1. Introduction
TiB₂ ceramic particle – reinforced aluminum matrix composites are high – performance materials. By dispersing TiB₂ ceramic particles in the aluminum matrix through in – situ reactions, the properties of the materials are significantly enhanced. These composites possess excellent characteristics such as high melting point, high hardness, high elastic modulus, good chemical stability, and corrosion resistance. As a result, they find extensive applications in various fields, especially in the underwater equipment industry, where they are commonly used for manufacturing underwater pressure – bearing thin – walled shells.
However, during the casting process of these thin – walled shells, some defects like small pores, micro – slag inclusions, and sand holes often occur. When these defects are present on the important sealing parts or surfaces of the shell, they can severely affect the subsequent processing efficiency and the reliability of the final product. Conventionally, castings with such defects are usually scrapped or recycled, leading to a waste of raw materials and an increase in production costs. Therefore, finding an effective and economic method to repair these defects is of great significance.
Welding is a potential solution for defect repair. However, welding TiB₂ – aluminum matrix composites is challenging due to the significant differences in physical and chemical properties between the ceramic reinforcement phase and the matrix alloy. For example, during the solidification of the molten pool, when the heating temperature reaches the melting point of the matrix, the reaction between Al and the reinforcement phase can form brittle compounds, which seriously deteriorates the joint performance. In addition, the solid – state reinforcement phase increases the viscosity of the molten pool, reduces its fluidity, and may cause defects such as pores and cracks after welding. Moreover, the large differences in expansion coefficients and thermal conductivities between the matrix and the reinforcement phase can lead to a large number of micro – area residual stresses at the interface after the welding thermal cycle, further weakening the joint performance.
Among the various welding methods, laser welding has unique advantages. It can precisely control the depth and width of the weld by adjusting parameters such as laser power and focal length. With its high energy density, low heat input, small thermal deformation, and narrow melting and heat – affected zones, laser welding can not only improve the welding speed but also minimize the heat diffusion to the surrounding area, reducing the size of the heat – affected zone. Additionally, the small welding deformation can greatly reduce the subsequent processing workload, shorten the production cycle, and improve production efficiency. Therefore, laser welding shows great potential for repairing the surface micro – defects of castings.
This article focuses on the welding repair of the important surface point – like defects of TiB₂ – ceramic – reinforced aluminum matrix composite castings. Through a series of experiments, we study the microstructure and properties of the welded materials, the performance changes of the surface – treated coatings, and the impact of laser repair welding on the corrosion resistance of the anodized and nickel – phosphorus – plated coatings of the castings.
2. Experimental Materials and Methods
2.1 Experimental Materials
In this experiment, TiB₂ – ceramic – reinforced aluminum matrix composite casting test pieces with a size of 100 mm×50 mm×5 mm were used. The surface of the test pieces was punched to simulate casting defects. The chemical composition of the test piece material is shown in Table 1.
| Component | Si | Mg | Zr | Ti | B | Impurities | Al |
|---|---|---|---|---|---|---|---|
| Content | – | – | – | – | – | – | Balance |
| Table 1: Main Chemical Composition of TiB₂ – Ceramic – Reinforced Aluminum Matrix Composite (Mass Fraction) (%) |
The choice of welding wire is crucial as it affects the composition, structure, liquidus temperature, solidus temperature, hot – crack resistance, corrosion resistance, and mechanical properties of the weld metal at normal, high, or low temperatures. Considering the principle that the content of corrosion – resistant elements (such as Mg, Mn, Si, etc.) in the welding wire should not be lower than that in the base material, an ER4043 aluminum – silicon welding wire with a diameter of 0.8 mm was selected as the filler material. The chemical composition of the ER4043 welding wire is shown in Table 2.
| Brand | Si | Fe | Cu | Mg | Zn | Ti | Al |
|---|---|---|---|---|---|---|---|
| Content | – | – | – | – | – | – | Balance |
| Table 2: Chemical Composition of ER4043 Welding Wire (Mass Fraction) (%) |
2.2 Experimental Methods
A GY – FLW1500 handheld laser welding machine was used in this experiment, and argon gas with a purity of more than 99.9% (φ(Ar)) was used as the shielding gas. Depending on the size and depth of the defects, laser single – spot welding, double – spot welding, and continuous – scanning repair welding processes were employed. The specific welding process parameters are shown in Table 3.
| Laser Power/W | Laser Frequency/Hz | Galvanometer Speed/(mm·s⁻¹) | Galvanometer Width/mm |
|---|---|---|---|
| 900 – 1500 | 7000 – 10000 | 300 – 600 | 1 – 4 |
| Table 3: Laser Welding Process Parameters |
Before welding, the defects were cleaned thoroughly using a triangular scraper or an electric milling cutter, and the sharp corners were removed. The oxide layer on the surface of the welding area was also removed until the metal luster was exposed, and then the area was cleaned with alcohol. Before starting the welding, the shielding gas was turned on to ensure that the welding area was completely surrounded by the shielding gas, expelling the surrounding air. After the welding was completed, the gas supply was stopped with a certain delay to fully protect the welded parts from being affected by other gas components.
3. Microstructure Analysis and Property Changes after Repair Welding
3.1 Macro – Morphology Analysis of the Laser Repair Layer
After the test pieces were repaired by welding, the solder joints were cut for analysis. The cutting position and the macroscopic morphology of the cross – section are shown in Figure 1.
Figure 1: Surface and Cross – Section Macro – Morphology of the Repair Layer
As can be seen from the surface and cross – section morphology after laser repair, whether it is single – spot welding, double – spot welding, or continuous – scanning repair welding, there are no defects such as pores, cracks, or inclusions in the laser – repaired cross – section layer. The solder joint is full, the joint is fully penetrated, the weld width is large, and there is no collapse phenomenon. The surface of the repaired joint has an aesthetic appearance, and the joint is smooth with a metallic luster, showing no obvious oxidation. From the perspective of macroscopic fusion, the welding wire filler and the base material are well – combined. The welding time in spot welding is short, and the base material is minimally affected by the welding thermal cycle. The microstructure of the heat – affected zone is basically unchanged and is similar to that of the matrix.
3.2 Microstructure Metallographic Analysis of the Repair Layer
The metallographic analysis was carried out in accordance with GB/T 13298—2015 “Metallographic Microstructure Inspection Method”. Three repaired welding samples were cut from the test pieces, and each sample had a size of 5 mm×5 mm with a length of 10 – 15 mm. The metallographic samples were prepared by the resin embedding method (Figure 2), and after preparation, they were ground, polished, and the obtained metallographic photos are shown in Figure 3.
Figure 2: Metallographic Sample
Figure 3: Metallographic Structure of the Solder Joint of the Repair Layer
According to Figure 3a, the white substance is the aluminum alloy matrix, and the black substance is the second – phase TiB₂ ceramic strengthening phase. Since the TiB₂ reinforcement phase is nanoscale, there are a large number of black agglomerated substances in the figure, which are distributed in a network in the matrix. The agglomeration of TiB₂ is severe, and the TiB₂ particles are in – situ generated with a small particle size and uniform distribution. The wettability between TiB₂ and the aluminum matrix is good, so no particle segregation occurs during the welding process. In Figure 3b, the agglomerated TiB₂ is much more dispersed compared to the base material structure. The laser pulse disperses the agglomerated TiB₂. This is because the pulse has a certain stirring effect on the molten pool. It may also be due to the presence of low – melting – point elements such as Mg and Zn in the matrix. During the welding process, these elements preferentially melt and vaporize, generating vapor pressure that can push the TiB₂ apart, making the second – phase distributed in a network in Figure 3a evenly distributed and eliminating the second – phase network structure. In Figure 3c, it can be seen that the interface structure between the repair layer and the matrix is evenly distributed, and there is no obvious reaction product generated at the interface. No brittle phases such as Al₃Ti and AlB₂ are formed. From the microscopic morphology of the solder joint, the structure after repair welding is relatively ideal. The structure of the solder joint is evenly distributed, no brittle phase is generated at the interface, and the solder joint is firmly and reliably combined with the matrix.
3.3 Micro – Hardness Measurement
A micro – hardness tester was used to measure the micro – hardness of the repaired solder joints and the surface of the surrounding base material. The load was 50 g, and the holding time was 10 s. The results are shown in Table 4.
| Welding Process | Hardness HV0.05 | Average Hardness HV0.05 |
|---|---|---|
| Single – Spot Welding | 94.131, 95.803, 92.676, 91.203 | 93.453 |
| Double – Spot Welding | 74.004, 81.975, 68.471, 74.816 | 74.816 |
| Continuous – Scanning Repair Welding | 96.598, 109.537, 92.013, 99.382 | 99.382 |
| Base Material Hardness | – | – |
| Table 4: Micro – Hardness Measurement of Repaired Solder Joints |
It can be seen from Table 4 that the hardness of the repair layer by single – spot welding and continuous – scanning repair welding is equivalent to that of the base material. This indicates that single – spot welding and continuous – scanning repair welding have little impact on the hardness performance of the TiB₂ – reinforced aluminum matrix composite. Generally, there is a positive – proportional relationship between the hardness and strength of metal materials. Therefore, it can be inferred that laser repair welding has little impact on the pressure – bearing performance of the shell casting. However, double – spot welding repeatedly heats the matrix, causing the material to anneal and soften, resulting in a decrease in hardness, which has an adverse effect and should be avoided.
4. Impact of Repair Welding on the Corrosion Resistance and Properties of Surface – Treated Coatings
4.1 Macro – Corrosion of the Coating
After spot welding, the test pieces were subjected to anodizing treatment. All the test pieces were placed in seawater at room temperature for 120 days. After the seawater immersion corrosion was completed, the macro – corrosion of the coating was observed, as shown in Figure 4.
Figure 4: Coating after Seawater Corrosion
In Figure 4, the solder joint is black, and the base material around the solder joint is silver – white. The color difference between the solder joint and the base material coating is due to material differences. After corrosion, neither the solder joint nor the base material around the solder joint shows coating peeling, and no corrosion products are generated. The anodized coating on the solder joint and its surrounding area is evenly attached, and the coating is continuous and reliable, still effectively protecting the matrix.
The test pieces after spot welding and chemical nickel – phosphorus plating were immersed in a 25% NaOH solution at 90°C for 2.5 h. After the strong – alkali corrosion was completed, the macro – corrosion of the coating was observed, as shown in Figure 5.
Figure 5: Coating after Strong – Alkali Corrosion
In Figure 5, the solder joint is silver – gray, and the base material is silver – white. The color difference between the solder joint coating and the base material coating is not significant. After corrosion, there are no defects such as blistering, peeling, or 起皮 around the solder joint, and no corrosion products are found on the surface.
4.2 Changes in Coating Thickness and Hardness
After the seawater immersion corrosion was completed, one anodized and corroded test piece was selected. Three solder joints (No. 1, No. 2, and No. 3) on this test piece were selected to measure the coating thickness and micro – hardness, and three points were also selected on the base material around the solder joints to measure the coating thickness and micro – hardness. Each point was measured 6 times, and the average value was taken. Tables 5 and 6 show the average thickness and average micro – hardness values of the anodized coating on the base material around the solder joints and at the solder joints.
| Location | 1# | 2# | 3# |
|---|---|---|---|
| Coating Thickness of Base Material around Solder Joints/μm | 32.2 | 32.3 | 32.4 |
| Coating Thickness of Solder Joints/μm | 29.0 | 27.3 | 30.3 |
| Table 5: Average Thickness of Anodized Coating/μm | |||
| Location | 1# | 2# | 3# |
| — | — | — | — |
| Micro – Hardness of Base Material around Solder Joints HV0.1 | 481 | 464 | 460 |
| Micro – Hardness of Solder Joints HV0.1 | 489 | 489 | 480 |
| Table 6: Average Micro – Hardness of Anodized Coating HV0.1 |
After the chemical nickel – phosphorus – plated test pieces were subjected to strong – alkali corrosion, similar to the anodized coating test method, the coating thickness and coating hardness of the base material around the solder joints and at the solder joints of one corroded test piece were measured. The measurement results are shown in Tables 7 and 8.
| Location | 1# | 2# | 3# |
|---|---|---|---|
| Coating Thickness of Base Material around Solder Joints/μm | 57.1 | 64.3 | 60.3 |
| Coating Thickness of Solder Joints/μm | 56.9 | 65.3 | 61.9 |
| Table 7: Average Thickness of Chemical Nickel – Phosphorus Coating/μm | |||
| Location | 1# | 2# | 3# |
| — | — | — | — |
| Micro – Hardness of Base Material around Solder Joints HV0.1 | 510 | 566 | 514 |
| Micro – Hardness of Solder Joints HV0.1 | 514 | 572 | 542 |
| Table 8: Average Micro – Hardness of Chemical Nickel – Phosphorus Coating HV0.1 |
By comparing the coating thickness and micro – hardness of the base material around the solder joints and at the solder joints, it can be seen that the changes in coating thickness and hardness before and after repair welding are very small. The corrosion resistance of the surface – treated solder joints after repair welding is equivalent to that of the matrix.
5. Conclusion
(1) The laser spot – welding repair process has a small heat input. The size of the TiB₂ reinforcement phase in the joint structure is significantly refined. The interface between the reinforcement and the matrix is clean, and the solder joint is firmly and reliably combined.
(2) After the repaired castings are surface – treated, except for the color difference at the solder joints, the indicators such as the continuity, thickness, and hardness of the coating can meet the requirements of seawater corrosion resistance.
6. Future Research Directions
Although this study has achieved certain results in the laser spot – welding repair of TiB₂ – aluminum matrix composite castings, there are still some aspects that can be further explored.
Firstly, the long – term corrosion resistance of the repaired parts in more complex environments needs to be studied. In actual service conditions, the underwater equipment may be exposed to a variety of chemical substances and different temperature and pressure conditions. Investigating the performance of the repaired parts under these complex conditions can provide more reliable data for practical applications.
Secondly, the influence of different laser welding parameters on the mechanical properties of the repaired parts can be further optimized. Although the current research has obtained some basic data on the influence of welding parameters on hardness, a more in – depth study on the impact of parameters on other mechanical properties such as tensile strength and fatigue strength is necessary to obtain more accurate welding process guidelines.
Finally, the development of new filler materials or surface treatment methods to further improve the performance of the repaired parts is also a promising research direction. For example, developing filler materials with better compatibility with TiB₂ – aluminum matrix composites can enhance the quality of the weld and improve the overall performance of the repaired parts.