This welding method can be applied for bonding not only between lumber and lumber, or lumber and plastics, but also between lumber and ceramic such as bricks, etc., or lumber and graphite, and between many other non-metallic substances. In this welding, after the pretreatment of the surfaces to be welded has been performed, welding is effected by use of a filler rod and a torch just in a similar manner to hot jet welding of plastics. As the filler rod, plastics is used, and as the welding gas, the air is utilized. As to the size of the beveling form of the weld, it is preferable to apply the regular size for an ordinary weld, but in practical usage, unevenness in the bonding surfaces would scarcely be an obstruction. In a broad sense, the plastic welding may be classified under the category of welding, but in a narrow sense, in as much as this welding method is not accompanied by the melting of lumber, etc., it may not belong to the category of welding. Presuming that the welding rod is a solder, this method may come under the category of brazing, while its operation can hardly be called brazing because it is virtually a kind of welding. For this reason, the present method is tentatively called plastic welding.
The mechanism of the plastic welding of lumber is considered as follows: Lumber is composed of cells. Since lumber is thus constituted with numerous cells having thick cell-walls, the vacuoles which are termed the lumens are formed between one cell and another. Now, in the pretreatment process, the pretreating agent is caused to penetrate sufficiently into the lumens, whereby an electrostatic bonding force, along with the mechanical anchor effect, is generated in relation to lumber, and thus hot jet welding is effected on the surfaces which have been pretreated. In this manner, by the pretreatment which involves the elimination of various stains, and the activation of the surface to be welded, the lumber surface becomes so much activated as to greatly facilitate the hot jet welding to be carried out. These effects may vary depending upon the condition of formation of lumens which are different in composition from one lumber origin to another. On the other hand, these effects differ in their specific adhesion in accordance with the classification of lumber.
It may be concluded: 1) The beveling area is the larger the better. 2) The moisture content has been measured to be 11.0-13.0 in pine wood, 9.8-13.0 in cryptomeria wood, 10.0-12.5 in hinoki wood, and 10.0-30.0 in lauan wood, and 9.0-13.0 in plywood. This necessitates a consideration for promoting the filler effect of the primer against the lumens by pre-drying. The drying condition of 150°C for one hour seems to bring about a good effect. 3) The transition phenomenon of the welding strength due to temperature and atmosphere resembles that of the primer and the welding rod. The more the properties of the bulking agents of both primer and welding rod are brougth forth, the stronger the weld will be. 4) The lumens are present in the direction of trunk axis, while the effect of the bulking agent is stronger in the axial direction. 5) The fatigue strength also resembles that of the bulking, where the quality of a bond largely depends upon the quality of the bulking agent. 6) According to the microphotographic observation, the condition of impregnation of the primer and the welding rod is good, and it has been made clear that sufficient keying action is present. 7) A welding joint efficiency of higher than 60% may easily be expected. A new bonding method for lumber has been developed, and it is considered that the initial objective has been achieved for the present.

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As regards the adhesion of metals with high polymer adhesives, the ADHESION THEORY seems to be most pertinent. This theory explains that adhesion is effected by the van der Waals force working between the adhesive agent and the base metal. Now, the van der Waals force refers to the aggregate of the interaction forces due to the dispersion effect, image induction effect and orientation effect. The present study was undertaken for the purpose of proving the characteristics of the adhesive force due to the image induction force.
The commonly used high polymer adhesives are polar, with their molecules having dipoles. Each of these dipoles on the metal surface points to various directions. If it is assumed that the image induction effect between these dipoles and base metal has something to do with the adhesive force, it may then be supposed that, when adhesion is performed with all these dipoles orientated in one direction, a substantial difference may be observed in the adhesive force, from the case when the dipoles are not orientated. Now, in order to accelerate the degree of the orientation of dipoles, adhesion was carried out with the electric field kept acting on the bonded part and the influence of the electric field on the adhesive strength was examined.
The results are summarized as follows:
(1) It became evident that the adhesive strength was influenced by the D.C. electric field. It is supposed that these phenomena are due to the change of interaction force, mainly image induction force, between the dipoles of adhesive and metal.
(2) The influence of A.C. electric field on the adhesive strength was not observed.

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Using three series of electrode wires, mild steels were welded in nitrogen and air atmospheres.
The effects of titanium, zirconium and aluminum in the electrode wires on the nitrogen contents and porosities of weld metals were systematically studied.
The results are summarized as follows:
(1) The nitrogen contents of weld metals decrease with an increase of welding current in both atmospheres.
(2) In the welding in a nitrogen atmosphere, the nitrogen contents of weld metals increase with increasing titanium and zirconium contents in electrode wires until certain amounts. Aluminum has no significant influence.
(3) In the low current welding in an air atmosphere, the nitrogen contents of weld metals decrease with a small addition of titanium, zirconium or aluminum to electrode wires.
(4) Among these three elements, zirconium is.the most effective to eliminate the porosity of weld metals in the welding in a nitrogen atmosphere, and aluminum is the least so.
(5) In the welding in an air atmosphere, a separate alloying of the electrode wire with these elements is not so effective to eliminate the porosity of weld metals as in a nitrogen atmosphere.

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In spot welding, tensile shear strength will increase exponentially as time goes on in the condition that weld time is set longer and will eventually saturate.
However, tensile strength reaches a maximum point at a certain weld time and it goes down abruptly after that time. Seeing that tensile shear strength increases as weld time goes on, we may consider that corona bond increases little by little, but the nugget itself will never grow.
Corona bond increases tensile shear strength, but not tensile strength.
As we reported in First Report, columnar structure (substructureless) is generated from the external boundary of nugget if weld time is set longer and the boundary becomes brittle and accordingly tensile strength goes down.

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