Metal 3D printing technology is widely used in aerospace, automobile manufacturing, biomedicine and other fields due to its high material utilization rate and mold-free near-net forming. However, although metal 3D printing can realize the forming of almost infinitely complex parts, common metallurgical defects such as pores and inclusions are still a constraint on the large-scale application of this technology. This article reviews the common defects and their formation mechanisms in the metal 3D printing process. It is hoped that on this basis, it can provide metal 3D printing R&D personnel with ideas to reduce or even eliminate internal defects.

Since the rise of metal 3D printing technology in the 1990s, after more than 20 years of development, it has been widely used in national key construction fields such as aerospace. In addition, since this technology can realize reverse modeling and customized production, it has also been greatly promoted in medical dentistry.

Laser forming technology is an important branch of metal 3D printing technology, which includes laser selective melting technology based on powder bed and laser directional energy deposition technology. Laser selective melting technology is to evenly spread a layer of powder with a thickness of 20~100μm on the substrate. The powder is evenly distributed through a liquid storage tank or hopper next to the work area. A laser beam with a power ranging from 20W to 1kW and a scanning speed of up to 15m/s is usually used. The powder is selectively melted based on the defined scanning strategy, the substrate is lowered, another powder layer is applied on top of the existing layer, and the melting process is repeated; laser directed energy deposition technology is based on a digital model, with a high-energy laser beam as the heat source. The laser beam is irradiated on the metal substrate to form a molten pool. At the same time, the metal powder is input into the molten pool and melted. The powder is melted in the molten pool. As the molten pool moves, the molten metal solidifies rapidly, and thus repeatedly, point by point and layer by layer, metal parts are formed. In the forming process of the above two technologies, an inert gas (such as argon or nitrogen) is fed into the process chamber to prevent the molten metal from reacting with other gases, especially oxygen. At present, titanium alloy, aluminum alloy, cobalt chromium and stainless steel powders are commonly used in laser forming processes. Compared with laser directed energy deposition technology, laser selective melting has lower forming efficiency, but the surface finish, dimensional tolerance and feature resolution of the formed parts are better than those of the parts made by laser directed energy deposition process.

Compared with existing metal forming technologies, metal laser forming process contains a variety of defects, such as metallurgical defects such as poor fusion and pores, as well as defects such as cracks caused by residual stress accumulated during the rapid solidification process unique to this technology. Since fatigue cracks usually start from stress concentrations such as pores and inclusions, the above defects will have a significant impact on the fatigue life of metal parts. In addition, these defects will further promote local corrosion, thereby stimulating fatigue cracking. Fatigue performance is one of the most important properties of alloy metal parts. Metal additive manufacturing parts in aerospace and biomedical applications usually need to operate under complex dynamic load conditions, and inherent laser forming process defects will significantly affect the fatigue performance of these components. Based on this, this paper reviews the typical metal laser forming process and the sources of defect formation, hoping to lay a theoretical foundation for research in this direction.

Table 1 lists the main defect categories observed in laser formed metal parts. Due to the layer-by-layer manufacturing strategy, local heating and rapid cooling, and consumption of powder material, common defects in laser-formed metals are mainly manifested as pores, unmelted powder, etc. The above defects are usually the source of fatigue failure detected on the fatigue fracture surface, as shown in Figure 1. The defects shown in the figure are usually caused by insufficient or excessive energy during the metal laser forming process. Process parameters have a great influence on the characteristics of defects such as type, location, shape, size, orientation and density. Defects can be minimized by using high-quality powder (higher sphericity and less hollow powder) and optimizing process parameters such as layer thickness, energy input, deposition direction, scanning strategy, filling spacing and scanning speed. They can also be minimized by post-processing of the manufactured parts (such as hot isostatic pressing), but they cannot be completely eliminated. Other methods such as preheating and keeping the substrate at a certain temperature during the process and maintaining an appropriate cooling rate may also affect the defect content of the final part.

As shown in Figure 1 (a), the main sources of spherical or elliptical pores are metal powders and residual air in the atmosphere protection chamber during the printing process of the part. In addition, the energy supplied to the molten pool should also be optimized. Any deviation from the optimal energy may result in poor fusion or highly unstable molten pool and vaporization. Excessive energy will lead to violent convection and significant vapor recoil, resulting in jet splashing and instability. The above phenomena will lead to the formation of pores in the molten pool (see Figure 1 (a)). Since the surface tension of the liquid metal is dominant, the shape of the pores is usually spherical or elliptical. In addition, spherical powder particles show higher density due to enhanced powder fluidity compared to shaped powder particles with the same process parameters. Spherical powders also reduce splashing, thus reducing the number of defects to a certain extent. In the case of excessively high energy density, circular/spherical defects are generated on the surface of the workpiece due to the shear force exerted by the surface tension on the liquid surface. Elongated narrow defects formed by insufficient energy density around unmelted particles perpendicular to the deposition direction.

Spherical powders also reduce splashing, thus reducing the number of defects. Researchers used elemental powder mixtures when studying laser selective melting of Al-Si and Ti-Ta alloys. The relatively low cost and ease of fabrication of elemental powders make them an interesting choice for laser forming and demonstrate that selective laser melting is feasible for these metals. In-situ prepared eutectic Al-Si alloys have lower ultimate tensile strength and higher plasticity compared to samples prepared from prealloyed powders. In addition, the preparation of dense Al-12Si alloys by selective laser melting of powder mixtures requires higher energy density compared to prealloyed powder feedstocks, which may be related to the complex in-situ reaction processes. Laser forming techniques such as selective laser melting also offer the possibility of multi-material processing by using powder mixtures or by adding special separators to the recoating agent, in which two or more materials can be selectively stored and deposited in each layer. Demir et al. studied multi-material processing by selective laser melting of pure iron and Al-12Si, Sing et al. on AlSi10Mg, and Chen et al. on 316L and CuSn10 interlayers for titanium alloys and stainless steel.

Oxide particles are considered to be one of the reasons for the formation of pores in some alloys (such as AlSi10Mg) during laser forming. These oxide particles may be formed due to oxidation of the evaporated alloy by residual oxygen in the argon atmosphere. More oxide particles were observed on the laser melted surface of the part during the process. The molten aluminum alloy cannot wet the oxide and oxide particles, thus inhibiting the consolidation of the molten metal. Oxidation of the laser processed metal leads to the formation of a metal-ceramic interface in the sintering pool, which reduces the liquid-solid wettability. The lack of wettability of the solid particles and the underlying matrix can cause balling in various metals (as shown in Figure 1(c)). Since the laser forming process is performed in a point-by-point and layer-by-layer manner, the balling effect can lead to the formation of discontinuous tracks, poor line-to-line bonding, porosity, and even delamination. Extremely high scanning speeds can also cause liquid ball splashing, resulting in incomplete fusion and defect formation. Balling can also cause the molten material to solidify as a row of droplets instead of a continuous layer, as shown in Figure 1(e).

These studies demonstrate the feasibility of this technology, although the resulting tensile strength is usually lower than that of at least one material and lack of fusion is often visible at the interface. In general, due to insufficient energy, there is a lack of bonding between layers, resulting in LOF defects. LOF defects are areas of unprocessed powder, as shown in Figure 1 (d), mainly appearing between layers or laser tracks with sharp edges. Typically, large LOF defects contain unmelted or partially melted particles. Cracks may also be generated during the process. Kasperovich et al. optimized the process parameters with the goal of minimizing defects in laser selective melting of Ti-6Al-4V. In the case of excessively high energy density, circular/spherical defects will be generated on the surface due to shear forces exerted on the liquid surface by surface tension. Elongated narrow defects formed by insufficient energy density around unmelted particles perpendicular to the deposition direction. Since the laser selective melting process is carried out in a point-by-point and layer-by-layer manner, the spheroidization effect may lead to the formation of discontinuous tracks, poor line bonding, porosity, and even delamination.

2  Effect of laser forming process parameters on surface roughness

Despite recent advances in laser forming technology, surface roughness remains a major issue. The inherent repeatability of the process, as well as the presence of semi-molten particles attached to the surface and subsurface and surface connection defects, lead to higher surface roughness compared to conventional manufacturing processes. Surface roughness is significantly affected by process type, powder size, part geometry and surface orientation, as well as process parameters such as laser power, scanning speed, pattern filling spacing and layer thickness.

El Sayed et al. proposed a linear model to explain the relationship between laser power and surface roughness for Ti-6Al-4V parts processed by selective laser melting. The study showed that increasing the laser power from 35 W to 50 W resulted in a significant decrease in the arithmetic mean surface roughness Ra value from 21µm to 9µm at a constant scanning speed and pattern filling spacing of 250mm/s and 78µm, respectively. Increasing the laser power can significantly reduce the roughness of the top and side surfaces of the laser selective melting parts. The results show that higher laser power produces greater recoil pressure, which causes the molten pool to flatten, creating better quality for the top surface. The increase in laser power and energy density also improves the wettability of the molten pool and reduces the chance of balling, which significantly improves the side surface roughness. In contrast, another study showed that the improvement of the Ra of the top surface is always accompanied by the deterioration of the Ra of the side surface, and vice versa. The above problems can be explained by the different surface tensions within the molten pool caused by thermal changes. Large molten pools and increased overlap areas between molten pools also show greatly improved surface roughness. At a certain filling spacing, the molten pool size gradually decreases with increasing scanning speed. The reduction in molten pool size leads to reduced overlap, thereby increasing the top surface roughness. In addition, when the molten pool solidifies, partially molten particles from the surrounding powder adhere to the edges of the layer and form the final surface texture. The layer-by-layer production process in laser forming produces a high degree of dependence between the surface characteristics and the inclination angle of each surface. In the upper and lower surfaces that are tilted toward the deposition direction, a phenomenon called the staircase effect occurs, which will increase the roughness.

For specimens manufactured in a diagonal direction relative to the deposited layer, the downward facing surface facing the deposited layer has a higher roughness than the upward facing surface. The lower heat dissipation rate on the overhanging side causes more powder particles to partially melt and adhere to the surface. The overhanging surface of the specimen is essentially built on powder, which has a thermal conductivity that is approximately an order of magnitude lower than the solidified part. This phenomenon especially increases the roughness of surfaces that are angled less than 45° downward relative to the build platform. Therefore, the surface topology of parts produced by laser forming technology is highly dependent on the orientation of the part. Downward facing surfaces with angles less than 45° are usually avoided by reorienting the part being printed. This is also to avoid the need to build support structures, as removing supports may cause burr formation, resulting in higher roughness.

Gravity can also affect the melt pool of the unsupported layer, causing it to sag into the unmelted powder below, resulting in the lower side of the component or lower skin being rougher than the upward facing surface or upper skin. A study by Gockel et al. investigated the relationship between process parameters and surface roughness, represented by the arithmetic mean height Ra and the maximum pit height Rv. They used various methods such as structured light scanning and CT measurements. Using alloy 718 L-PBF round bars with a constant layer thickness of 40µm, the results showed that both Ra and Rv decreased with increasing laser power between 80 and 120 (W). Rv also decreased with increasing laser speed in the range of 500~900 (mm/sec), but there was no obvious trend for Ra. The surface roughness of selective laser melted parts can be changed by different remelting strategies, with the remelting direction being the same or opposite to the first scanning direction. Yu et al. studied the effect of remelting strategy on selective laser melted AlSi10Mg parts using confocal microscopy, micro-computed tomography (CT), and optical microscopy (OM).

The top surface roughness Ra values ​​were improved from 20.67μm to 11.67μm and 10.87μm for the same and opposite direction remelting, respectively, but the side roughness showed a negative trend. Remelting also generally reduces porosity, depending on its direction and porosity distribution. Han and Jiao also demonstrated the positive effect of combining laser selective melting and laser surface remelting (LSR) processes to manufacture customized aluminum components in the automotive and aerospace fields. The method improved the laser selective melting AlSi10Mg-de creating-Ra value from 19.3μm to 0.93μm. The microhardness was increased by 19.5% by refining the microstructure. The top surface roughness Ra values ​​were improved from 20.67μm to 11.67μm and 10.87μm for the same and opposite direction remelting, respectively, but the side roughness showed a negative trend. Remelting also typically reduces porosity, depending on its orientation and porosity distribution.

In summary, the synergistic effect of process parameters on surface roughness is very important, and the impact of these parameters varies greatly depending on the level of application. Ultimately, the results are mainly attributed to powder energy interaction and the energy absorbed by the powder. Considering the changes in all process parameters, the final surface roughness is greatly affected by the interaction of surface inclination, geometric process parameters such as powder size, layer thickness and filling spacing, and process parameters such as power and scanning speed. Due to the complex geometry of laser formed parts, post-processing surface treatment can be difficult, so it is very important to obtain acceptable surface roughness of net parts by optimizing the process parameters and their interaction.

3  Conclusion

As an important branch of 3D printing technology, laser forming technology has process parameters that are decisive factors for the metallurgical quality and external roughness of laser formed parts. This article summarizes the relationship between common defects and main process parameters in the metal laser forming process, and summarizes the relationship between the external roughness of the parts and process parameters, which can provide certain solutions for industry-related personnel in clarifying the causes of defects in parts.