In the design and strength analysis of a spot welded structure, the mechanical property of spot weld, and the number, spacing or arrangement, etc. of spot welds in a joint are considered as the primary factors. However, elastic deformation of weld, that is displacement coefficient, which is one of the mechanical properties of spot weld is not sufficiently applied because the characteristics have not been made clear. This report deals with the relationship between the displacement coefficient value and the quality of spot weld, the influence of joint type, etc. in a mild steel spot welded joint. As the result of the investigation, it is found that the displacement coefficient value of mild steel spot weld is smaller than those of aluminum alloy and riveted joint of same size, and there is hyperbolic relationship between the displacement coefficient value and the nugget diameter of spot weld and so on. Moreover, the application of displacement coefficient to spot welded structure design is studied. And, it is shown with some examples that the displacement coefficient is applicable to analysis of torsional rigidity and calculation of load share of spot weld in a spot welded structure.
In spot welding, if there is friction in electrode force system, the actual electrode force increases at weld part, because the material can not expand itself during welding. As a result, the weld tensile strength decreases. If the apparent electrode force is set at a lower value taking the increase of this electrode force into consideration, a sound weld can be obtained. However, there are troubles such as poor contact of mating plates and inconsistency of friction itself, so friction is not desirable for welder.
The weld deposit metal is formed by a complicated process. Assuming the carbon content of the deposit metal to be about 0.15%, the weld molten metal solidifies in peritectic reaction, and it transforms into austenite. Then, the ferrite grain precipitates at the austenite grain boundary. Therefore, there are relations between the primary structure and the secondary structure. Authors already reported about the relation between the heat input and the primary structure. This time, they researched the relation between the primary structure and the austenite grains and the ferrite grains. Besides, the influence upon the mechanical property was investigated. The conclusions are summarized as follows; (1) The precipitation of the austenite grain is controlled by the primary structure, hence it forms a counter cone like a cellular structure. (2) Austenite grain growth mainly occurs at above 1200°C. (3) The ferrite grain precipitates at the austenite grain boundary. So, it can be said that the weld deposit metal is the assembly of a counter cone surrounded by the ferrite grains. (4) When the mass of the deposit metal is large, it exhibits mechanical anisotropy in the Charpy inpact test.
The authors have investigated the penetration mechanism of the base metal in TIG arc welding process in D.C.S.P. When the arc is fixed perpendicular to the base metal, three typical shapes of penetration, i.e. Simple, Central and Peripheral Penetration Types, are observed as shown in Photo.1. The mechanism of the first type can be clearly explained by the theory of heat conduction. The second type appears under heavy current. The strong plasma stream which becomes remarkable under heavy current digs deeply the central part of the molten pool, and consequently makes the penetration deep at the center. The last type is observed when the arc is held long and continued for a considerably long time. The formation of this type may be explained by the internal convection in the molten pool. As the surface temperature at the center of the molten pool is higher than that at the peripheral zone, a surface flow the center to the peripheral zone will be generated by the difference in surface tension. Thus the peripheral zone is melted effectively. The phenomena of the surface flow and the internal convection were examined with paraffin models. In the case when the arc is traveling, the relations between the welding conditions and the penetration of bead are as follows; i) Generally, the shape of the penetration becomes wide and shallow when the vertex angle of the cathode is decreased under small current and slow traveling speed. Under heavy current, the depth of penetration increases as the vertex angle becomes small. This is due to the strong plasma stream in the neighbourhood of the cathode with sharp vertex angle. ii) Photo 4. shows the relation of the bead form when the welding speed is increased. At the higher speed, an exposure of solid surface is observed at the bottom of molten pool due to the driving force of arc plasma.
It is well-known fact that cutting oxygen stream greatly affects gas cutting quality, but little attempt has been made to study the stream theoretically. On the other hand, the problems of jet stream have been studied fairly well in the gas dynamics fields. In this paper, Prandtl-Myer theory is introduced to study the phenomena of the stream at nozzle exit and some values of characteristics are calculated to compare with experimental results. Furthermore, cutting oxygen stream is disdussed from a standpoint of isentropic flow, and the generation of normal shock wave in the stream at high nozzle inlet pressure (P0) is estimated. To examine the results of theoretical considerations, some Schlieren photographs of the stream for various nozzle at various value of P0 were taken and also total head of the stream were measured. The results of these experiments agreed with the theoretical results, and the generation of normal shock wave in the stream was recognized very clearly at high value of P0 Some calibrations to the value of total head measurements of supersonic flow were made and satisfactory results have been obtained.