experimentally bonded according to the method of this invention as described above and by other comparative methods and each of the resulting joints was subjected to a tension test to fracture whereupon the results indicated in fig1 and 13 were obtained fig1 shows the shape of test pieces used the dimensions of the test pieces were as follows in the case of the test pieces obtained from the transition insert materials produced by the method of this invention a rod portion 1 of the stainless steel hard metal was interposed between a pair of rod portions 7 of the aluminum alloy soft metal.