When welding galvanized steel with a high energy density beam such as provided by a laser or electron beam, poor weld quality results from splatter and porosity in the weld zone. High energy density beams for welding have become of some importance because of the speed with which welding can be conducted. The high speed is attained by the unique welding process commonly called deep penetration welding associated with a high energy density beam. In this process the energy beam vaporizes a void into the material, the molten material surrounding the void being held apart by the vapor pressure in the void. Accordingly, if at least one steel piece to be welded has a coating of zinc, which has a boiling point lower than the melting point of steel, there will be a sudden drastic vaporization when the focused beam impinges on the zinc coating. If the zinc coating is disposed at the interface between pieces of the workpiece, the drastic vaporization occurs when the void, created by the beam melting process, penetrates to the zinc coating and exposes the zinc coating to the focused beam. This disrupts the stability of the surrounding molten metal about the void resulting in weld splatter and porosity; as a result there will be an insufficient weld puddle to fill the void upon removal of the beam.
Some satisfactory means must be provided to protect or prevent the coating material, such as zinc, from undergoing rapid vaporization and disrupting the stability of the weld puddle so that the void will fill properly. The problem does not present itself in connection with other types of welding such as resistance or other welding with these latter welding types, the heat from the welding mechanism can be conducted to the coating layer slow enough to preclude the sudden vaporization that can occur when a high energy density beam is employed.
One prior art approach to solve this problem has included alloying a substance with the zinc coating to change the vapor pressure, but this has not been successful since discontinuities still appear within the weld zone.
USPTO Backgrounds
USPTO Backgrounds