The interaction between the solder and the base metal is mainly wetting, which refers to the spread of liquid on solid surface.From the perspective of chemical thermodynamics, it refers to the process in which the solid-liquid interface replaces the solid-gas interface, thus reducing the free energy of the system.In other words, when the liquid filler metal comes into contact with the solid base metal, the filler metal displaces the gas at the base metal surface and spreads along the base metal surface to form a new solid-liquid interface.The liquid solder must be able to wetting the base metal well in order to fill the capillary gap of the brazing joint.

In the actual brazing process, as long as the filler metal wets the base metal, the liquid filler metal and the filler metal more or less dissolve and diffuse each other, resulting in the variation of the composition, density, viscosity and melting temperature range of the liquid filler metal.These changes will affect the wetting and capillary filling of liquid needle material during brazing.When the composition of the filler metal and the base metal changes, the wettability also changes and the interaction changes, that is, the wettability is not the same.

The heating temperature has an effect on wettability.Generally speaking, the higher the heating temperature, the smaller the wetting Angle, so that the wettability is improved, the better the wetting effect.However, the higher the heating temperature, the better.If the temperature is too high, it may cause the grain size of the parent material to grow too much, resulting in overheating, overburning and other problems.Moreover, when the spreading capability of the solder is too strong, the wettability of the solder is too good, which tends to cause excessive loss of the solder and not easy to fill the brazing joints, that is, the solder will flow to the place where brazing is not needed.At the same time, too high temperature will cause welding defects such as melting corrosion.The brazing temperature is too low, the viscosity of the solder increases, the fluidity becomes poor, the wettability is too weak, leading to the insufficient flow and penetration of the solder, making the gap of the joint not easy to be filled, thus forming a bad joint.Usually take the solder liquid line above 20~40℃, or take the solder melting point of 1.05~1.15 times.

In brazing, brazing temperature and holding time cannot be determined in isolation, but there is a certain complementary relationship between them to match each other.When the brazing temperature is high, the holding time can be shorter.When the brazing temperature is low, the holding time can be longer.This ensures that the solder can fully interact with the base metal and does not dissolve the base metal too much.