Application of Plasma Cladding Ni60A+35wc Wear-resistant Layer on Agricultural Machinery Knives（3））

       1. Experimental method: First use dilute hydrochloric acid to remove the rust on the surface of the sample, then boil the sample 5 min with NaOH lye at 70 80℃ to remove the oil on the surface of the sample, and finally rinse the sample with distilled water and use
The fan blows dry.
The alloy powder is adjusted into a paste with water glass, and the prepared cladding alloy powder is uniformly coated on the surface of the sample after degreasing and rust removal by the preset method, and the coating thickness is about 1 mm.
Place the coated sample in an oven, slowly heat from room temperature to 200°C and keep it for 3 hours to dry.
The cladding is performed under a plasma arc heat source with a current of 90 A and a voltage of 18 V. In order to ensure the stable arc, high melting efficiency, deep and narrow melting layer, and small shrinkage stress and strain of the workpiece, the plasma arc adopts direct current connection for lap welding.
; In order to avoid cracks in the cladding layer due to the excessive cooling rate, after the cladding, put the workpiece in the incubator and slowly cool it to room temperature.
Use a surface grinder to polish the cladding layer of the sample to Ra 1.6 umAfter sample preparation, observe the bonding state and microstructure of the cladding layer interface with a metallurgical microscope. Measure the micro hardness and Rockwell hardness of the cladding layer with HD 9245 hardness tester. The cladding layer abrasion test was performed on the M PX 2200 disc-pin abrasion tester. In order to investigate the wear resistance of the cladding layer relative to the current commonly used agricultural machinery tool material 65 M n, 65 M n steel quenched at 830°C and tempered at 540°C was selected for the pin. During the test, the pin was pressed on the sample with a load of 50 g, and the non-lubricated dry friction and wear test was carried out at a speed of 1120 rpm. Weigh with T G328A photoelectric analytical balance (with an accuracy of 0.1 mg) every 2 hours. Before weighing, rinse the sample with clean water, then rinse with anhydrous alcohol, finally rinse with ether, and blow dry with a hair dryer. Record all test data and calculate the cumulative wear and average wear. The test is carried out for a total of 12 hours.
2. Test results and discussion: Analysis of the bonding state of the cladding layer and the substrate. The metallographic diagram of the bonding state of the cladding layer and the substrate. It can be seen that the grain boundary between the surface of the substrate, the fusion zone and the cladding layer is continuous and the bonding is good. This shows that the bond between the cladding layer and the substrate is a metallurgical bond. This kind of metallurgical bonding is due to the low melting point of Ni60A alloy, good self-fusion, and the plasma arc heat source is sufficient to melt the coating and the substrate, thereby forming a metallurgical bonded cladding layer.

3. Analysis of the structure of the cladding layer
It is the metallographic diagram of the surface, middle and bottom of the cladding layer (the part near the interface).
It can be seen that the coating structure is mainly composed of dendrites whose growth direction is from the bottom to the top of the molten pool, and the order from the interface to the surface of the cladding layer is: coarse dendrites, finer dendrites and fine dendrites that grow in a certain direction
Of non-directional grains.
The structure of the cladding layer is mainly due to the extremely high temperature gradient and solidification rate in the molten pool.
Due to the large temperature gradient, the surface zone and the fusion zone will crystallize first and expand to the middle.Because the surface zone and the fusion zone cool faster, the growth rate of dendrites is also faster, while the cooling rate of the middle zone is slower, and the microstructure is composed of branches.
The crystal shape gradually changed to a cell shape.
Coarse dendrites grow against the direction of heat flow, while small dendrites have no obvious directionality and are much smaller in length than large dendrites. These dendrites are mainly distributed in the middle of the cladding layer.
The outermost surface of the cladding layer is a thin layer of crystal grains with irregular shapes, and the structure is relatively dense.