Study on overlap rate and machinability of selected laser melting of maraging steel

In this experiment, Vilory New Material Technology Co. vacuum-atomized 18Ni300 powder and rolled 18Ni300 substrate were used. The elemental composition of the powder is shown in Table 1. The powder’s appearance is shown in Figure 1, with a particle size of 45-105 μm. Before the experiment, the substrate was polished to prevent surface impurities and the oxide layer from affecting the experiment, and the powder was dried to remove moisture. After the experiment, the material was cut into convenient polishing shape using wirecutting technology, and the cladding layer was polished many times with 600–2000 mesh sandpaper, then polished with metallographic grinding and polishing tester and grinding paste. The surface of the polished cladding layer was etched with aqua regia, and the surface was cleaned and dried with industrial alcohol.

Fig. 1.

The equipment used for laser additive manufacturing is the YLK-3000 fiber laser integrated processing system, which consists of the YLR-3000 high-power fiber laser; the large-load, high-precision programmable KUKA-KR30H 6-axis robot; the PERCITEC YC52 laser cladding head with four coaxial powder feeding channels, a cooling water channel, a protective gas channel, and the FHPF-10 synchronous dual-channel powder feeder, synchronized dual-channel powder feeder, and other equipment. During the experiment, the powder is driven by the powder-feeding gas through the synchronous powder-feeding device. The powder is shot out from the powder-spraying orifice on the laser cladding head to the substrate, and at the same time, it interacts with the high-energy laser beam to form the molten pool. As the KUKA Robot moves along a preprogrammed path, the desired cladding coating is eventually formed. The experimental embodiment is shown in Figure 2, where helium gas is used as a protective gas in the additive manufacturing process. Based on the previous research, the experimental parameters of additive manufacturing are shown in Table 2.

Fig. 2.

Overlap rate is the degree of overlap between adjacent cladding channels in laser cladding. A change of overlap rate will directly affect the shape and performance of the workpiece after cladding, and a suitable overlap rate can make the material produced more compact [18]. An overlap rate that is too large or too small will have a negative effect, as shown in Figure 3(a). Here, the overlap rate is too low, the surface of the cladding layer is “wavy,” and there is a significant depression between the adjacent fusion channel. The surface smoothness of the cladding layer is poor, and cracks are appear easily between the melt channels, so the quality of the cladding layer will be affected by further processing. Figure 3(b) shows a moderate overlap rate: a flat surface is formed between adjacent melt paths, and the surface flatness is good, which is an ideal overlap rate for subsequent treatment. Figure 3(c) shows that if the overlap rate is too high, the melt channels overlap each other. Under these circumstances, the cladding layer will increase continuously with further processing, the change of the defocus of laser cladding may result in a change of the surface morphology and internal structure of the cladding layer. In Figure 3(d), H is the height of the cladding layer, h is the depth of the cladding layer, R is the overlap rate, D₀ is the lap width, and W is the single channel width. The calculation of overlap rate can be simplified as the ratio of the overlap width D0 between adjacent cladding tracks and the width W of a single cladding layer in multi-pass laser cladding [19], which can be simplified as equation (1). 1 R=D0/W $$\text{R}={{\text{D}}_{0}}/\text{W}$$

Fig. 3.

First, a single-layer, single-track laser cladding experiment is carried out, the height and width of the experimental results are measured, and the lap ratio is calculated with equation (1). According to previous investigations [20–22], most of the studies on the overlap rate are in the range of 30% to 70%, and our research is based on this range of experiments.

In this research, the material produced by laser additive manufacturing is processed by milling. The cutting tool is a double-edged end mill with a diameter of 0.8 mm, and the cutting mode is dry cutting. The manufacturing equipment uses the Carver 400GA CNC precision machine tool. The experimental parameters were obtained from the 18Ni300 material processing line of the Wuxi Yingpu Precision Industry Co Ltd. Machining parameters are shown in Table 3. As shown in Figure 4, the finished material is cleaned with an ultrasonic cleaner to remove surface debris and other impurities.

Fig. 4.

The machined surface was observed by the MR5000 inverted metallographic microscope and the Sigma500 scanning electron microscope. Aztec X-Max 50 energy spectrometer was used to detect the element distribution. The surface morphology was characterized by a HYBRID C3 real color confocal microscope, and the roughness was calculated. The composition of the material image was observed by the D/Max-2500/PC X-ray diffractometer. The hardness of the material was measured by an HVS-1000 digital microhardness tester with a diamond probe.

Figure 5 shows the macroscopic and 3D real surface morphology of the laser cladding layer with different overlap rates. It can be seen that the surface of the cladding layer is smooth, and if there is no big porosity or crack, the experimental effect is better. As predicted before, when the overlap rate is 30%, the morphology of the cladding layer is “wavy,” which proves that the overlap rate is too low, and the channel gap is large. When the overlap rate is 70%, the protruding circle can be seen on the surface of the cladding layer, which proves that the overlap of the cladding layer is caused by the high overlap rate. When the lap rate is 40% to 60%, although there will be a “wavy” morphology and cladding overlap, it will not be obvious.

Fig. 5.

Figure 6 shows the section morphology of the cladding layer under a different overlap rates. It can further be seen from the figure that the cladding layer section is wavy when the overlap rate is 30%, some of the incompletely melted powder will be retained. This results in nonfusion defects, which can lead to stress concentration and crack initiation [23, 24]. When the overlap rate is 70%, it is obvious that the height of the cladding layer is higher than the other overlap rates, and the height difference of the cladding layer is obvious. When overlap occurs, there is a big gap between the base material and the bottom of the cladding layer, and the bonding effect is not good. When the overlap rate is 40% to 60%, the lapping effect is good, and there is no obvious crack or overlapping part in the adjacent melt path. When combined with the macro-morphology and three-dimensional real surface morphology of the cladding layer, the lapping effect is not good when the overlap rate is 30% and 70%, so it is not considered. The analysis is continued when the overlap rate is 40% to 60%.

Fig. 6.

Figure 7 shows the metallographic structure of the cladding layer at different positions in the cross-section of the laser cladding experiment under the overlap rate of 40%, 50% and 60%. (a) shows the metallographic structure at the top of the cladding layer, (b) shows the metallographic structure at the middle of the cladding layer, (c) shows the microstructure at the lap of the cladding layer, (d) shows the microstructure at the bottom of the cladding layer, and the microstructure evolution is mainly determined by the ratio of a temperature gradient to solidification rate [25]. The metallographic structure at (a) consists of fine cellular and equiaxed crystals because of the large area of heat exchange between the top of the cladding layer and the outside during fabrication, resulting in smaller and denser crystals [26]. The microstructure at (b) is composed of columnar dendrites and a small number of cellular crystals. As a result of the high cooling rate, uneven temperature distribution, and thermal diffusion of the molten pool, some of the columnar dendrites were biased or even stopped growing. The reason why the dendrites are clustered and directional is that the distance between adjacent melt paths is close during laser cladding, and some materials will have secondary crystallization during lapping. The growth direction of columnar dendrites is changed, and these crystals grow in the direction of the previous coating surface, that is, in a negative direction along a temperature gradient [27]. The microstructure at (c) is composed of columnar dendrites with different orientations and mixed cellular crystals along the arc of the lapping surface. Due to the different temperatures at different positions on the lapped surface, during the next lapping process, the second laser heat input and the residual heat on the lapped surface act together, making the heat flow direction at different positions on the lapped surface different, influencing the direction of dendrite growth [28]. The interlaced dendrite growth is also beneficial to prevent the crack from forming through the whole cladding layer when the thermal stress is too high in the laser cladding experiment. The microstructure of (d) is composed of columnar dendrites growing perpendicular to the fusion line and mixed with cellular crystals. Because of the decrease of heat input at the bottom of the molten pool, the temperature gradient increases, and the solidification rate is small. Therefore, the temperature gradient is larger than the solidification rate, and the crystallization of the cladding layer grows in a dendritic manner with fine crystal structure interspersed between them. An obvious curve appears at the interface between the cladding layer and the substrate, which is the boundary between the cladding layer and the substrate. It shows that there is a good metallurgical effect between the cladding layer and the substrate.

Fig. 7.

The metallographic structure of the cladding layer at different positions in the laser cladding experiment under the overlap rate of 40%, 50%, and 60% was compared. When the overlap rate is 40%, the bonding porosity between the cladding layer and the substrate is larger, which is caused by an overlap rate that is too small and gap between melt paths that is too wide. When the overlap rate is 50%, the bonding between the cladding layer and the substrate is better, the microstructure is distributed evenly, and there is no obvious defect. The microstructure at the top of the cladding layer is smaller when the overlap rate is 60%, but the grain size of the other parts is larger because there are more overlapping parts between the melt paths. This leads to the growth of the grain of the other parts, and, finally, the coarse grains are formed. The results show that the microstructure of the cladding layer is the same in the same area under conditions of different overlap rates, and the change in overlap rate has no effect on the microstructure.

Figure 8 shows the metallurgical bonding zone at the bottom of the cladding layer by EDS scanning at the rates of 40%, 50%, and 60%. As can be seen from the diagram, the metallurgical bonding zone at the bottom of the cladding layer is continuous and compact, without obvious cracks and gaps. The element distribution in the metallurgical bonding zone at the bottom of the cladding layer was observed, and the distribution of the elements is consistent at each overlap rate. Fe is the main element. By scanning from the substrate to the cladding layer, it was found that Cr decreased to a stable trend, and Ni increased to a stable trend, which proved that the elements exchanged at the junction between the substrate and the cladding layer, which constitutes a good metallurgical effect.

Fig. 8.

Figure 9 is the SEM micrograph of the top of the cladding layer under the overlap rate of 40%, 50%, and 60%. Panels a, b, and c are observed under 1k magnification; panels a1, b1, and c1 are observed under 3k magnification. It can be seen from the diagram that the microstructure of each lap rate consists of rough martensite with uniform distribution and obvious grain boundaries. The microstructure morphology of the cladding layer is the same as that of the 18Ni300 made by Chaolin Tan additive [29]. Under a 3k magnifying mirror, it can clearly be seen that there are pores and short cracks on the surface when the overlap rate is 40% and 50%,. There are penetrating cracks when the overlap rate is 60%: the crack length is long and the crack width is large. At the same time, many branches are extending in other directions at the end of the crack, which may be due to the large residual stress of surface compression when the overlap rate is too high [30], which leads to the increase of the crack growth rate [31, 32], thus producing a larger crack.

Fig. 9.

Figure 10 shows the SEM-EDS map of the overlap of the cladding layer under 40%, 50%, and 60% laser cladding experiments. Table 4 shows the content of elements under each overlap rate. According to the content of each element (shown in Table 3), it can be seen that the distribution and content of the five elements are approximately the same: the content of Fe is the highest, followed by Ni, Co, MO, and C. The microstructure of the lapped joint is small and grows alternately along the forming direction, because the microstructure of the lapped joint is optimized by laser remelting. The columnar crystal and nonequilibrium structure are eliminated, resulting in finer and more uniform grain morphology and distribution [33]. A few pores and short cracks occur at a 40% overlap rate, the microstructure morphology is good at a 50% overlap rate without obvious defects, and the microstructure morphology is very bad at a 60% overlap rate, when a large number of through–through cracks occur at the top of the microstructure of the cladding layer.

Fig. 10.

Figure 11 shows the hardness curves of the cladding layer under conditions of 40%, 50%, and 60% overlap rates. The hardness values were measured from the top of the cladding layer, with a load of 100 g applied to each measurement, and an interval of 0.15 mm, for a total of 17 measurements. It can be seen from the figure that the hardness of each overlap rate changes following the same trend: the hardness of the cladding layer is higher than that of the substrate due to the different preparation technology of the substrate and the cladding layer. The smaller the grain size, the larger the area of grain boundary; and the stronger the bonding force between metals, the higher the hardness and the higher the mechanical properties of the material [34]. In conclusion, the hardness of the lap joint has an increasing trend [35–37], there is an upward trend in the hardness of the lap region because of the its secondary processing [38]: the grains are refined and the distance between grains is reduced, the dislocation movement was significantly inhibited, and the hardness and strength were increased [39]. The average hardness of 50% is higher than that of 40% and 60% because of the cracks and pores in the additive manufacturing. Based on the above analysis, the cladding effect is best when an overlap rate is 50%. The cladding layer with an overlap rate of 50% is processed.

Fig. 11.

Milling experiments were conducted on cladding parts with 50% overlap rate, Figure 12 shows the surface section morphology after milling. It can be seen from the figure that the milled surface of the additive material is very smooth without major defects, and, in contrast with Figure 7(2), the distortion and tensile deformation of the grains of the metallographic structure are very small. It may be that the smaller the depth of cutting and the smaller the diameter of the tool, the smaller the deformation. As can be seen from Figure 12(a), small pits appear after the cutting surface because the cladding layer contains some small, unmelted particles mixed into it, which fracture when the particles come into contact with the tool under maximum stress. Some of the particles remain bonded to the matrix, and some are pulled out entirely, causing the cutting surface to sag. As can be seen from Figure 12(c), during milling, the material of the workpiece undergoes elastic and plastic deformation with the local temperature rising, and induced milling surface residual stress has a new effect on the workpiece surface [40–42], resulting in small cracks.

Fig. 12.

Figure 13 shows the surface morphology after milling at a 50% overlap rate. Panel a is the surface after milling under a metallographic microscope, panel b is the true surface morphology, and panel c is the macroscopic surface after milling. From Figure 13a, we can see that there is a small defect on the surface: the friction in the contact area after milling increases, resulting in an increasing cutting force and cutting temperature. The increased plastic flow ability of the workpiece results in plastic flow defects (Fig. 13a Mark I), pores in the cladding experiment (Fig. 13a Mark II), micro-cracks in the cladding experiment (Fig. 13a Mark III), scratches on the cutting surface caused by forward milling of some chips or unmelted powder (Fig. 13a Mark IV), and pits formed when the bulk of the cladding surface is removed (Fig. 13a Mark V). After milling, the surface of the workpiece is smooth. The surface roughness of Figure 13b was measured to be 0.342 μm. The improved surface quality after milling is convenient for subsequent use [43].

Fig. 13.

Scanning electron microscopy of the milled surface is shown in Figure 14, with burrs growing unevenly along the cut direction of the channel [44]. From Fig. 14(a), it can be seen that the fine chips pushed out by the milling side are scattered at the top to form long whiskers, occasionally forming large burrs with continuous curls. From Figure 14(b), it can be seen that the side burr of reverse milling is large and has a continuously wavy shape, while the action mode between tool and workpiece material is mainly extrusion, friction, and plow effect. The burr on the reverse milling side is larger than that on the down milling side, and the shape of the burr is irregular. The burr is more complicated because the continuous advance of the milling cutter results in the wear of the carbide cutter and an increase in the radius of the cutter tip. The tool extrudes from the side of the workpiece, some material moves to the upper surface under the condition of plastic deformation, and some chips are removed on the down milling side. Most material enters the cutting end along the rake face, bending and fractures then occur on the reverse milling side [45]. As a result, the burr on the reverse milling side is larger and more irregular. Compared to reverse milling, the down milling side is subjected to less impact, which results in smaller burrs and less impact on tool wear [46]. A larger ratio of the cutting depth to the radius of the edge circle can be chosen in milling to reduce burrs [47]. Figure 14(c) is the milling surface topography, from which it can be seen that there are obvious tool marks on the surface, and some parts of the surface have spalling, but the overall smoothness is good, with no large obvious defects. Figure 14(d) shows the defect of the cutting surface. It mainly involves the chopping of the air hole and the unmelted powder in the laser cladding. The laser power can be increased to melt the powder sufficiently in order to reduce the surface defect. Defects in additive manufacturing can also be avoided by varying the depth of the cut.

Fig. 14.