In view of the poor surface quality of Fe45 alloy remanufactured by laser cladding additive manufacturing, which cannot meet the functional and assembly requirements of precision mechanical parts, the laser cladding layer is milled and processed. This paper studies the influence of milling cutting process on milling force, surface roughness and micromorphology, and uses orthogonal test method to carry out milling cutting test. The influence of spindle speed S, feed speed F and cutting depth ap on milling force and surface roughness of cladding molded parts is evaluated by variance and range analysis methods; the influence of milling cutting process on surface morphology and chips is analyzed from a microscopic perspective. The results show that the milling cutting process parameters have a great influence on the surface quality of the molded surface, among which the milling depth has the greatest influence on the milling force, and the feed speed has the most significant influence on the surface roughness; after milling processing, the surface roughness Ra of Fe45 laser additive molded parts can be reduced from 13.68 μm to 1.7 μm, which is reduced by 87.6%. It can be seen from the experiment that the milling reduction process can greatly improve the surface quality of Fe45 alloy remanufactured by laser cladding additive manufacturing, which has guiding significance for the mechanical processing of laser cladding coatings.
Laser cladding technology is a layer-by-layer cumulative forming method that forms a high-performance alloy coating on the surface of the mechanical structure, realizing the surface strengthening and failure repair of high-end mechanical parts. It has the characteristics of efficient and flexible manufacturing, and can meet the direct forming of complex structural parts. With the continuous maturity of laser cladding technology, it has been gradually applied to aerospace, weapons and equipment, and coal chemical industry.
However, there is still a certain gap between the surface of metal parts laser cladding additive forming and the surface of traditional mechanical processing, which is mainly reflected in the surface roughness and geometric accuracy that are difficult to meet the use requirements. Therefore, it is necessary to perform subtractive post-processing on the laser additive cladding coating to improve the surface quality. Traditional mechanical subtractive processing methods mainly include turning and milling. Li S et al. studied the turning performance of nickel-based alloy cladding layer. The results showed that the machinability of the cladding layer gradually deteriorated with the increase of cutting depth, and the cutting vibration had a great influence on the surface roughness. Böß V et al. used laser cladding and ball-end milling technology to repair nickel-based alloy parts and studied the influence of uneven substrate and cladding shape on milling force and final part surface quality. Zhao YH et al. used milling force time domain and frequency domain signal analysis methods and processing vibration signal analysis methods to study the milling characteristics of laser cladding layers and analyzed the influence of cutting parameters and microhardness changes on chip morphology. Shu L S et al. studied the dry cutting performance of nickel-based laser cladding alloy coatings. The results showed that the magnitude of milling force during dry milling is closely related to grain structure and cutting temperature. Zhao YH et al. [7,9] analyzed the chip morphology and processing vibration during end milling and side milling of laser cladding layers. Wang Qingqing conducted a systematic study on the formation and evolution mechanism of microstructure during TC4 cutting. Zhang Yuanjie et al. compared the surface roughness and residual stress of laser selective melting additive molded parts before and after milling. The results showed that the surface roughness of the additive parts decreased from 10 μm to 1 μm after milling, and the surface residual stress was compressive stress. Bai Haiqing et al. conducted a small hole drilling test on laser cladding parts. Hua Y et al. studied the influence of process parameters on surface roughness during dry turning deformation of IN718. The study showed that cutting speed and fillet radius are the main factors affecting surface roughness. Polishetty A et al. milled TC4 titanium alloy additive parts and forgings, respectively, and compared the milling force and surface roughness of the two test pieces after milling. The results showed that the milling force of TC4 titanium alloy SLM molded parts is greater than that of forgings, and the surface roughness is lower. Although domestic and foreign scholars have begun to study the laser cladding subtractive manufacturing process, there is little research on the mechanism of laser cladding iron-based alloy powder milling subtractive process to improve the surface quality of additive parts, and there is still a lack of theoretical research on the mapping relationship between the molding process parameters and the processing surface quality.
Iron-based alloy powder has the characteristics of strong self-fluxing and good welding performance. The cladding coating prepared by it has high hardness and good wear resistance, and has a very broad application prospect in the field of surface strengthening of mechanical parts. Therefore, this paper uses Fe45 alloy powder as raw material, adopts an annular coaxial synchronous powder feeding method to prepare Fe45 laser cladding parts, and designs orthogonal milling experiments to analyze the influence of milling process parameters on milling force, processing surface roughness, surface morphology and chip morphology, and studies the mechanism of milling subtractive process on the surface quality of Fe45 laser cladding parts.
1 Experimental principle and scheme
1.1 Preparation of laser cladding additive specimens
The preparation of laser cladding parts adopts circular coaxial synchronous powder feeding. The cladding substrate is a 40Cr steel square specimen with a size of 100 mm×80 mm×30 mm. The specimen is milled before cladding to remove impurities such as surface oxide layer. The cladding powder is Fe45 alloy powder with a diameter of 42-128 μm. Under the ultra-depth microscope, the particle surface is relatively smooth and the shape is approximately regular spherical. There is no adhesion between particles and the powder fluidity is good, as shown in Figure 1.
Before the test, the powder is dried to prevent the powder from sticking to each other and affecting the final forming quality. The chemical composition of Fe45 alloy powder is shown in Table 1. The protective gas in the preparation of laser cladding forming specimens uses 99.99% pure industrial argon. The cladding process parameters are: laser power 2 400 W, scanning speed 20 mm/s, powder feeding speed 20 g/min, defocusing amount +5 mm, overlap rate 50%, powder feeding method annular coaxial synchronous powder feeding, bow-shaped scanning path, layer-by-layer accumulation to prepare 75 mm×60 mm×3.5 mm Fe45 laser cladding coating, the forming system diagram is shown in Figure 2, and the final Fe45 laser cladding molded part is shown in Figure 3. From the macroscopic analysis of Figure 3, it can be seen that the surface of the Fe45 cladding molded part is flat and crack-free, the surface roughness value Ra=13.68 μm, the cladding layers are closely connected, and a metallurgical bond is formed with the surface of the 40Cr substrate.
1.2 Milling cutting test and force measuring device
As shown in Figure 4, the milling test is based on the DMG five-axis machining center DMU50, the maximum spindle speed of the machine tool is 14,000 r/min, and the spindle drive power is 23 kW; the milling tool is Walter MC377-06.0A4-BC-WK40EA with a diameter of 6 mm and a straight shank solid carbide 4-tooth end mill; in order to reduce the milling force, milling heat and coolant during the processing process. The influence of surface quality and processing performance, this test adopts the down-milling dry milling method. The milling force signal is collected by the dynamometer
(KISTL-ER 9272A), and the sampling frequency is set to 20,000 Hz. Since the surface of the laser cladding specimen is rough, the base surface of the specimen is rough-milled before the orthogonal test to make its surface smooth.
1.3 Design of milling cutting test plan
This test studies the influence of three factors, spindle speed n, feed speed F, and back cutting depth ap, on the milling force, surface roughness, and surface morphology of 40Cr laser cladding parts. The test adopts the orthogonal test method of three factors and three levels L9(3’3). The selected factor level table is shown in Table 2, where A, B, and C represent the spindle speed n, feed speed F, and back cutting depth ap, respectively. The test milling width is 1 mm. The orthogonal test plan is shown in Table 3. After the sample is processed, the surface roughness Ra of the milled specimen is measured using a portable roughness meter TR200. Five points are selected for each group of data to measure the surface roughness, and the arithmetic mean is taken as the surface roughness of the group of processing parameters. The machined surface and chip morphology are observed and analyzed using the Keyence VHX-S750E ultra-depth of field microscope system.
2 Experimental results and analysis
2.1 Milling force analysis
2.1.1 Milling force measurement results
Milling force is one of the important indicators for evaluating material machinability. The magnitude of the milling force directly affects the tool life, workpiece machining quality and machining accuracy, and plays an important role in studying the milling mechanism; it can also indirectly evaluate the quality of the cladding layer. If the laser cladding quality is poor, the cladding layer will peel off from the base material when the milling force is too large. Therefore, the analysis of the milling force is crucial. Figure 5a is the original milling force curve. It can be seen that the milling force shows a periodic change. This is because the milling process is discontinuous, and the cutter teeth continuously cut in and out of the workpiece, causing the milling force to change periodically. Figure 5b is the processed milling force curve. It can be seen that the milling force can be divided into three stages. The first stage is from the moment the blade is involved in milling to the moment the entire tool is fully involved in milling. At this time, the radial force Fx, the main milling force Fy and the axial force Fz gradually increase. The second stage is stable milling. At this time, the milling force remains basically stable with slight fluctuations. This is because in the stable milling process, the cutter teeth continuously cut in and out of the workpiece. When the chips are peeled off from the workpiece, the milling force decreases. As the tool feeds, the new milling layer participates in milling, and the milling force increases, and the fluctuations are cyclical. The third stage is when the cutter teeth cut out the workpiece. At this time, the tool gradually separates from the workpiece, and only part of the cutter teeth participate in milling, and the milling force gradually decreases. In order to eliminate the interference of various external factors on the measurement results, the milling force measurement value takes the incremental mean value △F of the milling force. By calculating the milling force after filtering, the milling force components Fx, Fy, Fz and the milling resultant force F in Table 4 are obtained. From the table, it can be seen that the fluctuation of Fx and Fz is small, indicating that the milling parameters have little effect on the radial and axial components; the fluctuation of Fy is more obvious, indicating that the milling parameters have the most significant effect on the main milling force. This is because the component force Fy is along the feed direction during the milling process, and the workpiece always presses the tool. During the milling process, according to the changes of different milling parameters, Fy fluctuates the most.
2.1.2 Analysis of the extreme difference of milling force
The results of the orthogonal test of milling force are analyzed, and the results are shown in Table 5. From Table 5, it can be seen that the order of influence of each factor on the radial milling component force Fx is A>C>B; the order of influence on the main milling force Fy is C>A>B; the order of influence on the axial milling component force Fz is A>B>C; the order of influence on the milling resultant force F is C>A>B.
According to the results of the extreme difference analysis, the relationship between each component force and the resultant force and the milling parameters is shown in Figure 6. From the analysis of Figure 6a, it can be seen that the spindle speed has little effect on Fx and Fz, and Fx decreases with the increase of spindle speed. Although Fz also increases with the increase of spindle speed, the increase is small. Fy and F increase significantly with the increase of spindle speed, and are close to linear increase. From the analysis of Figure 6b, it can be seen that the radial milling force and the axial milling force also increase with the increase of feed speed, but the increase is very small, indicating that the feed speed has little effect on Fx and Fy. The main milling force increases with the increase of feed speed. When the feed speed increases from 120 mm/min to 150 mm/min, the milling force changes slightly. When the feed speed increases from 150 mm/min to 180 mm/min, the growth rate becomes faster, indicating that the increase of feed speed has a greater impact on the main milling force. The milling resultant force is approximately parallel to the main milling force curve. At each level, the main milling force accounts for 93.999% to 96.75% of the milling resultant force, indicating that the milling resultant force is greatly affected by the main milling force and less affected by the radial milling force and axial milling force. From the analysis of Figure 6c, it can be seen that with the increase of milling depth, the radial milling force Fx also increases, and its increase is small; when the milling depth changes within the test range, the axial milling force first increases and then decreases, but the increase and decrease are small. Therefore, the milling depth has little effect on the radial milling force Fx and the axial milling force Fz; with the increase of milling depth, the main milling force increases. When the milling depth increases from 0.1 mm to 0.5 mm, the increase of the main milling force is 37.916 N, which is 7.2 times the milling force when the milling depth is 0.1 mm. This shows that the milling depth has the most significant effect on the main milling force. This is because the greater the milling depth, the larger the chip volume that needs to be removed per unit time, and the greater the energy required; although the axial milling force has a decreasing trend, when the milling depth increases from 0.1 mm to 0.5 mm, its proportion in the milling resultant force increases from 0.1 mm to 0.5 mm. 69.41% rapidly decreases to 10.63%. Therefore, the reduction of axial milling force does not affect the increase of milling force. When the milling depth increases from 0.1 mm to 0.5 mm, the proportion of main milling force in the milling force increases rapidly from 68.99% to 99.05%. The analysis shows that the milling depth has the most significant effect on the milling force. As shown in Figure 6d, the increase of spindle speed, feed rate and milling depth will increase the milling force, but the milling force is most sensitive to the milling depth. With each increase of the milling depth, the increase rate of the milling force is also increasing. The spindle speed and feed rate have little effect on the milling force. As the factor level increases, the milling force shows a slow upward trend. As the spindle speed increases, its influence on the milling force is greater than the influence of the feed rate on the milling force. From the above analysis, it can be seen that the greatest influence on the milling force is the milling depth, followed by the spindle speed and feed rate.
2.1.3 Analysis of variance of milling force
Table 6 is the variance analysis table of milling force. From the results of variance analysis, it can be seen that the order of influence of various factors on the test index milling force is: C (milling depth)>A (spindle speed)>B (feed speed). The F value of factor C (corresponding to the ratio of factor mean square to error mean square, used for the significance test of the difference between two or more sample means), that is, FC=18.59 is close to F0.05 (2,2) = 19, so factor C has the most significant effect on the milling force. The conclusions obtained by variance analysis are consistent with those obtained by range analysis.
2.2 Analysis of surface roughness
2.2.1 Range analysis of surface roughness
The results of the orthogonal test of surface roughness were subjected to range analysis. The results are shown in Table 7. From Table 7, it can be seen that the order of influence of each factor on surface roughness is B>C>A (feed speed>milling depth>spindle speed), and the optimal milling parameter combination is A2B2C1.
According to Table 7, an intuitive diagram of the influence of each factor level on surface roughness is drawn, as shown in Figure 7. From Figure 7, it can be seen that the surface roughness first decreases and then increases with the increase of spindle speed and feed speed; the surface roughness first increases and then decreases with the increase of milling depth. When the feed speed increases from 120 mm/min to 150 mm/min, Ra decreases by 33.8%, and when the feed speed increases from 150 mm/min to 180 mm/min, Ra increases by 13.2%. The reason is that the increase in feed speed increases the tool movement distance per unit time, increases the amount of material removed, and thus increases the milling force, so the surface roughness value also increases. As the spindle speed increases, the Ra value decreases first and then increases, and the decrease is 14.9%. Overall, the Ra value tends to increase. The reason is that the increasing speed increases the action and reaction force between the machine tool and the tool, and the tool vibration is strengthened, causing the milling system to be unstable, thus forming a trend of increasing surface roughness value. As the milling depth increases, the Ra value gradually increases, because as the milling depth increases, the required milling force increases, the tool force increases, and the vibration is strengthened, so the surface roughness value increases.
2.2.2 Surface roughness variance analysis
Due to the inevitable errors in the test system, the extreme difference analysis cannot exclude the influence of the errors. Therefore, variance analysis is used to further analyze the significance of the influence of each factor on the roughness, see Table 8.
From the results of variance analysis, it can be seen that the order of influence of each factor on the surface roughness of the test index is: B (feed speed)>C (milling depth)>A (spindle speed). The F value of factor B (corresponding to the ratio of the factor mean square to the error mean square, used for the significance test of the difference between the mean values of two or more samples), that is, FB=11.19 is greater than F0.10 (2,2) = 9, so the factor feed rate has the most significant effect on surface roughness. The conclusions obtained by variance analysis are consistent with those obtained by range analysis.
2.3 Surface morphology and chip analysis
2.3.1 Surface morphology analysis
The surface morphology of the orthogonal milling test was observed by ultra-depth microscope, as shown in Figure 8. It can be seen from Figure 8a and Figure 8b that when the speed is 2 000 r/min, under different feeds and cutting depths, there is an obvious transverse equidistant texture along the feed direction. Further analysis shows that the displacement of the ridge of each uniform protrusion of the tool trajectory is equal to the feed rate per tooth in the milling parameters. As can be seen from Figure 8c, the tool trajectory becomes blurred when the speed increases. This is because when the speed increases and the feed remains unchanged, the number of milling times of the tool at the same point per unit time increases, the milling deformation decreases, and the increase in milling speed causes the chips to fly away from the workpiece surface at a higher linear speed without scratching the machined surface, and the surface roughness decreases. As can be seen from Figure 8d, when the speed increases to 3200 r/min, the surface texture is not clear, and many scratches can be seen. This is because the temperature increases during the milling process, causing the chips to stick to the tool, causing scratches on the machined surface, and the surface quality decreases, resulting in an increase in the roughness value.
Figure 9 shows the morphology observed after the surface is magnified 1 000 times at different feed speeds and milling depths at a speed of n=2 600 r/min. It can be seen that when n=2 600 r/min, F=120 mm/min, ap=0.3 mm, the high temperature generated by milling on the surface makes the surface burn less; when n=3 200 r/min, F=180 mm/min, ap=0.3 mm, the high temperature generated by milling on the surface when the feed speed increases and the milling force increases makes the surface burn more. The burning phenomenon on the surface of the part will cause the corrosion resistance of the part to decrease, and the contact fatigue performance will also decrease, resulting in a decrease in the service life of the part. Therefore, in order to obtain better surface quality, the milling parameters must be reasonably selected to increase the service life of the milled parts.
2.3.2 Chip analysis
The experimental chips were observed by ultra-depth microscope, as shown in Figure 10. As can be seen from the figure, the chip morphology under different milling parameters is basically the same. The inner surface of the chip is smooth, the outer surface is fuzzy, and there are serrations on the edge of the chip. It can be judged that the chips are all C-shaped chips. The formation of C-shaped chips is closely related to the milling parameters. This type of chip is easy to produce when the milling speed is low and the back cutting amount is large. When the milling depth is 0.1 mm, the milling force is small and the sawtooth of the chip is not obvious; when the feed speed and milling depth are increased to 150 mm/min and 0.3 mm respectively, the sawtooth is more regular and evenly distributed, indicating that the milling process is relatively stable at this time; when the milling depth is increased to 0.5 mm, the milling force is large, the sawtooth of the chip becomes larger, and the sawtooth distribution of the chip is uneven, and even obvious cracks appear at the root of the sawtooth. It can be concluded that the milling speed, feed speed and milling depth all play an important role in the formation of chips. As the feed speed increases, the sawtooth degree of the chips first becomes uniform and neat, then forms an irregular shape, and the difference in the tooth shape of adjacent sawtooth is also large.
3 Conclusion
This paper conducts subtractive milling processing on the Fe45 alloy parts formed by laser additive manufacturing on the surface of 40Cr substrate. The subtractive milling performance of Fe45 additive manufacturing parts is analyzed by orthogonal test, and the influence of milling process on the milling performance of Fe45 laser additive manufacturing parts is explained. The following conclusions are obtained.
(1) The milling process parameters have little effect on the radial milling force and axial force of Fe45 parts; the main milling force fluctuates significantly within the test parameter range, indicating that the milling parameters have the most significant effect on the main milling force. The milling depth is the milling process parameter that has the greatest influence on the milling force, followed by the spindle speed and feed speed.
(2) The order of significance of the milling process parameters on the roughness of Fe45 molded parts is: F (feed speed) > ap (milling depth) > n (spindle speed). By comparing the surface roughness of the molded parts after milling and before milling, it can be found that the maximum value of the surface roughness after milling is 3.48 μm and the minimum value is 1.7 μm, both of which are higher than the surface roughness of the molded parts before milling, indicating that milling can significantly improve the surface quality of laser additive manufacturing parts.
(3) The shape of the chips under different milling parameters is C-shaped chips. As the milling parameters increase, the degree of sawtooth of the chips first becomes neat and uniform, and then changes to an irregular shape. The difference in tooth shape is also large, indicating that the spindle speed, feed speed, and back cutting amount are the key process parameters affecting the chip morphology.
