Discussion on the characteristics of laser selecti

2022-08-08
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Discussion on the characteristics of selective laser sintering process of metal powder Abstract: Based on analysis and experiments, this paper systematically expounds some basic characteristics of metal parts formed by high-power laser direct sintering of metal powder. This mainly includes the spheroidization characteristics of molten pool, melt flow characteristics and dynamic solidification structure characteristics in the process of laser sintering, and further discusses the basic laws of the sintering process, which provides a basis for the direct laser sintering of metal powder

key words: laser selective sintering, metal powder, process, characteristics

since the birth of rapid prototyping technology, rapid manufacturing of metal parts has become a remarkable research direction [1~3]. Among them, the selective laser sintering (Selectiv Jinan testing machine factory takes you to know the testing machine e laser sintering, referred to as SLS) technology has attracted much attention because it can carry out rapid manufacturing of metal parts. So far, people's research mainly focuses on mixing two kinds of metal powders with high and low melting points or mixing a certain binder in the metal powder, sintering and forming metal parts with low laser power [4~5], while there is less research on directly sintering metal powders with high power laser to form metal parts. Based on the previous research work [6~7], this paper starts with the analysis of the sintering phenomenon, clarifies the basic characteristics of the metal powder laser direct sintering process, and further discusses the basic laws of the sintering process, so that people have a relatively complete and clear understanding of the metal parts formed by directly sintering metal powder with high-power laser

1 basic principle of SLS

the basic principle of SLS is shown in Figure 1. Its process is to divide the CAD model through layered slicing software to form several thin layer planes. During sintering forming, the powder is first paved with a powder spreading device, and then the laser selectively scans the material according to the geometry of the layer to melt the powder and bond it to the lower material, while the powder not sintered by laser scanning is used as the support of the part. After one layer of sintering is completed, the cutting force and displacement of the workbench drop by one, which are immediately recorded by the instrument and maintain the same sheet thickness, and then pave powder again and sinter. Repeat this process until the whole part is sintered

2 discussion on the characteristics of forming process

2.1 spheroidization characteristics of molten pool

the shape of molten pool formed after metal powder melting depends on the gas, liquid and solid three-phase contact of three different substances, which can also be said to depend on the interfacial tension between the contacting substances. For the liquid particle on the three-phase intersection, let its force by the gas particle be F1; The force of the liquid receiving body point is F2; The force exerted by solid particles is F3. F1, F2 and F3 point to the interior of gas, liquid and solid phases respectively, as shown in Figure 2. The magnitude and direction of the resultant force of F1, F2 and F3 depend on the position of the molten pool. In the molten pool on the powder, because the powder is loose, there is a gap between the particles, and their bonding force is small, making the total resultant force F of F1, F2 and F3 point to the interior of the liquid. As shown in Figure 2 (a), the included angle between the liquid surface and the solid-liquid interface is an obtuse angle. At this time, the interfacial tension will shrink the liquid surface into a sphere, and the shape of the molten pool is a sphere. Under this condition, sintering forming is carried out. Because the molten pool flows in multiple directions, the surrounding powder is bonded and a spherical molten pool is formed. As the laser moves to the next melting zone, the cooling and solidification of the molten pool of the bonded surrounding powder are accelerated. If there is not enough powder material in the next melting zone under the action of laser (because the powder here has been bonded by the previous melting zone), a new melting pool cannot be formed. Only after moving to a certain distance, can enough powder materials be melted to form a new molten pool, so as to repeatedly form new spheres. Therefore, the sintering line is composed of a string of balls. In the molten pool of the matrix part, due to the large force F3 of solid particles, the total resultant force F of F1, F2 and F3 points to the solid. As shown in Figure 2 (b), the included angle between the liquid surface and the solid-liquid interface is an acute angle, and the interfacial tension will open the liquid surface along the solid surface. The shape of the molten pool appears in a fan shape in the vertical scanning direction, while the molten pool is continuous along the scanning direction, making it difficult to form a single sphere. The shape of the whole molten pool is determined by the melting depth of the matrix. The greater the melting depth of the matrix, the greater the ovality of the molten pool, which is more conducive to sintering forming

2.2 melt flow characteristics

in the process of laser sintering, the forces acting on the fluid unit in the metal molten pool are mainly volume forces and surface forces. Its volume force is mainly the temperature difference in the molten pool( Τ) And concentration difference (c), and the surface force is mainly the temperature difference on the surface of the molten pool( Τ) And the difference in surface tension caused by the concentration difference (c). Due to the small spot used in sintering forming (d=1mm), the surface tension in the molten pool plays a major role. In this case, the role of volume force can be ignored

the surface tension is affected by the temperature change on the surface of the molten pool and the change of solute concentration [8]

where: σ 0 is a constant independent of temperature and concentration. It is the surface tension value of pure metal at the melting point. Obviously, when there is a temperature gradient or solute concentration on the surface of the molten pool under the action of laser, it is bound to produce a surface tension gradient σ/ r. This causes the convection driving force F of the melt σ。 The driving force of surface tension is:

where:( σ/ T) * t is the surface tension difference caused by temperature gradient; ( σ/ c) * t is the difference in surface tension caused by concentration gradient; δ (z) For δ- Function; H (D R) is the Heaviside function. δ- Function and Heaviside function show that the driving force of melt convection only exists on the surface of molten pool, which is a surface force

under the action of the laser beam, the surface temperature of the melt near the center of the energy beam spot is the highest, and the farther away from the center area, the lower the surface temperature. Accordingly, for the metal melt, the distribution law of the surface tension field is that the surface tension near the central surface of the molten pool is the lowest, while the surface tension near the edge of the molten pool is the highest. In this way, a forced convection mechanism is generated in the molten pool. In the plane perpendicular to the scanning direction, multiple couples flowing in a certain direction will be generated, as shown in Figure 4. According to the existing research results [8], the surface melt flow rate can reach 8.2m/s. The high-speed melt not only accelerates the heat and mass transfer of metal, but also can bond the surrounding powder. The new powder enters the molten pool, which increases the temperature difference between different parts of the molten pool and accelerates the melt flow. The thicker the powder layer, the more adhesive powder, and the forming size is not easy to control. Along the scanning direction, as the laser moves forward, part of the metal in the solidification zone behind the molten pool comes from the reflux of the front melting zone. The amount of reflux mainly depends on the maximum heating temperature of the central surface of the molten pool spot. The higher the central surface temperature of the molten pool spot, the greater the surface tension gradient of the molten pool, and the more the reflux of the melt. The amount of reflux determines the sintering forming results, and the typical forming results may be: ① a string of balls; ② Sintering line with different thickness; ③ Smooth sintering line. After a large number of experiments, the author found that only under appropriate technological conditions, such as using smaller powder layer thickness and larger laser power, increasing the temperature difference between the spot center and the solidification zone behind the molten pool, increasing the melt flowback in front of the molten pool, can sintering form a smooth straight line

2.3 morphological characteristics of solidification structure

the morphology of solidification structure is related to the location of the whole molten pool and the location inside the molten pool. When the molten pool has a certain depth on the substrate, the whole substrate can be used as its heat transfer body, which will have a great impact on the microstructure of the sintered body. For each sintering line, the melting zone can be divided into melting transition zone and melting zone. The melting transition zone refers to the junction between the molten pool and the matrix. In this region, the grains are in a partially molten state, and there are a large number of grain debris and micro molten grains. It is not a line, but a region, that is, a semi molten region. The grain debris and micro molten grains in the semi molten zone may be used as the nucleation core of new grains at the beginning of solidification [9], and the main nucleation mechanism is that the micro molten nuclei are used as heterogeneous epitaxy. In this region, due to the weak effect of melt convection on this region, the growth orientation of crystals is weakly disturbed by melt convection, and the dendrite orientation formed follows the normal direction of the solid-liquid interface, as shown in Figure 5

Fig. 5 crystallization characteristics of melting transition zone Fig. 6 crystallization characteristics of melting zone

due to the high laser power used, it can completely melt the powder in the molten pool. The melting zone is mainly homogeneous nucleation, forming equiaxed crystals, which are strongly affected by melt convection. The orientation of dendrites is greatly affected by melt flow, and the orientation of crystals is relatively disordered, as shown in Fig. 6. Therefore, the microstructure of the whole sintered body has structural heterogeneity, which is mainly reflected in the formation interface between layers (i.e. along the normal direction of the stacking surface). Its microstructure is mainly dendrite shape, and the dendrite direction is along the normal direction of the layer, which also indicates the joint growth of the matrix; In other places, the microstructure is mainly equiaxed crystal, so each layer is composed of dendrite and equiaxed crystal, in which the volume fraction of equiaxed crystal is much larger than that of dendrite. The whole sintered body is superimposed layer by layer, and the two organizational structures appear alternately and repeatedly along the stacking direction. When the molten pool is on the powder, because there are pores between the powder particles, the thermal conductivity of the metal layer powder is affected by the gas porosity, and its expression is:

where: λ T - thermal conductivity of metal powder; φ A=1- φ;φ— Volume percentage of metal powder; φ A is the volume percentage of air; λ A - thermal conductivity of air

according to formula (4), the thermal conductivity of metal powder is only proportional to the thermal conductivity of surrounding air, and there is no thermal conductivity term of metal powder. This shows that the thermal conductivity of metal powder is quite low. In this case, the matrix can not be used as heat transfer body, the change of heat transfer conditions in the molten pool affects the morphology of solidification structure, and there is no dendrite growing with the matrix in the sintering line; The microstructure is mainly equiaxed crystal

because the rapidly manufactured parts are superimposed layer by layer, and the upper and lower layers must be firmly bonded with thermosetting plates, the volume occupied by the alternating repetition of the two organizational structures is the main, and a single organizational form will appear only in the initial part such as the boss of the part

3 conclusion

(1) spheroidization of molten pool and melt flow will have an impact on sintering forming. As long as the laser power is properly increased and the matrix melts to a certain depth, the adverse effect of spheroidizing force on sintering forming can be avoided, and the return flow of melt in the molten pool is also increased, which is conducive to smooth and straight sintering forming

(2) the microstructure of sintered body is mainly composed of dendrite and equiaxed crystal alternately, forming the periodic repetition of the microstructure of the whole workpiece

references

[1] abdolreza simchi, Frank Petzoldt and Haiko pohl Direct metal laser sintering: Material considerations and mechanisms of particle bonding[J]. The International Journal of Powder Metallurgy. 2001,37(2): 49~61

[2] Mukesh Agarwala, David Bourell, Joseph Beaman, Harris Marcus and Joel Barlow. Direct selective laser sintering of metals[J], Rapid Prototyping Journal. 1995,1(1): 26~36。

[3] Ping Shen, Jiandong Hu, Zuox

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