In order to achieve the goals of improving product performance indicators, technological innovation, reducing costs, shortening manufacturing cycles, and personalized customization, at this stage, in-depth technical research has been conducted on the metal 3D printing laser selective melting (SLM) process for making aluminum alloy (AlSi10Mg) and stainless steel (316L) material parts. The main research is on the optimization of metal 3D printing support process, exploration of non-support technology, research on mechanical properties of parts, and application in automotive products, and its actual application scenarios have been explored; based on the exploration results, the metal 3D printing support technology process guidance specifications and non-support technology process exploration rules are summarized, and the specific values ​​that can be achieved by the mechanical properties of printed parts are summarized. At the same time, the application scenarios of automotive parts are developed, and the metal 3D printing laser selective melting (SLM) process is used to replace the complex, long-cycle, and high-cost traditional process parts, which provides a theoretical basis and practical application case references for the further application of metal 3D printing process technology for automotive parts.

1 Introduction

With the increasing demand for personalized and customized automotive products and the difficulties of long production cycles and high costs of some traditional complex process parts, metal 3D printing technology has begun to be explored and applied in the field of automotive parts. Metal 3D printing technology is an innovative technology in additive manufacturing technology. Many countries are also working hard to develop and apply this technology. At present, the automotive field is also exploring the use of this technology, opening up the exploration and application of small-batch, customized and personalized production of automotive products. When using traditional casting, forging and other processes to produce blanks, molds need to be opened, which has a long cycle, high cost, low efficiency, and high resource consumption. The supply chain needs to be optimized [1]. Metal 3D printing technology has a short cycle and low cost. It has great freedom in processing parts with free shapes and complex features. It is based on digital models and can directly print finished products according to three-dimensional data. It uses powdered metal pellets as raw materials and performs layer-by-layer printing and sintering to form parts. It can directly print complex and special-shaped parts without the need for molds. Like other processes, metal 3D printing technology has a short cycle and low cost. It has great freedom in processing parts with free shapes and complex features. It is based on digital models and can directly print finished products according to three-dimensional data. It uses powdered metal pellets as raw materials and performs layer-by-layer printing and sintering to form parts. It can directly print complex and special-shaped parts without the need for molds. The printing process also has limitations. In order to prevent deformation of parts during the printing process, it is necessary to add printing supports to the parts according to the actual situation. The supports need to be removed after printing. The fewer supports there are, the easier it is to remove the supports later [2]. Therefore, support design optimization and support-free technology exploration have become the direction of technical development. At the same time, due to the diversity of printing materials and printing processes, the mechanical properties of printed parts will vary. Therefore, conventional parameter powder materials and conventional printing parameters are used to print and test part test bars, establish the minimum mechanical performance indicators of aluminum alloy and stainless steel, and compare the mechanical properties with traditional process parts, and develop metal 3D printing technology application scenarios in automotive parts.

2 Exploration of metal 3D printing process technology

At present, metal 3D printing technology is constantly innovating. For metal 3D printing technology, it has its own advantages and difficulties. In the process of selective laser melting (SLM), metal powder is locally melted and solidified instantaneously under the action of high-energy laser, and its temperature gradient is extremely large. There is a large and complex internal stress inside the workpiece, which will cause serious warping and deformation of the workpiece, resulting in the inability to continue the SLM process. In order to ensure that the workpiece can be sintered smoothly, it is necessary to provide necessary support for the workpiece to be sintered. At the same time, at the edge of the workpiece contour, because the outside of the workpiece contour is all powder with poor thermal conductivity, the heat conduction process of the molten pool is different from that of the inside, so the edge of the workpiece contour in the same layer is higher than the inside of the workpiece, forming a contour protrusion; in order to deal with these unstable factors in the above-mentioned SLM process, the workpiece structure must be reasonably designed, and a certain support structure must be added to the workpiece before sintering to stabilize the sintering process. Therefore, a reasonable optimization support scheme is the basis of metal 3D printing. This paper introduces the basic principles of metal 3D printing, printing support optimization, and support-free technology.

2.1 Basic principles of metal 3D printing

There are 6 main working modes of metal 3D printing: SLM, Electron Beam Melting (EBM), Laser-Engineered NET Shaping (LENS), Binder Jetting (BJ), Double Insulating Wall (DIW), Ultrasonic Additive Manufacturing (UAM); the main molding principles of these 6 working modes are as follows.

2.1.1 SLM process principle

Through laser scanning, the preset metal powder is quickly melted to directly obtain parts of any shape, with a density of more than 99.9% and an accuracy of ±0.05 mm/100 mm (within 100 mm length, ±0.05 mm tolerance). The light beam is emitted from the laser (Laser), and the light beam is reflected to the platform through the galvanometer system (X-Y scanning mirror) for layer-by-layer melting. After each layer of melting is completed, the printed part is lowered layer by layer until the entire part is printed out; it can manufacture molded parts with complex structure, metallurgical bonding, dense organization, high dimensional accuracy and good mechanical properties. The mechanical performance indicators are better than castings, and can even reach the level of forgings; at the same time, it can shorten the part production cycle, without the need to open molds, and reduce costs. The specific principle diagram is shown in Figure 1.

2.1.2 EBM technology principle

EBM is a powder bed melting technology and needs to be processed in a protective gas atmosphere created by inert gas. The main differences from SLM are as follows (Figure 2).

a. The energy source is different. EBM uses high-energy electrons generated by filaments to melt powder, and the maintenance cost is relatively low.

b. Before EBM processing, each layer of powder needs to be preheated according to the material properties. This behavior can reduce the temperature gradient during the manufacturing process, thereby reducing the internal stress of the additively manufactured parts.

2.1.3 LENS Technology Principle

LENS is another process that uses high-power lasers for additive manufacturing (Figure 3). Unlike SLM, LENS uses an inert gas flow to drive the flow of raw materials to achieve powder delivery and injection (the powder particle size is generally larger than that of SLM materials). The powder injection axis coincides with the axis of the laser. The laser melts the injected powder in real time and solidifies it on the substrate. As the substrate or laser head moves, the three-dimensional manufacturing of the part is realized. The ideal melting point is at the intersection of the laser and the injected powder, which can directly realize the additive manufacturing of various materials. LENS can also realize the additive manufacturing of metal wires. The unique advantage of LENS is that it can repair damaged parts or transform parts on the existing basis.

2.1.4 BJ Technology Principle

BJ is another additive manufacturing technology based on powder bed (Figure 4). In the BJ process, no external high-energy heat source is required to melt the metal powder. Instead, the powder is sprayed with a suitable binder to the powder bed so that the powder is bonded according to the specified morphology and stacked layer by layer to achieve the manufacturing of three-dimensional parts. However, the metal parts manufactured by BJ need to be degreased and sintered at high temperature to achieve the expected performance. This process will cause the size of the metal parts directly obtained by BJ manufacturing to shrink to a certain extent. One of the outstanding advantages of BJ manufacturing technology is that it can realize the direct manufacturing of foam parts or high-porosity parts.

2.1.5 Principle of DIW Technology

DIW is an additive manufacturing technology that uses air pressure to extrude a mixed slurry from a syringe and deposits it layer by layer into a three-dimensional part according to a specified morphology (Figure 5). The slurry is made by mixing raw material powder with a binder, solvent or dispersant. The quality of the printed parts depends largely on the properties of the mixed slurry (such as uniformity, viscosity, etc.). Metal parts directly obtained by DIW manufacturing also need to be degreased and sintered, and the final metal parts can also have high porosity.

2.1.6 Principle of UAM Technology

The initial material of UAM technology is metal foil/sheet. Under the action of positive pressure, the ultrasonic head applies ultrasonic vibration on the surface of the metal foil to cause rapid friction between the two layers of metal foil to break the original state of the material and produce metallurgical bonding to achieve interlayer connection. The three-dimensional parts are finally manufactured by layer-by-layer accumulation (Figure 6). The application of UAM to foil/sheet gives it a unique advantage in the preparation of metal laminated parts. At present, it has been applied to the preparation of honeycomb structure parts and sandwich structure parts. The shortcoming of UAM is that it cannot directly obtain complex three-dimensional structures. It is necessary to remove the excess parts through mechanical processing after each layer or the whole is processed. This is one of the reasons why it has not been widely used.

Combined with the current exploration and actual application of the additive manufacturing industry, the two technologies of SLM and LENS are relatively mature. Other process forms need to continue to explore technology upgrades to meet different process forms and performance requirements. The metal 3D printing process currently used is SLM selective laser melting. The following support structure optimization and support-free technology exploration are based on the SLM process form.

2.2 Metal 3D Printing Support Process Exploration

The following process support exploration is based on the SLM selective laser melting process form, mainly including support optimization exploration and support-free technology exploration. 2.2.1 Support Optimization Exploration The reason why the SLM metal 3D printing process needs to add support during the printing process is the following two reasons.

a. Internal stress. Under the action of high-energy laser, metal powder melts and solidifies locally and instantaneously, and its temperature gradient is extremely large. There is a large and complex internal stress inside the workpiece, which will cause serious warping and deformation of the workpiece.

b. Warping deformation. At the edge of the workpiece contour, because the outside of the workpiece contour is all powder with poor thermal conductivity, the heat conduction process of the molten pool is different from that of the inside. Therefore, the edge of the workpiece contour in the same layer is higher than the inside of the workpiece, forming a contour protrusion. At the same time, in the suspended structure of the workpiece, due to the tensile stress generated by the sintering of the top layer of the workpiece, the edge of the workpiece will warp upward. As the warping accumulates layer by layer, obvious overall deformation of the workpiece will occur (Figure 7).

Based on the above reasons, in order to cope with the uncertainties in the above-mentioned SLM printing process, the workpiece structure must be reasonably designed, and a certain support structure must be added to the workpiece before sintering to stabilize the sintering process. Adding process support mainly plays the following roles.

a. Heat conduction. When sintering a suspended structure, if the laser scans directly on the powder, the heat will not be dissipated in time, and the temperature of the molten pool will be too high, which is prone to spheroidization, oxidation and other phenomena. The support structure under the suspended structure can play a relatively good heat conduction role, making the sintering process stable.

b. Prevent deformation. During the printing process, a large and complex internal stress will be generated inside the workpiece. Usually, the top layer always produces a large tensile stress, which is manifested as the edge of the workpiece warping upward. As the layers accumulate, the deformation of the workpiece is macroscopically manifested as the most serious edge warping. Especially when there is a suspended structure, this warping is more obvious. Making a support structure under the suspended structure can effectively hold the workpiece and resist warping.

c. Prevent the fixed workpiece from being scraped away by the scraper. In order to ensure the smooth processing of unstable workpieces (such as workpieces with a large height-to-diameter ratio), additional support can be made on one side of the workpiece scraper moving direction.

d. Support the molding of small angle surfaces and suspended surfaces.

e. Support the molding of horizontal holes and runners.

For the role of the above process support, the following two aspects are mainly explored.

a. When different diameter cylindrical supports are stably sintered, what is the range of height-to-diameter ratio, and the processing process can proceed smoothly;

b. Reasonable optimization of support.

2.2.1.1 Exploration of the range of height-to-diameter ratio when different diameter cylindrical supports are stably sintered

Support stability means that if the height of a support with a smaller cross-sectional area is relatively high, there will be a risk of being hit and swung by the scraper, thereby affecting the printing effect of the part. Therefore, the support strength and support surface size directly affect the support stability. The higher the support strength, the smaller the projection length of the support surface in the scraper direction, and the larger the projection length of the support surface in the direction perpendicular to the scraper, the higher the height at which the support can be stably sintered. In order to explore the maximum printing height of cylindrical supports at different diameters, ten groups of cylindrical supports with different diameters were printed, and scraper impact and swing tests were performed. Finally, the actual test results were obtained to guide subsequent practical applications. The specific test results are shown in Table 1.

2.2.1.2 Reasonable optimization of support

For the maximum height of the cylindrical support that can be stably sintered through actual printing tests, some parts need to be designed with a support height greater than the maximum height during the actual printing process. For such supports, if they are not reasonably optimized, the supports will be scraped down by the hanging knife and the printing strategy will fail. Therefore, the supports with actual support heights greater than the maximum height of stable sintering are reasonably optimized to achieve the purpose of support reinforcement. At the same time, the relevant printing data parameters will be explored to guide subsequent actual production. The specific printing process exploration process of printed parts is as follows.

Figure 8 shows a cylindrical support with a large height-to-diameter ratio. The lower right surface of the upper part of the part is a suspended surface with a small area and a high position. The high small area support is at risk of being knocked off by the scraper during the sintering process. At this time, a conical support structure with a large bottom and a small top can be made to ensure its stability, but the angle between the bevel generatrix of the cone and the ground plane needs to be explored. Therefore, after printing exploration at different angles, when the angle between the bevel generatrix and the ground plane is less than 70°, the conical support can prevent being knocked off by the scraper, as shown in Figure 9.

The above is the practical application of two aspects of support optimization, which is used to guide printing work. The subsequent printing process needs to continue to be explored and studied in depth, and there are still many difficulties to be solved.

2.2.2 Exploration of unsupported technology

One advantage of metal 3D printing technology is that it can print internal channels and internal holes. It is difficult to make internal channels and internal holes through traditional processes. However, the unsupported suspended holes will have a worse closure effect as the aperture increases. Similar to unsupported structures, the downward surface will begin to distort as the width of the hole or channel increases. In other words, due to the gravity of the top of the circular hole in the circular hole structure, the top will sink during the printing process, and the circular hole structure will eventually become an elliptical structure. Therefore, there are two aspects to explore the printing of circular hole structure parts. On the one hand, the critical value of the aperture that does not change during the printing process of the suspended hole is explored; on the other hand, when the size of the circular hole structure of the part is greater than the critical value, the common solution is to design the circular hole channel into an ellipse, teardrop or diamond shape. Therefore, the ellipse size and shape corresponding to the specific numerical aperture is also an aspect of exploration.

2.2.2.1 Exploration of critical value of suspended hole printing

First, the critical value of the aperture that remains unchanged during the printing process of suspended holes was explored. By printing circular holes of different diameters of aluminum alloy (Al-Si10Mg) material, the roundness of the circular holes was measured. The roundness tolerance was qualified within the free tolerance range. A group of circular holes of Φ1~Φ10 were printed and the roundness tolerance was measured. It was concluded that the roundness between Φ1~Φ8 met the free tolerance requirements, while the roundness tolerance of circular holes with a diameter greater than Φ9 did not meet the free tolerance requirements. Therefore, in actual printing work, the aperture less than or equal to Φ8 does not need to add support. The roundness tolerance measurement is shown in Table 2.

The printing effect of the critical value of suspended holes of different diameters is shown in Figure 10.

Through actual printing exploration, it is recommended that the diameter of the flow channel should not exceed 8 mm for subsequent printing production. If it exceeds 8 mm, support structures are required to assist in molding. However, the removal of the inner flow channel support is a process problem. Therefore, how to print an inner flow channel with a diameter exceeding 8 mm without adding support is currently a process difficulty and a development direction of support-free technology.

2.2.2.2 Exploration of unsupported structure of elliptical inner flow channel

When the inner flow channel to be printed has a hole diameter greater than 8 mm, explore an elliptical inner flow channel printing scheme (Figure 11). Select the hole diameter of the exploration printing to be 10 mm, place the short side of the ellipse horizontally, set the short side size to 10 mm, and place the long side size vertically. The specific size needs to be explored. First, the basic size explored is 12 mm. It is increased or reduced according to the actual printing situation. Finally, the size used to guide engineering practice is obtained. After printing, the inner hole roundness tolerance is measured. The roundness tolerance meets the free tolerance requirements. After the printing test, when the long side of the ellipse is 11.4~11.8 mm, the inner hole roundness tolerance can meet the requirements, and the average value is 11.6 mm, therefore, the ellipse equation that can guide subsequent production is: see formula (1) in the figure

After the exploration of the above process, it is used to guide the preparation of the printing process plan for subsequent actual production. However, different circular hole diameter sizes need to be further explored, and finally a database is formed to guide practice.

3 Performance indicators of metal 3D printed parts

In-depth research has been conducted on the mechanical properties of metal 3D printed parts. The ultimate development goal of metal 3D printing technology in automotive parts is application; currently, metal 3D printed parts mainly include aluminum alloy (AlSi10Mg) and stainless steel (316L). The mechanical properties of the two materials are different. Therefore, the mechanical properties of the two materials are very important parameter indicators. In-depth exploration and research have been conducted on the mechanical performance parameter indicators, density level (flaw detection level), surface roughness, etc. of the parts to provide data reference and support for the application of metal 3D printing technology in automotive products, and form standard documents at the same time.

3.1 Test of mechanical properties parameters of printed test bars

The mechanical properties test bars were printed at 0°, 45°, and 90° respectively by metal 3D printing process, and aluminum alloy material AlSi10Mg and stainless steel material 316L were printed respectively. The mechanical properties were tested at 0°, 45°, and 90°. The minimum value was taken as the standard numerical reference value to provide standard data support for automotive product application and automotive product research and development. The specific test results are shown in Table 3 and Table 4.

According to the above mechanical test bar test results, the conclusions in Table 5 are obtained.
The specific printed test bar is shown in Figure 12.
The specific metallographic diagram is shown in Figures 13 and 14.

3.2 Part density and surface quality grade parameter indicators

At present, the trial production institute has established the self-made metal 3D printing capability, and has conducted test statistics for all the parts that have been printed. The density grade and surface quality test statistics of metal 3D printed parts are as follows.

The surface roughness of aluminum alloy AlSi10Mg is Ra6~10 μm, density is 99.7%, flaw detection level is 1~2, and the size tolerance is (100 ± 0.10) mm;

The surface roughness of stainless steel 316L is Ra4~8 μm, density is 99.8%, flaw detection level is 2~3, and the size tolerance is (100±0.10) mm;

The above parameters are the reference results of the statistics of various parameters of the printed parts. The performance index parameters of metal 3D printed parts will continue to improve with the improvement of equipment performance and material properties. At present, the statistics are made for the indicators that can be achieved for application reference.

4 Overview of Metal 3D Printing Applications

The main value of metal 3D printing lies in its application. The current metal 3D printing process technology can manufacture complex structural parts that are difficult to complete with traditional processes, and can achieve advantages such as functional integration, lightweight, and topological optimization. It has low cost, short cycle, and does not require molds. The materials that can be printed include stainless steel, nickel-based alloys, aluminum alloys, and other materials. Aluminum alloy AlSi10Mg and stainless steel 316L are the two most widely used materials in the automotive industry. There are many examples of metal 3D printing that have been printed. The more representative parts are as follows.

4.1 Engine hood

The engine hood is made of AlSi10Mg, with dimensions of 410 mm×320 mm×190 mm. The conventional manufacturing process is casting, with a cycle of 3 months and a cost of 800,000 yuan (opening the mold). The metal 3D printing process is low-cost and short-cycle; the manufacturing accuracy and mechanical properties of the parts are better than the casting process; the trial production cycle of the hood blank is shortened from 3 months to 0.5 months, saving 83% of the trial production cycle; the mechanical properties of the blank are better than those of the casting blank, the dimensional accuracy of the blank reaches the casting CT7 level, and the density reaches 99.7%; the printing time of the hood blank is 216 hours, the raw materials require 15.7 kg, and the total printing cost is 115,000 yuan/piece. This solution saves about 685,000 yuan compared with the metal mold trial production process, saving 85.63% of the cost. The printed parts are shown in Figure 15.

4.2 Transmission oil shower assembly

The transmission oil shower assembly is an important component of the lubrication system in the transmission. It is made of PA66 and has a size of 210 mm×80 mm×80 mm. The conventional manufacturing process is injection molding. It includes two components, oil shower structure 1 and oil shower structure 2. The two parts are connected as a whole through a sealing gasket interference fit. After connection, they are used as a whole. The assembly accuracy of the connection is high to prevent oil leakage from causing the oil shower pressure to decrease. In view of the complex assembly process of oil shower structure 1 and oil shower structure 2 and the characteristics of long injection molding cycle and high cost, the cycle is 2.5 months and the cost is 300,000 yuan (opening the mold); the metal 3D printing process is used to make the parts. For the two components of the oil shower, the metal 3D printing integrated process is used to print the two parts of the oil shower into a whole, realizing the exploration of integrated functional printing process; the printing time is 8 hours, the raw material required is 0.24 kg, and the total printing cost is 0.41 10,000 yuan/piece, this solution saves about 295,900 yuan compared with the metal mold trial production process, saving 98% of the cost and shortening the cycle by 2.4 months.

4.3 Engine single cylinder cylinder head

The material of the engine single cylinder cylinder head is AlSi10Mg, with a size of 210 mm×190 mm×110 mm. The conventional manufacturing process is casting, with a cycle of 3.5 months and a cost of 350,000 yuan (opening the mold). The metal 3D printing process is low-cost and short-cycle; the manufacturing accuracy and mechanical properties of the parts meet the design requirements; the cylinder head of Al⁃Si10Mg material was explored in the early stage, and the single cylinder cylinder head of stainless steel 316L material was explored this time. The blank trial production cycle is 0.5 months, saving 86% of the trial production cycle; the printing cost of the single cylinder cylinder head blank is 41,000 yuan/piece. After testing, the blank mechanical properties are better than the casting blank, and the blank size accuracy reaches the casting CT7 level; this scheme saves about 309,000 yuan compared with the metal mold trial production process, and the cost is saved by 88%. The specific printed parts are shown in Figure 17.

4.4 Catalyst intake cone

The catalyst intake cone has a complex structure, with a blade structure at the end, and a size of Φ190×170. It is difficult to achieve with traditional casting technology, with a cycle of two months, high cost, and a cost of 100,000 yuan. Currently, the intake cones of many products have been made by metal 3D printing technology. We use high nickel alloy (HX Hastelloy high temperature alloy) material for printing, with a cycle of 1.5 days/piece and a cost of 8,000 yuan/piece, saving 57 days of cycle and 84,000 yuan in cost. The specific printed parts are shown in Figure 18.

4.5 Differential oil baffle and oil guide groove

Differential oil baffle and oil guide groove are important parts of the lubrication system in the differential. The material is PA66, and the size is approximately 140 mm×70 mm×30 mm. The conventional manufacturing process is injection molding, with a cycle of 2 months and a total cost of 150,000 yuan (opening the mold) for three parts. The cycle is long and the cost is high; therefore, the metal 3D printing process is used for process exploration, and aluminum alloy material (AlSi10Mg) is used for printing. The cycle is 2 days and the cost is 6,000 yuan, which saves nearly 2 months of cycle and 144,000 yuan of cost. The strength is better than the original process plan, and it has been applied in small batches. The specific printed parts are shown in Figures 19 to 21.

5 Conclusion

This study mainly includes three aspects: the basic working principle of metal 3D printing process technology and exploration of metal 3D printing support process, exploration of parameters such as mechanical performance index and density of metal 3D printed parts, and actual printing application cases; these three aspects provide practical reference and theoretical basis for subsequent metal 3D product printing production, subtractive processing, part design and test verification, especially the accumulation of metal 3D printing process support data and mechanical performance index of printed parts are more important, which provides theoretical and practical guidance for the application of this technology in the automotive field. At the same time, a metal 3D printing automotive parts application database will be established in the future and accumulated continuously.

The application of metal 3D printing technology in the automotive field is being explored and is developing in the direction of topological optimization, lightweight, functional integration, etc. from the design end, especially in the field of functional integration, which has great prospects; currently many car companies are using metal 3D printing technology in the research and development and production stages of automotive products, making full use of the characteristics of this technology to promote the development of parts technology in the direction of customization, personalization, lightweight, etc.

There is still a need to continue exploring support-free technology for metal 3D printed parts. By developing support-free printing technology, metal 3D printing can break through existing technical bottlenecks and completely solve the problem that the internal flow channel support cannot be removed, so that it can be more widely used in automotive parts. The further development of support-free technology and support optimization technology and the gradual improvement of the entire industrial chain can realize the application of metal 3D printing technology in automotive parts in the future. Many product parts with complex internal flow channel structures can be continuously explored and applied, reducing costs, shortening trial production cycles, and better supporting product research and development.