Improving the adhesion resistance of injection molding machineextruder screw to
2018-09-12 10:13:04
2 experimental analysis
2.1 microhardness analysis of alloying layer
The improvement of surface hardness is beneficial to improve the wear resistance of materials. Fig. 1 is the microhardness distribution curve of 40Cr treated by laser alloying. The technological parameters are as follows: the thickness of pre-coated alloy powder is 0.4mm, the laser power is 2kW, the laser scanning speed is 300mm/min, and the single channel scanning. Fig. 1 shows that the hardness of the laser treated sample reaches HV876, and the microhardness of the treated layer decreases gradiently from the surface to the inside, which indicates that the microhardness of the laser alloyed sample increases greatly.
The martensite with laser heat treatment has higher dislocation. Due to the improvement of the hardness of martensite itself, the refinement of martensite and the high dislocation density, the martensite after laser transformation has higher hardness.
On the premise that the laser energy density is enough to melt the surface, the hardness of the alloyed layer and the HAZ increases obviously with the increase of laser scanning speed. When the scanning speed increases to a certain value, the surface temperature is not enough to melt the material, the surface does not produce high hardness dendrite, and the heat affected zone temperature is still enough to ensure the transformation hardening.
2.2 wear resistance test of alloying layer
The wear resistance of materials can be evaluated by relative wear resistance method, which can avoid the systematic error caused by parameter change and measurement error in wear process to a certain extent, and can evaluate the wear performance of materials more scientifically and accurately. The mass changes before and after wear were measured with an electronic balance with accuracy of one-thousandth of a million. The wear amount was measured accurately. The samples were cleaned with acetone before each weighing.
In order to analyze the wear resistance of laser alloyed screw, a comparative test of disc-pin wear was carried out. The standard specimen was taken from the untreated part and the test sample was taken from the laser alloyed part. The results of wear comparison test are shown in Table 2.
Table 2 shows that the relative wear resistance of laser alloyed screw is 0.449, that is, the wear resistance is 2.2 times higher than that of nitriding treatment, thus prolonging its service life.
It is observed that some hard phases such as WC and MC exist on the wear surface after laser alloying, and the plough wrinkle is narrow. Ceramic phase WC is the main wear resistant phase, which contributes greatly to the wear resistance of the alloyed layer. When plastic deformation occurs, Co can induce the transformation from FCC to densely packed hexagonal martensite. The densely packed hexagonal martensite has lower friction coefficient and adhesion tendency, which is beneficial to improve adhesion wear and the reaction is uniform and delicate on the grinding surface. The grinding surface of matrix metal is broad and ploughing.
In addition to extrusion to the edge of the groove, a considerable part of the groove was dragged down, and even a large scale of undeveloped wrinkles were found, indicating that a large elastic-plastic deformation occurred on the secondary surface during the wear process.
3 conclusion
(1) The main failure modes of 40Cr are adhesive wear and corrosion wear. Laser alloying is an effective method to improve the surface quality and wear resistance of screw.
(2) Pre-coating method is used to add alloy materials, and the thickness of pre-coating has a great influence on the quality of laser alloying. When the precoating thickness is 0.4mm, the surface hardness reaches up to HV876, and the laser alloying effect is the best.
(3) Using C2 ultrafine alloy powder (1-5 micron), Al ultrafine alloy powder (1-muxm) and carbon nanotube mixed alloy (mass ratio 1:1:1) as alloy materials, the alloying elements in the alloying layer can be highly dispersed and uniformly distributed. By using the optimized laser processing parameters, excellent surface properties of the workpiece were obtained. The wear resistance of alloyed layer increased by about 2.2 times. Field test results show that the service life of the screw strengthened by laser alloying has been increased by at least two times.
