In this paper, two new Cr18 series cladding materials with a thickness of 5~6 mm were clad on the 38ClMoAl substrate by laser cladding process, and the microstructure of the cladding metal was analyzed, and its wear resistance and hardness changes were measured; by comparing the spline size of the inner hole before and after the repair of the original screw, the deformation during the laser cladding process repair process was determined. The research results show that the cladding metal is well combined with the substrate, mainly composed of dendritic martensite, and hard carbides are dispersed between the dendrites; a small heat-affected zone is formed between the cladding layer and the substrate, and the molten pool liquid grows rapidly along the maximum heat dissipation direction perpendicular to the interface, forming obvious upward-growing cellular crystals and dendrites. The hardness of the 1# cladding metal is between 50~52 HRC on average, and the hardness of the 2# cladding metal is between 54~57 HRC on average; through wear comparison, it can be seen that the wear weight loss of the two cladding metals is 68% and 36% of that of 45# quenched steel, respectively. The dimensions of the screw element before and after repair were compared. The average deformation of the parts after laser repair was within 0.12 mm, which met the repair requirements.
The screw extrusion granulator is a dry granulation process that uses pressure to agglomerate solid materials. The equipment converts polymer raw materials into granules through mixing, extrusion, granulation and other processes, effectively improving and enhancing the product performance and making subsequent metering, transportation and other operations more convenient [1]. As a continuous mixing equipment, the twin-screw extruder is mainly used for plastic modification. It has developed with the development of the plastics industry [2]. The extrusion granulator consists of an extrusion system, a transmission system and a heating and cooling system. The extrusion system includes a screw, a barrel, a hopper, a die, and a mold.
The screw is the most important component of the extruder. It is directly related to the application range and productivity of the extruder. It is made of high-strength and corrosion-resistant alloy steel. The screw extruder is the core equipment in plastic forming and blending modification. In the actual blending and modification production process, the screw of the extruder is in a harsh high-pressure and high-temperature environment and is subjected to huge friction and shear forces. Due to the special working environment, the extruder screw is not the common friction between metal and metal, but between metal and polymer, so the wear of the screw surface is often serious.
The wear of the screw increases the distance between it and the barrel, affecting the compression and shearing of the material by the screw, which will lead to a decrease in product quality. On the other hand, frequent replacement of worn and failed screws not only increases costs, but also delays production plans and reduces production efficiency. Therefore, the worn screw is usually repaired rather than replaced to reduce costs and improve production efficiency.
Laser cladding technology is an advanced material surface modification technology with the advantages of low dilution rate, dense cladding layer structure, good bonding between coating and substrate, and pollution-free working environment [3~4]. It can solve the limitations of material selection, thermal stress in the process, thermal deformation, coarse material crystals, and difficulty in ensuring the bonding strength of the substrate material in traditional repair methods. Therefore, this paper verifies the feasibility of using laser cladding technology to repair screw elements through experiments.
1 Sample preparation and test method
1.1 Sample preparation
The base material used in this experiment is 38CrMoAl, with a specification of 100 mm×50 mm×20 mm. Two laser cladding materials, 1# and 2#, are used. The chemical composition of the cladding material is shown in Table 1, and the laser cladding process parameters are shown in Table 2.
1.2 Test method
The sample is sampled on the specimen by wire cutting, with a specification of 20 mm×15 mm×15 mm, and the sampling direction is from the cladding layer to the cross section of the base. The microstructure of the cladding layer is observed by a CLYMP VF-DEM optical microscope. The hardness gradient distribution of the sample is measured by a HV-3000 microhardness tester. The room temperature wear resistance test was carried out using an ML-10 wear tester with a test load of 3 kg, a rotation speed of 120 r/min, and a wear time of 10 min. The test results were compared with the quenched (51.2 HRC) sample of 45# steel. The microstructure and composition changes of the cladding layer after use were observed using an S-3400N scanning electron microscope.
2 Test results and analysis
2.1 Microstructure of cladding metal
Figure 1 shows the microstructure of the cladding layer and the base material under different magnifications of an optical microscope. Figure 1(a) shows the microstructure of the 1# pattern, and Figure 1(b) shows the microstructure of the 2# pattern. The microstructure of the cladding layer under different passes can be clearly seen from the figure. The light-colored part is the cladding layer, and the dark-colored part is the base material. It can be seen at the interface that a relatively tight metallurgical bond is formed, with a thin transition zone in the middle, and the size of the transition zone is about 5 μm. This is because during the laser cladding process, the temperature of the base material is low and the temperature of the molten pool is high, forming a huge temperature gradient perpendicular to the interface between the molten pool and the substrate. The molten pool liquid grows rapidly along the maximum heat dissipation direction perpendicular to the interface, forming obvious dendrites.
At the same time, with the increase of alloying elements such as B and W in the cladding metal, its structure has also changed. Fine hard phase compounds are evenly dispersed around the dendrite-like martensite, playing a role of dispersion strengthening.
2.2 Cladding metal composition
Figure 2(a) shows the line scanning area division of the specimen from the cladding layer to the substrate. Figure 2(b) shows the content changes of each element with different positions.
From the change of the Fe content, the transition layer is thin and the dilution rate of the cladding layer is very low, indicating that the laser cladding process helps to control the dilution rate. The content of Cr and Ni elements does not change much, and the element burnout is small.
2.3 Hardness test
Tables 3 and 4 show the surface hardness distribution of the two cladding metals. Three ranges were selected, and 5 points were taken in each range to calculate the average value. According to the statistical results, the surface hardness of the 1# cladding metal is between HRC50~52, and the surface hardness of the 2# specimen is between HRC54~57. The hardness variation range of the two cladding metals is not much different, indicating that laser cladding has little effect on the fluctuation of hardness. The hardness of the 2# specimen is higher than that of the 1# specimen, indicating that the hard reinforcement phase on the internal surface of the 2# specimen has increased, and the wear resistance can also be improved.
2.4 Wear resistance test
The test was carried out on an ML-10 disc pin abrasive wear tester. The sample size is Φ6×25 mm, the test load is 3 kg, the corundum sandpaper is 20#, the rotation speed is 120 r/min, the wear time is 10 min, and the 45# steel quenching (51.2 HRC) sample is used as the standard for comparison. Among them, 1# and 2# are cladding metal samples, and 3# is a 45# steel quenching sample. The wear test data is shown in Table 5.
From Table 5, it can be seen that under the same wear conditions, the average weight loss ratio of the 1# sample is 2.063 9%, and the average weight loss ratio of the 2# sample is 1.097 3%, which is 68% and 36% of the weight loss of the 45 steel quenching sample. At the same time, the wear resistance of the 2# cladding metal material is higher than that of the 1# specimen, indicating that these two new wear-resistant materials have good wear resistance.
3. Repair of spiral elements
The spiral elements to be repaired were selected for verification test (wear loss was selected as ≤ 4 mm), and 1# and 2# laser cladding powders were used for repair, and flaw detection and size detection were performed. The results are shown in Table 5 and Figure 3.
After testing and analysis, the screw elements made with the above process and materials had no cracks after PT flaw detection, and the size change of the inner hole spline was basically within 0.12 mm (Table 6), which met the original process design requirements. Therefore, the process method of using laser cladding to repair the original screw is feasible.
4 Conclusion
(1) The cladding metal is well bonded to the substrate, mainly composed of dendritic martensite, with hard carbides dispersed between the dendrites.
(2) A fine 5 um heat-affected zone is formed between the cladding layer and the substrate. The molten pool liquid grows rapidly along the maximum heat dissipation direction perpendicular to the interface, forming obvious upward-growing cellular crystals and dendrites.
(3) The hardness of the 1# cladding metal is between 50 and 52 HRC on average, and the hardness of the 2# cladding metal is between 54 and 57 HRC on average. The wear weight loss is 68% and 36% of that of 45# quenched steel.
(4) The finished parts passed the test welding, and the average deformation of the parts was within 0.12 mm, which met the tolerance requirements.
