laser-cladding

Cladding VS Welding Overlay

Compared with the traditional surface cladding methods, laser cladding can make cladding layer Microstructure even and beautiful. The thermal deformation of the workpiece small, the heat-affected zone of base metal small, and the dilution of the cladding layer small, more conducive to improving the material surface fatigue resistance, wear resistance, and corrosion resistance, has attracted extensive attention.

The FE-based high chromium wear-resistant alloys WF372 and D667 are compared by laser cladding and laser surfacing, respectively.

1. Experiment

D667 surfacing electrode of 4.0 is selected, AX7-500 DC ARC welding generator is selected, surfacing welding in flat welding direction, sequence welding method, cooling after welding. Welding current control in 200A or so is not determined.

The alloy composition (WT%) : WF372 is C3.3 ~ 4.3% , CR23 ~ 27% , B1.0 ~ 2.0% , SI1.0 ~ 2.0% , NI4.0 ~ 6.0% , the others are iron

D 667 is divided into c 2.5ー5.0% , CR 25ー32% , SI 1.0ー4.8% , ni 3.0ー5.0% , Mn ≤8.0, the others are iron.

The laser cladding alloy is WF372 alloy powder, and the pre-powder method is used for laser cladding. The preset powder thickness is 1mm. The maximum output power of the laser is 2.0 kw, and the usable output power is 1.8 kw — multimode output. The molten pool is maintained by Ar gas blowing pressure. The diameter of the spot is 4mm; the lap ratio is 30%, the scanning speed is 4mm / s; the width of the cladding layer is about 20mm.

The deformation and surface forming of the specimen was examined, and metallographic Analysis was carried out. Application of Buchler III type Vickers microhardness tester was made in the USA to measure the depth distribution of micro-hardness of the cladding layer.

The laser cladding specimens were annealed at 450 °C for 1.5 hours and then compared with the surfacing samples. MM200 wear test machine is used in the wear test. The speed of the grinding wheel is 200 RPM, the working pressure is 50N, and the friction pair is cooled by dropping mechanical oil. Each specimen worked for 8 hours. Weighing by electronic balance (standard deviation 0.001 G). The difference in weight before and after the experiment is the weight lost by grinding. The wear surface was analyzed by Quanta 200 environmental scanning electron microscope (SEM).

2. Results and Analysis

2.1 contrast of laser cladding layer and surfacing layer surface forming and deformation of the specimen

The thickness of the laser cladding layer is 0.9 mm. The depth of the surfacing layer is 2.8 mm. The uniform warpage of the surfacing samples is 0.858 ~ 10-2 mm / MM2. The laser cladding specimens show no visible deformation and warpage, and the 0.03 mm thick plug ruler can hardly be inserted into the base surface of the samples. The surface of the laser cladding layer is flat and smooth. The groove between the two passes is very shallow, which is due to the high lap ratio of 30%. The welding ripple of the surfacing layer is visible, and there are small pits in some directions, and the groove between the two welding passes is deep.

2.2 comparative analysis of Microstructure of cladding layer

The Microstructure of the surfacing layer is dendrite carbide with CR distributed on the pearlite Matrix. The primary dendrite rod is more continuous, the other small pieces are secondary carbide, the distribution of carbide is not uniform, and the central Dendrite rod near the fusion line is visible. Most of the Microstructure of the laser cladding layer is non-equilibrium and sub-crystallization. i. e. non-equilibrium austenite and carbide eutectic of M7C3 alloy with a high content of alloy elements. and the Microstructure is uniform and excellent. The solid phases, such as carbides, are distributed uniformly, most of which are dispersed in fine grains, and even a few of which are in the form of lamellar Dendrite are in the way of beautiful uniform and cellular crystals.

The cladding layer of surfacing welding and laser cladding forms a strong metallurgical bond with the substrate. However, the influence of arc welding on the melting of the substrate is more significant than that of laser cladding. The heat-affected zone (Haz) of surfacing is more significant than that of laser cladding. The dilution rate of the cladding layer to the substrate is less than that of the ARC surfacing. This is because in the laser cladding, high-energy laser beam scanning substrate to its surface after heating and melting away, substrate surface only a thin layer of melting. However, ARC surfacing requires a large amount of energy output to melt the surface of the substrate to form a molten pool, which has a significant effect on the substrate.

The laser cladding technology makes the cladding layer alloy fully melt in an instant. Still, only a skinny layer can be dissolved on the surface of the base material, which not only ensures excellent metallurgical bonding but also changes the alloy composition without too much dilution. And the high cooling rate of 106 ~ 108K / s leads to rapid condensation, forming a beautiful and uniform non-equilibrium, sub-crystalline dendritic eutectic structure. The refractory phase is metastable austenite with high supersaturation content of alloy elements. And the strengthening period is metastable carbide with high hardness and fine Microstructure. This kind of structure shape is far superior to the Microstructure of the surfacing layer, which has an effect on the function. Therefore, the mechanical function and application function of the wear-resistant layer can be improved and advanced by selecting the laser cladding technology.

2.3 Comparative Analysis of microhardness of cladding layer

The hardness of the laser cladding layer is 300HV higher than that of the surfacing layer. At the fusion line, the microhardness of both samples decreased obviously. But the microhardness of the substrate was lower. This is because the Microstructure of the laser cladding layer is non-equilibrium-austenite and carbide eutectic of M7C3 alloy with a high content of alloy elements. And the Microstructure is uniform and beautiful. The solid phases such as carbides are distributed uniformly, and most of the hard periods are dispersed in fine grains. The Microstructure of the surfacing layer is dendrite carbides and small secondary carbides with CR distributed on the pearlite Matrix. The distribution of carbides is uneven, and the grains are coarse. Under the action of-austenite and high hardness eutectic carbide, the microhardness of the laser cladding layer is higher than that of the surfacing layer.

2.4 wear contrast test of the cladding layer

Under the same wear condition and wear time, the wear loss weight of laser cladding tempering wear specimen is smaller than that of surfacing wear specimen. The laser cladding layer after tempering is more wear-resistant than the surfacing layer.

The results show that the internal stress, Brittleness, hardness, and cracking resistance of the laser cladding layer decrease after tempering treatment. The heat treatment process of tempering and heat preservation at 450 °C for 1.5 hours makes the laser cladding layer reach the better condition of strong Resistance. At this time, the Rockwell hardness of the laser cladding layer is HRC61, which is much higher than that of hardfacing layer HRC54 and higher than that of grinding wheel HRC60. It can be seen that the higher the hardness of the material, the better the wear resistance. Wear Resistance of laser cladding layer after tempering.

2.5 Microcosmic contrast analysis of wear surface

The wear marks of the laser cladding tempering specimens show small pits and irregular dendrites and grains. However, there are many long and thin grooves in the wear marks of the surfacing welding specimens. These grooves have obvious directionality, which is the same as the conflict direction. With a high degree of compliance. Together, there are numerous small holes. The laser cladding substrate has good cooperation of strength endurance, and there is almost no micro-furrow, micro-cutting, or micro-crack. However, the surfacing specimens have high flash tolerance, and there are many micro-furrow and micro-cutting drawings, but there is no micro-crack flash. The Abrasion marks are related to the original structure and function of the specimen.

The Microstructure of the laser cladding layer is thin Dendrite and cellular crystal, while that of the surfacing layer is thick Dendrite. The Microstructure of the laser cladding layer is austenite, which has higher hardness than the pearlite in the surfacing layer. After tempering, the AUSTENITE’s endurance is strengthened. Therefore, under the action of Matrix and hard carbide, the wear trace of laser cladding appears irregular dendritic and granular, and the wear trace of surfacing welding sample shows many long and thin grooves and holes. The pearlite is soft and has strong adaptability. The surfacing specimens from many deep and shallow ditches, which stick together with the conflicting direction and have considerable length. The remaining area of the supporting Matrix is less, and more of it is worn away. In the laser cladding specimens, the remaining area of the supporting matrix structure is higher, and the worn-out structure is mainly the soft structure, which was originally distributed in the fine dendrite vacant land, many highly resistant tissues provide wear-resistant support.

Through the investigation and Analysis of Esem, the micrograph of wear surface of the sample flash, and the laser cladding layer is more wear-resistant than the surfacing layer after heat preservation at 450 °C for 1.5 H.

3. The verdict

1. The laser cladding layer has more ambition than the surfacing layer. There was no visible distortion and warpage in the laser cladding specimen.

2. Compared with the Microstructure of the surfacing layer, most of the Microstructure of the laser cladding layer is non-equilibrium and sub-crystallization and has the characteristics of two-phase Microstructure. The Microstructure of the cladding layer is fine. Then the mechanical function and the application function of the machine are improved and advanced.

3. The microhardness of the laser cladding layer is 300HV higher than that of the surfacing layer. This is mainly determined by the microstructure function and shape of the alloy in the cladding layer.

4. After the laser cladding layer was tempered at 450 °C for 1.5 h, the strong endurance reached a better cooperation condition. Under the same wear condition and wear time, the wear loss weight of laser cladding wear specimen is 50% less than that of surfacing wear specimen. The wear surface was analyzed by SEM. The laser cladding layer WF372 after tempering has better wear resistance than the surfacing layer D667.