1. Introduction
Regulations on automotive fuel efficiency and emissions have been reinforced due to global environmental problems, and vehicle weight reduction has become increasingly important along with changes in electric vehicle trends. Materials, engineering methods, and structural technologies are required for vehicle weight reduction. Among them, technology to apply lightweight materials is effective1).
The main materials used for vehicle weight reduction are alloy metals (e.g., aluminum, magnesium, titanium, and high strength steel) and non-metallic materials (e.g., plastics and ceramics). Aluminum is suitable for weight reduction as its specific gravity is approximately a third of that of iron, and it has been widely used in automotive parts due to its high stiffness and ease of recycling2). Aluminum alloy extrusions are produced by subjecting aluminum plates to the rolling process. They are used for the car exterior and structural parts that require high strength and machinability. On the other hand, casting materials are used for engines and chassis parts because they can provide complex geometry and high strength3). A combination of aluminum alloy extrusions and cast materials can implement optimal performance, including vehicle part weight reduction, strength optimization, and process efficiency improvement, using the complementary physical properties and manufacturing processes of the two materials. To this end, various joining technologies, such as mechanical joining, adhesives, and welding, are used. Mechanical joining can avoid the thermal deformation problem between the two materials, but the weight reduction rate decreases compared to other methods due to incidental materials, such as rivets and screws. High-strength adhesives are used when welding and mechanical joining are difficult, but additional environmental factors can occur. Welding is eco-friendly and involves low process cost, but mechanical strength can be reduced by the difference in thermal expansion coefficient between dissimilar aluminum alloys, the internal pore defects caused by high solubility, and the change in the microstructure of the heat affected zone4,5). In particular, internal pore defects significantly reduce joint quality and durability. Pores occur because a large amount of hydrogen is generated in the form of bubbles due to temperature changes when molten aluminum solidifies6-9). Aluminum castings contain micropores in the material itself, and welding them generates a large amount of internal pores due to hydrogen expansion. Internal pores occur non-uniformly under the influence of welding conditions and the surrounding environment, resulting in quality variation6,7). Fig. 1 shows the geometry of the internal pores caused by the welding process. Fig. 2 shows the hydrogen solubility of aluminum according to the temperature.
On the other hand, the insert casting method injects the molten cast material after placing an insert in the mold during the aluminum casting process. This method is effective for metal-non-metal bonding as well as metal-metal bonding, and can manufacture products with complex geometry10). It is also favorable for joint strength improvement because it causes no mechanical damage or deformation and the quality variation caused by internal pore defects is low6,11). The application of the insert casting method for joining aluminum extrusions and cast materials can reduce the quality variation of the casting product caused by internal pores and improve the durability of the joint. To enhance joint quality through the insert casting method and maximize its effect, the mechanical bonding at the joint interface can be reinforced by applying the laser structuring process before joining12,13). Fig. 3 shows the process for joining aluminum extrusions and cast materials through the laser structuring process and the insert casting method. Laser structuring can control roughness by implementing specific geometry of the material surface, and it is commonly used as a technology to increase the joint strength between metals and other materials14).
Fig. 3
A process for joining aluminum extrusions and cast materials using laser structuring and insert casting methods
In this study, the laser structuring process was applied as the processing method before joining through insert casting, and the surface geometry implemented according to major process parameters was analyzed.
2. Experimental Method
The materials used for research on laser structuring process parameters were Al 6061 (extrusion) and A 365 (cast material, Al9SiMgMn), and specimens with a thickness of 3 mm were used. The specimens used had a size of 30 mm × 80 mm, and a joint area of 10 mm × 30 mm was applied in the joining test to verify the effectiveness of the laser structuring process.
Fig. 4 shows the laser system used for the laser structuring process.
For the laser, TruMicro7060 (Trumpf) with a maximum power of 1 kW, a pulse duration of 30 ns, a repetition rate of 5 to 100 kHz, and beam quality ≤ 10 mm·mrad was used. The beam size of approximately 250 μm implemented through the laser and optical system was used. Specimens were prepared using resin mounting and polishing equipment to analyze the surface geometry implemented through laser structuring, and cross-sectional geometry was analyzed using a confocal microscope (OLS4100, Olympus). Fig. 5 shows specimen preparation and analysis equipment for laser structuring cross-sectional analysis.
Fig. 5
System set-up for cross sectional analysis (a) Mounting equipment, (b) Polishing equipment and (c) Confocal microscope
Laser process parameters include the power, scan speed (SS), jump speed (JS), number of repeat, beam irradiation interval, and beam irradiation direction.
In this study, an experiment was performed by fixing the number of repeat at two, the beam irradiation interval at 400 μm, and the beam irradiation direction at a single direction. Fig. 6 shows the beam irradiation interval and direction. Detailed process conditions are listed in Table 1.
Table 1
Laser parameters
The geometry implemented through the laser structuring process can be divided into the width, depth, and recast layer size. The width is related to the beam size and pattern interval while the depth is associated with the power, SS, number of repeat, and pattern interval. The recast layer is the geometry created by the melting and re-solidification of the material during laser irradiation. It is closely related to undercut geometry implementation through the laser structuring process and the joint strength between materials. Fig. 7 shows the surface geometry implemented by the laser structuring process. The width, depth, and recast layer of the geometry can be seen.
Insert casting specimens were prepared to verify the effectiveness of the laser structuring process, and the joining test method was evaluated based on the MS 715-60 structural adhesive test standard, a standard of Hyundai Motor Company and KIA Corporation. When joining was performed using structural adhesives, the average shear strength was 25 MPa for the same joint area. Fig. 8 shows the region of laser structuring for the joining test of aluminum alloy extrusions and cast materials as well as the joint area and tensile direction for joint strength analysis.
3. Experiment Results and Discussion
Among the laser process parameters, JS represents the speed at which the beam moves to the next pattern when it is irradiated according to the pattern. When the geometry according to JS was analyzed, there was no significant difference in the implemented geometry as JS 5 mm/s resulted in a depth of 353.83 μm and a width of 231.35 μm and JS 600 mm/s led to a depth of 355.45 μm and a width of 210.51 μm. The geometry according to JS is shown in Fig. 9, and the process conditions and geometric data are listed in Table 2.
Fig. 9
Sectional images of laser structuring processed specimen with different JS; (a) 5 mm/s and (b) 600 mm/s
Table 2
Laser structuring parameter and shape of undercut (depth, width) different with JS
| Parameter | JS 5 mm/s | JS 600 mm/s |
|---|---|---|
| Power (W) | 600 | 600 |
| Repetition | 1 time | 1 time |
| Scan speed (mm/s) | 600 | 600 |
| Pattern pitch (μm) | 400 | 400 |
| Depth (μm) | 363.83 | 355.45 |
| Width (μm) | 231.35 | 210.51 |
The most influential factor in implementing depth geometry is the amount of heat supplied to the area of laser machining, which is highly correlated with the laser power and SS. The amount of heat increases if the laser power increases and SS decreases, and it decreases if the laser power decreases and SS increases.
Laser structuring geometry could be examined in the 400-1,000W laser power range and 400-1,000mm/s SS range. It was difficult to implement geometry when the laser power was too low. When the laser power was too high, the recast layer covered the geometry entrance due to overheating. Fig. 10 shows the specimen with overheating and the specimen with low heating among the specimens according to the power and SS.
Fig. 10
Sectional image of laser structuring processed specimen with heat input (a) Overheating specimen (Power 900 W, SS 500 mm/s), (b) Low heating specimen (Power 400 W, SS 1,000 mm/s)
Fig. 11 shows the depth according to the laser power and SS. In this instance, cases in which the cast material could not enter due to overheating and conditions that could not implement geometry due to low heating were excluded. It can be seen that the depth of the surface geometry increased as the laser power increased and SS decreased. The highest depth of 623 μm was observed when the laser power was 500 W and SS was 400 mm/s. On the other hand, the depth decreased as the power decreased and SS increased. It was found that laser structuring geometry was implemented at a laser power of 400 W or higher. The lowest depth of 56 μm was observed when the laser power was 400 W and SS was 1,000 mm/s. Specimens were prepared by applying a laser power of 600 W, SS 600 mm/s, and JS 5 mm/s to examine whether the surface geometry implemented under the application of the laser structuring process can improve the bonding force with the cast material. The cross-sectional analysis results confirmed the formation of hook-shaped undercut. (Fig. 9(a))
When the joining test was conducted using the specimens, a shear strength of approximately 44.4 MPa was confirmed. Fig. 12 shows the aluminum alloy extrusion and cast material joint specimens as well as the sectional image of the joint part. In the joining test results, the joint part was fractured, and the geometry was separated without the base metals being broken in most cases. No internal pore in the casting product was observed through the microscope. Pores were observed from the recast layer in the undercut geometry of the extrusion. It is judged that they did not affect the joint strength because there was no geometric deformation at the time of the fracture.
Fig. 12
Joint image of aluminum alloy extrusion and cast (a) Joint specimens (b) Sectional image of joint part (Power 600 W, SS 600 mm/s, JS 5 mm/s)
For the joint specimens that applied the laser structuring process and the insert casting method, the joint strength increased by approximately 1.78 times compared to the conventional structural adhesive method, confirming the effectiveness of the laser structuring process. It was also found that a depth of 363.83 μm and a width of 231.35 μm are sufficient conditions for the bonding of the molten cast material. It is deemed necessary to analyze the optimal geometry and conditions implemented according to the laser process parameters in detail through joint strength evaluation in the future.
4. Conclusion
In this study, the insert casting method and the laser structuring process were applied to enhance the joint quality of aluminum alloy extrusions and cast materials, and research was conducted on surface geometry according to laser process parameters (laser power, scan speed (SS), and jump speed (JS)).
1) There was no significant difference in surface geometry depending on JS, but further analysis is required to identify microcracks in the recast layer and fine width changes. Considering the time required for the process, a high JS value is favorable.
2) The surface geometry was correlated with the laser power and SS, and a maximum depth of 623 μm was observed when the power was 500 W and SS was 400 mm/s. Depth geometry was implemented at a laser power of 400 W or higher, and a sufficient width for the entry of the cast material could be implemented when SS was 400 mm/s or higher. It was, however, difficult to implement surface geometry when the amount of heat supplied was excessive or too small.
3) When the aluminum alloy extrusion and cast material joint was evaluated, a shear strength of 44.4 MPa was confirmed. This verified the effectiveness of the laser structuring process.
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