Basic research on fast and flexible manufacturing

Basic research on laser rapid and flexible manufacturing of metal parts

Abstract: for laser cladding rapid and flexible manufacturing of metal parts (LRFM), a new coaxial powder feeding nozzle was developed, and some simple shape NiCrSiB alloy parts were manufactured. The microstructure, composition uniformity, hardness distribution and tensile strength of these metal parts were studied. In order to improve the accuracy of laser cladding rapid manufacturing parts, the stability of the manufacturing process must be guaranteed, so the main influencing factors of the whole manufacturing process are controlled in a closed loop

key words: laser rapid flexible manufacturing; Metal parts; Laser cladding; NiCrSiB

rapid prototyping technology is an excellent advanced manufacturing method. It can directly use computer CAD models to produce 3D models or entities with complex shapes within a few hours without specific molds. At present, rapid prototyping technology includes stereo lithography (SLA), laminated manufacturing (LOM), selective laser sintering (SLS), molten metal deposition (FDM), three-dimensional printing (3DP), orbital particle manufacturing (BPM) and other methods. However, at present, RP technology can only produce non-metallic prototypes (such as polymers, waxes, paper or coated powder). Although they have good shape accuracy, they do not have mechanical strength and mechanical properties. Therefore, developed countries are vigorously developing new rapid manufacturing technologies that can directly manufacture full-density and high-strength functional metal parts []. Laser rapid flexible manufacturing technology (LRFM) or direct metal deposition (DMD) based on laser cladding is a further development of rapid prototyping technology. It promotes the existing rapid prototyping technology to a new level of metals and superalloys, and is the integration of the basic principle of rapid prototyping and the essence of laser processing technology

basic considerations of laser rapid manufacturing

lrfm is an additive manufacturing technology that is completely different from the traditional manufacturing method (removing useless parts) (the electrical equipment on the experimental machine should be installed and tested by the manufacturer on the experimental machine to add useful parts). A high-power laser is focused on the surface of the workpiece to form a molten pool, and the metal powder is coaxially sprayed into the molten pool to form a cladding layer. Scanning the laser beam back and forth according to the route given by CAD, the metal part entity can be manufactured line by line, layer by layer. Due to the rapid solidification characteristics of laser cladding, the metal parts produced have excellent quality and strength. LRFM is an integrated manufacturing technology based on laser cladding, including laser, CAD, cam, sensor and powder metallurgy

laser cladding was originally a coating technology. High performance surface coatings can be obtained by lapping the laser cladding track horizontally along the plane. Three dimensional metal parts can be manufactured by stacking the laser cladding surface in multiple layers along the vertical direction to the space. As a manufacturing technology, laser cladding must consider the following factors:

a high-quality coaxial powder feeding nozzle that can obtain isotropic cladding results; Three dimensional forming ability of laser cladding; Manufacturing accuracy and its influence mechanism; Detection of key influencing factors in the manufacturing process; Closed loop control of key parameters in manufacturing process; Development of special cad/cam software; Special requirements for working full Chinese system, simple table motion system and CNC system; Specific requirements for laser beam quality and peripheral equipment

development of new coaxial nozzle

in this paper, a new multifunctional coaxial powder feeding nozzle (see the cover photo) is developed, which can meet the requirements of rapid laser manufacturing. The coaxial nozzle has the following characteristics:

flexible and convenient vertical loading and unloading function; Dynamic homogenization function of powder flow; Good cooling of the internal parts of the nozzle ensures long-term stable and reliable operation; The adjustment function of the size of the outlet light spot; The suitable focal length range is mm; The recycling function of reflecting laser energy and bouncing metal powder

laser manufacturing experiment

the experiment adopts rpc3000 fast axial flow CO2 laser, numerical control workbench, positive pressure pneumatic powder feeder and the aforementioned coaxial powder feeding nozzle. The cladding material is ni25b nickel base alloy, the composition is shown in Table 1, and the matrix is low carbon steel. Table 1 Composition and properties of ni25b powder

experimental results and discussion

PM manufacturing simple metal parts

based on the laser cladding technology of new coaxial nozzle, some simple metal parts (such as thin wall and cylinder) are manufactured by layer by layer superposition method (as shown in Figure 1). The main process parameters are: laser power 2kw, spot diameter 4.5mm, scanning speed 0.1m/min, powder feeding speed 11.2g/min, gas flow l/h. When manufacturing cylindrical and simple thin-walled parts, the surface quality and stability can also be produced by laser cladding process without special control. 3 Parts that are quite good in line with international standards. However, when manufacturing corner parts such as Pentagon, the height of cladding stack is different, which shows that it is difficult to ensure the quality of metal parts only by using conventional laser cladding process methods and parameters. Laser cladding manufacturing is a step-by-step manufacturing process from point to line to surface. The manufacturing completion of each part involves the superposition of many cladding layers, so the process of laser cladding, especially the stability of the process, is very important. Only by implementing real-time closed-loop control on the key influencing factors of cladding manufacturing, such as bath temperature, cladding speed, cladding height and so on, can we ensure the stability of the whole manufacturing process and the high accuracy of manufacturing parts

Figure 1 simple metal walls and cylindrical parts manufactured by LRFM technology

2 Microstructure of metal parts

the microstructure of cross-section and vertical section of the manufacturing sample was analyzed by SEM, as shown in Figure 2. The microstructure of laser manufactured parts is a very fine typical solidification dendrite, and the structure is very dense without cracks and obvious pore defects (Fig. 2a). The latter cladding layer is based on the former cladding layer and grows in an epitaxial growth mode against the direction of heat flow. The structure of the overlapping growth zone is solved by designing flexible, light and heat conductive plastics, which is bound to provide many rare opportunities for the wire and cable industry (Fig. 2b) and is slightly thicker than the cladding layer itself (Fig. 2C). The organization of cross sections is similar to that of profiles

Fig. 2 microstructure of laser manufactured samples

3 Component analysis of manufacturing parts the component distribution of sample parts was analyzed by SEM line scanning. Figure 3A shows the distribution of Ni elements along the line between the two overlapping areas and the cladding layer. Figure 3B shows the line distribution of Ni and Cr elements along the longitudinal section of the sample. Both figures show that the elements are evenly distributed in the part

Figure 3 element distribution

4 Hardness distribution

the hardness distribution of metal parts manufactured by cladding is also quite uniform, as shown in Figure 4

Figure 4 hardness distribution

5 Tensile strength test

the tensile strength test was carried out on the cladding wall cut from the substrate. The maximum tensile strength was 451mpa and 454mpa respectively, higher than the calibrated metal powder strength

6. Discussion

lrfm is a manufacturing technology based on laser cladding. Its forming ability and manufacturing accuracy are affected by many factors, including process parameters (such as laser power, spot diameter, scanning speed, powder feeding rate), geometry of clad parts, temperature characteristics of multi-layer bonding, changes in cladding speed, etc. These factors will affect the stability of the cladding process, and thus directly affect the geometry, manufacturing quality and manufacturing accuracy of the clad parts. In order to ensure the stability of the cladding process, closed-loop control must be realized for the main influencing factors, such as the temperature accumulation effect in the manufacturing process, the change of the geometry of the molten pool and the distance between the nozzle and the cladding surface

conclusion

the rapid flexible manufacturing technology based on laser cladding can produce net formed metal parts with full density and full strength. The forming ability and manufacturing accuracy of lrpm technology mainly depend on the stability of laser cladding process, which requires the realization of closed-loop control of key process parameters

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