Cracks can be divided into longitudinal cracks, transverse cracks, large mouth cracks, arc pit cracks and heat-affected zone cracks according to the different parts where they are produced.
According to the difference of temperature and time of crack generation, it can be divided into two kinds of hot cracks and cold cracks.
The main reasons for cracks are the following:
(1) spray welding alloy hardness is high, poor crack resistance
Spray welding alloys such as containing high amounts of carbon, silicon and boron, they form alloy compounds with other alloying elements. These compounds have high hardness, poor toughness, and the tendency to produce cracks. For the rigidity of the workpiece, such as not to take effective process measures, it is very easy to cause cracks.
(2) The rigidity of the workpiece is large
If the difference between the expansion coefficient of the alloy and the expansion coefficient of the base material is too large, for the rigid workpiece, when the liquid alloy solidifies, the resulting tensile stress and deformation increases, exceeding the strength limit of Taikin and generating cracks.
(3) The workpiece spray welding surface has defects
Workpiece spray welding surface with cracks, notches, casting trachoma and other defects, the weld channel in the defect is easy to form stress concentration and produce cracks.
(4) The width-thickness ratio of the weld channel is small, and the weld channel itself is poor crack resistance.
(5) Improper selection of process specifications
Due to improper selection of process specifications, spray welding layer and workpiece fusion table is not good, poor forming, or spray welding layer has a lack of meat, slag and other defects, easy to produce cracks.
(6) In the pick-up part of the current decay too fast
In the weld lap, such as current decay too fast, the weld channel rapid heat and cold, liquid metal rapid contraction, easy to cause large mouth cracks or arc pit cracks.
(7) Spray welding process affected by cold airflow spray welding process such as cold airflow is too large (so-called “through the wind”), so that the weld cooling rate is too fast, easy to produce cracks.
(8) Preheating before welding and welding after the slow cooling measures are not appropriate.
According to the various reasons for the above sub-fold, the main measures to prevent the plasma spray welding machine cracks are:.
Improve the crack resistance of alloy powder materials.
Eliminate defects on the spray welding surface of the workpiece, and from the rough spray welding surface structure design as far as possible to improve the rigidity, such as leaving tabs, deformation grooves, etc.
Reasonably decide the welding channel forming size, and control the process specification, so that the formation of smooth and smooth, to eliminate quality defects.
According to the structural rigidity of the workpiece and spray welding materials, take appropriate preheating before welding, slow cooling measures after welding, if necessary, also take spray welding process insulation and post-welding heat treatment measures.