Powder-feeding metal 3D printing, referred to as LDM, is a process that uses a powder feeder to transport metal powder to a laser melting pool, and uses the radiation of the laser to melt and re-condense the powder, thereby directly producing metal parts. It has the advantages of large product molding size, rapid processing, and the ability to achieve complex component gradient printing. At the same time, it also has disadvantages such as difficult part quality control and rough part appearance. This article summarizes the technical status of powder-feeding metal 3D printing and analyzes its further development direction.

1  Comparison of several common metal 3D printing processes

For metal 3D printing, the commonly used process methods are arc wire printing (abbreviated as WAAM), powder-feeding printing (abbreviated as LDM) and powder-laying printing (SLM). A brief comparison of their characteristics and suitable uses is shown in Table 1.

As can be seen from the above table, powder-feeding printing has unique advantages: it can mix multiple materials to complete gradient printing, as well as large parts with weight reduction requirements and internal flow channel structures. This makes it irreplaceable at present.

2  Technical issues of powder-feeding 3D printing and their countermeasures

2.1  Analysis of technical issues of powder-feeding 3D printing

In powder-feeding 3D printing, during the processing, the laser spot and the powder-feeding nozzle move synchronously under the drive of the motion mechanism. Where the spot moves, a molten pool is formed, and at the same time, the powder falls into the molten pool to form an additively printed workpiece. In the printing process after the process parameters are determined, there are still several issues that need special attention:

Printing is often carried out on a substrate; and the substrate is initially cold. After a period of printing, the substrate and the newly printed workpiece have a certain temperature. A cold substrate will inevitably absorb a large amount of heat quickly, resulting in a low molten pool temperature during initial processing. This is the main factor that causes quality problems near the substrate.

The motion mechanism is uniform in linear motion, but not uniform at bends and corners. Over time, a printing bulge will be formed at the bends and corners; correspondingly, a collapsed waist will be formed at the straight printing part.

The height difference of the workpiece surface will cause the powder feeding nozzle to be closer or farther from the workpiece surface. The light reflection of the molten pool will cause the powder to melt and recondense at the powder feeding nozzle to form agglomerates. Once the agglomerates fall into the molten pool, they will cause individual protrusions on the surface of the workpiece, where pores are easily formed; the presence of agglomerates at the nozzle for a long time will easily cause the nozzle to be blocked, causing the powder feeding parameters to change passively.

The protrusions and depressions on the workpiece surface cause the focusing state of the laser on the workpiece surface to change, which will affect the effect of the laser on the workpiece surface, and then cause the laser power density to change during the workpiece processing.

The passive change of process parameters due to the unsatisfactory surface state of the workpiece is a major factor in the cracks of the workpiece.

Therefore, under the condition of reasonable process parameter settings, the unsatisfactory powder feeding process and uneven spot movement speed during 3D printing are the main essential factors causing workpiece defects. The speed of the light spot movement is limited by the dynamic response characteristics of the mechanical structure, and it is impossible to achieve a completely ideal state; the formation of agglomerates on the powder feeding nozzle, the shedding of agglomerates, and the clogging of the powder feeding nozzle are also accidental phenomena without specific rules. Both of these points are difficult to deal with directly.

2.2  Countermeasures for the difficulties of powder feeding 3D printing

In current engineering applications, dual colorimetric temperature sensors or molten pool detection cameras are often used to monitor the temperature or morphology of the molten pool and perform closed-loop control of the laser power, which is beneficial to offset the impact of the cold state of the substrate on the molten pool.

Without considering the substrate temperature, it can be seen from the analysis of 2.1 that the stability of the light spot movement speed and the passive change of the powder feeding state will affect the molten pool, thereby causing quality defects. Since the passive changes of the light spot movement speed and powder feeding state parameters are difficult to manage directly in engineering practice, the molten pool can also be adjusted in a beneficial direction by monitoring the molten pool state and controlling other process parameters accordingly.

The light spot moves slower at the corner, which will cause the laser to act on the workpiece surface for a longer time, so the temperature of the molten pool will increase accordingly. In contrast, for straight sections, the temperature of the molten pool should be slightly lower than that of the corner.

When the workpiece surface has collapsed, when the light spot passes through the collapsed area, the laser energy is dispersed at the molten pool because the convergence point of the laser after passing through the focusing lens is above the workpiece surface, and the temperature of the molten pool will also be low.

Based on the above analysis, taking the detection of the molten pool temperature as an example, a new control strategy can be derived – by monitoring the temperature of the molten pool, when the first few layers of the substrate are cold, the molten pool temperature monitoring is used as input to perform closed-loop control of the laser power to offset the influence of the cold state of the substrate on the molten pool; and after the substrate temperature enters the hot state, when the molten pool temperature is detected to be too low, it means that the light spot is passing through the collapsed area. According to the deviation between the target temperature and the ideal temperature, the equipment is controlled to reduce the movement speed ratio according to certain rules to extend the laser action time on the workpiece surface and the time for the powder to fall, which can automatically fill the collapsed area.

In fact, the molten pool is not static, like boiling water. Under the impact of the laser, the molten metal at the bottom keeps rolling upwards. The temperature of the molten metal rolling up from the bottom must be lower than the original molten metal temperature on the surface, which will cause a certain deviation in the temperature measurement of the molten pool. In addition, if a camera is used to monitor the morphology of the molten pool, the surface morphology of the molten pool is also unstable due to the rolling and rippling of the molten pool, as well as the blowing effect of the powder feeding gas and the shielding gas on the molten pool. Therefore, when designing the control algorithm, attention should be paid to filtering and averaging to avoid adverse effects of measurement deviation on closed-loop control.

3  Industrial prospects of powder feeding 3D printing

At present, wire arc feeding printing (WAAM) is developing rapidly because of its fast speed and the performance of some parts even surpassing traditional castings; SLM is expanding rapidly because of its fine parts, outstanding weight reduction for workpieces with internal structure requirements, and almost no manual participation in the production process after the process parameters are determined; while powder feeding printing (LDM) is gradually squeezed out because it requires more manual intervention in the processing process and is not particularly outstanding in production speed. However, powder feeding printing is irreplaceable: gradient functional parts with changing printing components and large structural parts with special components can only be completed by powder feeding printing. In addition, the characteristics of powder feeding printing that can adjust the composition of parts at any time can develop high-throughput material testing equipment for new material development, which is of great significance to the development of domestic materials science. Therefore, powder feeding printing (LDM) has its specific development space and necessity.

Through the comparison of the three 3D printing processes listed in Table 1, the disadvantage of powder feeding 3D printing (LDM) compared to wire arc printing (WAAM) is the processing speed, and the disadvantage compared to laying powder 3D printing (SLM) is the part accuracy; the processing speed of powder feeding 3D printing is difficult to achieve large-scale improvement, but by improving the part accuracy, the application scenarios of powder feeding 3D printing (LDM) will be rapidly expanded. Combining powder feeding 3D printing with traditional milling and cutting, designing and manufacturing multi-station additive and subtractive integrated equipment, realizing the alternation of additive printing and subtractive manufacturing. While one station is doing powder feeding 3D printing or milling and cutting, other stations are used to cool the printed workpieces to wait for milling and cutting. By using a set of additive and subtractive equipment to alternately perform additive printing and milling and cutting on multiple parts, the production efficiency will not be significantly reduced while improving the precision of product parts. This will form an overwhelming advantage over powder laying 3D printing (SLM) and greatly expand the application scenarios of powder feeding 3D printing (LDM).

The integration of powder feeding 3D printing and milling and cutting is a difficult point. It is necessary to solve the problems of product spatial positioning and coordinate transformation in the process of additive and subtractive. This requires development and innovation in trajectory planning software development; it is also necessary to solve the problem of process combination between additive and subtractive: for example, cutting fluid cannot be used in the process of milling and cutting, otherwise the residual cutting fluid will affect the composition of the part in the next printing process.

4  Conclusion

Powder feeding 3D printing has the advantages of fast printing speed, basically unlimited part size, and gradient printing, which makes it irreplaceable in the field of 3D printing. Targeted research on molten pool control technology and additive and subtractive material technology for powder feeding 3D printing can reduce the dependence of powder feeding 3D printing on process personnel, effectively expand the application scenarios of powder feeding 3D printing, and bring extensive social benefits.