A cold extrusion-laser cladding integrated forming process for pistons

Abstract

This invention relates to the field of mechanical manufacturing technology and discloses an integrated cold extrusion-laser cladding forming process for pistons. Through the design of this integrated cold extrusion-laser cladding process, the invention deeply integrates substrate forming and surface strengthening, and pre-prepares an activation layer before extrusion, providing a clean and active ideal interface for subsequent cladding. This improves the metallurgical bonding quality and reliability between the cladding layer and the aluminum alloy substrate by utilizing the residual heat of extrusion, avoiding the risk of thermal damage to the strengthened substrate caused by traditional post-cladding. This process can ensure the density and overall strength of the piston substrate while directionally constructing functional alloy layers such as wear-resistant and heat-resistant layers according to the service requirements of different parts. It achieves proactive design and localized control of material properties on a single part, replacing the traditional insert process, simplifying production steps, and forming a comprehensive manufacturing solution for high-performance pistons.

Description

Technical Field

[0001] This invention relates to the field of mechanical manufacturing technology, specifically to an integrated cold extrusion-laser cladding forming process for pistons. Background Technology

[0002] Pistons, as a key reciprocating component, are widely used in various power machinery, pressure devices, and fluid transmission systems, such as internal combustion engines, compressors, and hydraulic cylinders. Their typical structure includes a head, annular grooves, and a skirt. The annular grooves and head end face, in particular, often operate under harsh conditions of high temperature, high pressure, and poor lubrication, enduring intense friction, wear, and thermal shock.

[0003] Currently, the mainstream manufacturing process for aluminum alloy pistons mostly involves casting or forging to obtain a blank, followed by machining to the final dimensions. To improve the wear resistance of critical areas such as the ring grooves, a common practice is to insert austenitic cast iron rings into the aluminum alloy piston. While this method can partially alleviate wear, the ring insert and the aluminum substrate are mechanically bonded to dissimilar materials, and their physical properties, such as their coefficients of thermal expansion, differ significantly. Under alternating thermal loads, the interface is prone to becoming a source of thermal resistance and stress concentration, posing a risk of loosening or promoting crack initiation. Furthermore, the ring insert process adds extra parts and assembly steps, leading to increased production costs and a more complex process.

[0004] To overcome the aforementioned drawbacks of the piston ring insert process, the industry has explored the use of laser cladding technology to enhance the performance of specific parts of the piston. This technology, as a surface modification technique, can prepare high-performance alloy coatings on the substrate surface. However, in existing technologies, laser cladding is typically applied as a post-processing step to already formed piston blanks or finished products. This simple process combination, with its high energy input during laser cladding, may cause over-aging softening of the already heat-treated and strengthened aluminum alloy substrate, impairing its mechanical properties. Furthermore, it is difficult to form a high-quality, defect-free metallurgical bond between the cladding layer and the substrate, especially with aluminum alloy substrates possessing a dense oxide film on the surface. Insufficient bonding strength can easily lead to coating peeling off under harsh operating conditions. Therefore, directly using laser cladding as a post-processing step does not fundamentally solve the problems of interface bonding reliability and substrate performance preservation.

[0005] On the other hand, from the perspective of blank forming technology, cold extrusion, as a near-net-shape forming technology, can be used to manufacture piston blanks with denser microstructure and higher material utilization. However, this technology itself focuses on the precise forming of macroscopic shapes, and its inherent process characteristics determine that it is difficult to achieve proactive design and differentiated distribution of material composition and properties in different parts of a single blank. This means that simple cold extrusion forming also cannot meet the special requirements of critical areas of the piston for wear resistance, heat resistance, and other properties.

[0006] In summary, existing technologies lack an integrated manufacturing method that can deeply integrate the precision forming of pistons with the performance enhancement of key components. This method needs to ensure the performance of the base material while achieving a high-strength metallurgical bond between the cladding layer and the base material in the areas requiring reinforcement, and effectively simplify the production process to meet the needs of a wider range of industrial applications. Therefore, a cold extrusion-laser cladding integrated forming process for pistons is proposed. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides an integrated cold extrusion-laser cladding forming process for pistons, thereby resolving the problems in the background technology.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a cold extrusion-laser cladding integrated forming process for pistons, comprising the following sequential steps: S1. Blank prefabrication and activation layer preparation: Provide aluminum alloy piston blanks, and prepare a pure aluminum or aluminum-silicon alloy activation layer on the blanks at a preset position corresponding to the piston finished ring groove area and / or top and / or skirt by cold spraying technology. The activation layer has a thickness of 50-200μm and a surface roughness Ra>10μm. S2. Cold extrusion composite molding: The blank with the activated layer is placed in a mold preheated to 150-300°C and cold extruded to obtain a piston matrix blank, wherein the activated layer undergoes plastic deformation during the extrusion process to achieve micro-mechanical interlock with the matrix. S3. In-situ laser cladding: Under the protection of inert gas, laser cladding is performed on the activation layer area on the piston substrate blank. The cladding material is a metal-based composite powder compatible with aluminum alloy. The high roughness, high activity and strain energy stored by cold extrusion of the activation layer are used to achieve low-temperature and high-quality metallurgical bonding between the cladding layer and the substrate. S4. Controlled heat treatment: The piston blank that has been clad is subjected to a heat treatment in one pass. This heat treatment process simultaneously optimizes the mechanical properties of the piston aluminum alloy matrix and the residual stress state and microstructure of the laser cladding layer. S5. Composite finishing: The heat-treated piston is machined, and the aluminum alloy substrate and laser cladding layer are simultaneously machined to the final dimensions.

[0009] Preferably, in step S1, the parameters of the cold spraying process are: the carrier gas is nitrogen or helium, the gas temperature is 300-600℃, the gas pressure is 2.0-4.0 MPa, the spraying distance is 10-30 mm, and the composition of the activation layer matches the composition of the piston substrate aluminum alloy but the oxygen content is less than 1.0 wt%.

[0010] Preferably, in step S3, laser cladding is performed when the surface temperature of the piston substrate blank is in the range of 80°C to 250°C after it is taken out of the mold; the laser cladding uses a oscillating laser head with an oscillation frequency of 50-500 Hz and an oscillation width of 1.5-3 times the spot diameter.

[0011] Preferably, in step S3, the metal-based composite powder is an Al-Si alloy powder as the binder phase, wherein 20-50 vol% of a ceramic reinforcing phase is uniformly dispersed therein. The ceramic reinforcing phase includes, but is not limited to, one or more of SiC, Al2O3, TiB2, and TiC, and its particle size is 5-50 μm.

[0012] Preferably, the metal-based composite powder adopts a core-shell structure design: its core is the ceramic reinforcing phase, and its shell is a eutectic alloy layer with the same composition as the Al-Si alloy powder but with a lower melting point.

[0013] Preferably, in step S4, the heat treatment in one pass is a graded aging treatment, specifically: first, holding at 150-180℃ for 2-4 hours, then raising the temperature to 190-220℃ and holding for 1-3 hours; this process can effectively reduce the thermal residual tensile stress in the cladding layer while ensuring that the substrate reaches the peak hardness of T6 or T7, and induce compressive stress.

[0014] Preferably, in step S5, a combination of PCD (polycrystalline diamond) cutting tool and CBN (cubic boron nitride) grinding wheel is used to process the piston, wherein the PCD cutting tool is used to turn / mill the aluminum alloy substrate and the cladding layer, and the CBN grinding wheel is used to perform precision grinding on the side of the annular groove formed by the cladding layer.

[0015] Preferably, in the laser cladding process of step S3, a gradient cladding strategy of dividing regions and powders is adopted: firstly, a layer of metal-based composite powder with a reinforcing phase volume fraction of 20-30 vol% is clad on the activated layer, and then a layer of metal-based composite powder with a reinforcing phase volume fraction of 30-50 vol% is clad on it.

[0016] Preferably, in step S3, by controlling the laser power and scanning speed, the depth of the molten pool of laser cladding is ensured to be at least 1.2 to 1.8 times the original thickness of the activation layer, so as to achieve full metallurgical bonding of the cladding material, the activation layer and the substrate.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention integrates cold extrusion and laser cladding into a single process, deeply fusing substrate forming and surface strengthening. An activation layer is pre-prepared before extrusion, providing a clean and active interface for subsequent cladding. This utilizes the residual heat from extrusion to improve the metallurgical bonding quality and reliability between the cladding layer and the aluminum alloy substrate, avoiding the thermal damage risk to the strengthened substrate associated with traditional post-cladding. This process ensures the density and overall strength of the piston substrate while directionally constructing functional alloy layers, such as wear-resistant and heat-resistant layers, according to the service requirements of different parts. It achieves proactive design and localized control of material properties on individual parts, replacing the traditional insert process, simplifying production steps, and forming a comprehensive manufacturing solution for high-performance pistons.

[0018] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures pointed out in the description, claims and drawings. Attached Figure Description

[0019] Figure 1 This is a flow chart of the integrated cold extrusion-laser cladding molding process for the piston of the present invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Please see Figure 1 This invention discloses an integrated cold extrusion-laser cladding process for pistons. This integrated process design deeply integrates the precision forming of the piston with the performance enhancement of key components, ensuring the performance of the base material while achieving a high-strength metallurgical bond between the cladding layer and the base material in areas requiring reinforcement. Specifically: Example 1: Taking wear-resistant reinforcement of the piston ring groove region as an example Step S1: Pre-fabrication of billet and preparation of activation layer Raw material: Commercially available cast A4032 aluminum alloy bars (main components: Si: 11-13.5%, Cu: 0.8-1.3%, Mg: 0.8-1.3%, Ni: 0.8-1.3%, balance aluminum) were selected and machined into cylindrical blanks of Φ80mm × 60mm. Subsequently, they were ultrasonically cleaned with two tanks of acetone and alcohol to remove surface oil and dirt, and then dried with cold air.

[0022] Preparation of activation layer: The Kinetics 4000 series cold spray system was used. The sprayed powder was gas-atomized Al-Si12 alloy powder (particle size range 15-45μm), whose composition was similar to the matrix to ensure compatibility, but the higher Si content could improve the subsequent interfacial flowability.

[0023] Specific operation: Clamp the blank on the CNC rotary table. Set the cold spraying process parameters: carrier gas is nitrogen, preheating temperature is 500℃, pressure is 3.2 MPa, spraying distance is 25 mm, spray gun moving speed is 200 mm / s, and overlap rate is 50%. Perform reciprocating spraying on the annular area on the outer surface of the blank corresponding to the first and second annular grooves of the piston product.

[0024] After eight cycles, an activation layer approximately 120 μm thick was formed, exhibiting a gray metallic luster and a measured roughness Ra of 13.5 μm. Sampling analysis revealed that this layer was dense, with a porosity of less than 1%, and due to its solid-state deposition, the oxygen content was controlled below 0.6 wt%, lower than the oxide film that forms instantaneously on aluminum alloys in the atmosphere. This activation layer provides a clean, rough substrate with high surface activity and good thermal conductivity, ideal for subsequent steps.

[0025] Step S2: Cold extrusion composite molding Die Preparation: The combined piston extrusion die is mounted on a 1000-ton hydraulic press. Using a cylindrical heater and thermocouple integrated inside the die, the entire die is preheated and stabilized at 180°C. As a preferred reinforcement measure, the cavity area corresponding to the piston ring groove in the die can be locally preheated to 230-250°C, approximately 50-70°C higher than other areas.

[0026] Specific operation: The billet with the activated layer is placed in the mold cavity with the sprayed side facing outwards. The local preheating of the mold enhances the plastic flow capacity of the billet material in the annular groove area, ensuring the filling of the complex annular groove profile. This also allows the activated layer to form a more complete and dense micro-mechanical interlock with the base aluminum alloy under high pressure, improving the bonding of subsequent interfaces. The press is then started for extrusion. The extrusion ratio is 25:1, and the extrusion speed is approximately 10 mm / s. Under the action of enormous multi-directional compressive stress, the billet undergoes plastic flow, filling the mold cavity and forming a piston base blank containing features such as annular grooves, pin holes, and internal cavities.

[0027] During extrusion, the activated layer and the base aluminum alloy undergo intense plastic deformation together. The microscopic protrusions on the surface of the activated layer are pressed into the base material, while the base material also flows into the pores of the activated layer, achieving a microscopic mechanical interlock at the interface. This interlocking structure increases the bonding area, providing a basis for physical bonding. After extrusion, the blank temperature rises due to deformation.

[0028] Step S3: In-situ laser cladding Transfer and Protection: The extruded piston blank is quickly gripped by a robotic arm and rapidly transferred to a sealed protective chamber filled with 99.999% high-purity argon gas. This ensures that the surface temperature of the blank remains within the range of 80°C to 250°C.

[0029] An IPG YLS-4000 fiber laser was used, equipped with a Precitec YW52 oscillating welding head and a GTV PF2 / 2 dual-cylinder powder feeder. The cladding material was a core-shell structured AlSi25 / 30 vol% SiC metal-based composite powder. The core consisted of polygonal SiC ceramic particles with an average particle size of 25 μm, and the outer shell was an Al-Si20 eutectic alloy layer approximately 2 μm thick.

[0030] The laser power was set to 2600W, spot diameter to 2mm, oscillation width to 4mm, oscillation frequency to 150Hz, scanning speed to 12mm / s, and powder feed rate to 20g / min. The laser beam was focused on the surface of the activated layer in the annular groove region, and powder was fed simultaneously for single-pass cladding.

[0031] Laser energy is absorbed by the activation layer and powder, forming a molten pool. The outer shell of the core-shell powder melts preferentially, and the liquid phase exhibits excellent wettability between the SiC core and the underlying activation layer / matrix, promoting uniform distribution of ceramic particles and metallurgical reactions at the interface. The molten pool depth is controlled to approximately 200 μm using parameters, ensuring that the molten region completely penetrates the entire activation layer and extends approximately 50 μm into the matrix, achieving a thorough metallurgical bond between the cladding layer, activation layer, and matrix. Utilizing the residual heat of the blank reduces the total energy input required to achieve good bonding, mitigating the thermal impact on the matrix.

[0032] Step S4: Controlled heat treatment The clad piston blank is loaded into a box-type resistance furnace and subjected to a single-stage aging treatment: first, it is held at 165℃ for 3.5 hours, then the furnace temperature is raised to 205℃ and held for another 2 hours. After completion, it is taken out and air-cooled to room temperature.

[0033] This process serves both the substrate and the cladding layer. The first stage of low-temperature aging aims to initially form a high-density nanoscale reinforcing phase (precursor) in the A4032 aluminum alloy substrate, providing initial strength to the material. Subsequently, the second stage of higher-temperature treatment promotes the transformation of these precursors into a large number of uniformly dispersed and stable nanoscale reinforcing phases, thereby enabling the substrate to reach its peak strength (T6 state). At the same time, this slow heating and holding process helps to relax the residual tensile stress in the laser cladding layer and induces some of it to be converted into beneficial compressive stress.

[0034] Step S5: Composite finishing Specific operations: Roughing and semi-finishing: PCD (polycrystalline diamond) tools are used to turn and bore the main aluminum alloy parts of the piston, such as the top combustion chamber, skirt, and pin hole.

[0035] Precision machining of the ring grooves: Using electroplated CBN (cubic boron nitride) grinding wheels, the upper and lower sides of the first and second ring grooves formed by laser cladding are precision CNC ground to control the width, parallelism and surface roughness of the ring grooves, and to produce the finished piston.

[0036] Example 2: Gradient Cladding and Precise Interface Control This embodiment is based on Embodiment 1 and is a gradient cladding strategy for pistons with higher service requirements.

[0037] Steps S1 and S2 are exactly the same as in Example 1.

[0038] Step S3: Gradient Cladding Material preparation: Prepare two core-shell composite powders with different ceramic contents: A: AlSi25 / 25 vol% SiC; B: AlSi25 / 45 vol% SiC.

[0039] Specific procedures: Under argon protection, the first cladding layer is performed using process A. The process parameters are: laser power 2400W, scanning speed 15mm / s, powder feed rate 18g / min. This layer aims to create a strong and tough transition layer. Immediately after completing the first layer, the second cladding layer is performed using process B, with the parameters adjusted to: laser power 3000W (higher energy input is required due to the high ceramic content), scanning speed 10mm / s, and powder feed rate 22g / min. This layer aims to create a working surface with extremely high wear resistance.

[0040] Interface control: By adjusting the laser power and scanning speed, the depth of the first molten pool is controlled to be approximately 1.5 times (about 180 μm) of the activation layer thickness (120 μm) to ensure that the bonding interface is fully melted. The depth of the second molten pool is sufficient to fully remelt the surface of the first cladding layer (about 50-80 μm). This forms a composite structure with a gradient increase in ceramic content from the substrate to the surface (0% -> 25% -> 45%) and a smooth transition in hardness, which alleviates stress concentration caused by abrupt changes in performance and improves the coating's resistance to peeling under heavy loads.

[0041] Steps S4 and S5: Same as in Example 1 Example 3: Heat-resistant reinforcement of the piston top Step S1: Pre-fabrication of billet and preparation of activation layer Procedure: The billet is the same as in Example 1. An activation layer is prepared on one end face of the billet (corresponding to the combustion chamber surface of the finished piston) using cold spraying. The material is pure aluminum powder (purity >99.7%), with a thickness of about 100 μm.

[0042] Step S2: Cold extrusion composite molding In this embodiment, since the goal is to strengthen the top of the piston, the top cavity of the combustion chamber of the mold is locally preheated to stabilize the temperature of the mold in this area at 250°C, while the temperature of other parts of the mold is 180°C, achieving a local temperature difference of 70°C.

[0043] Step S3: In-situ laser cladding Materials and Operation: The cladding material is a heat-resistant nickel-based alloy powder (such as Inconel 625). Laser cladding is performed at a top temperature of approximately 120°C. Because nickel and aluminum readily form brittle intermetallic compounds, precise control of heat input is required. A high-power (3200W) and high-scanning-speed (25mm / s) rapid cladding strategy is employed to form a thin and dense heat-resistant layer.

[0044] Subsequent steps: S4 and S5 are similar to those in Example 1. The heat treatment regime needs to be fine-tuned according to the characteristics of the nickel-based alloy, and appropriate cutting tools should be selected for the nickel-based alloy during finishing.

[0045] Summary of Examples: This piston cold extrusion-laser cladding integrated forming process creates ideal conditions for subsequent laser cladding by preparing an activation layer before cold extrusion and using extrusion molding to achieve microscopic mechanical interlocking at the interface and retain residual heat. Based on this process platform, an aluminum-based composite powder can be used to construct a high wear-resistant composite layer in the annular groove region through a gradient cladding strategy, or a nickel-based alloy powder can be used to prepare a heat-resistant layer on top. This achieves precise performance enhancement of different functional parts of the piston within the same process system, forming a deep integration of substrate forming and surface modification.

Claims

1. A cold extrusion-laser cladding integrated forming process of a piston, characterized in that, Includes the following sequential steps: S1. Blank prefabrication and activation layer preparation: Provide aluminum alloy piston blanks, and prepare a pure aluminum or aluminum-silicon alloy activation layer on the blanks at a preset position corresponding to the piston finished ring groove area and / or top and / or skirt by cold spraying technology. The activation layer has a thickness of 50-200μm and a surface roughness Ra>10μm. S2. Cold extrusion composite molding: The blank with the activated layer is placed in a mold preheated to 150-300°C and cold extruded to obtain a piston matrix blank, wherein the activated layer undergoes plastic deformation during the extrusion process to achieve micro-mechanical interlock with the matrix. S3. In-situ laser cladding: Under the protection of inert gas, laser cladding is performed on the activation layer area on the piston substrate blank. The cladding material is a metal-based composite powder compatible with aluminum alloy. The high roughness, high activity and strain energy stored by cold extrusion of the activation layer are used to achieve low-temperature and high-quality metallurgical bonding between the cladding layer and the substrate. S4. Controlled heat treatment: The piston blank that has been clad is subjected to a heat treatment in one pass. This heat treatment process simultaneously optimizes the mechanical properties of the piston aluminum alloy matrix and the residual stress state and microstructure of the laser cladding layer. S5. Composite finishing: The heat-treated piston is machined, and the aluminum alloy substrate and laser cladding layer are simultaneously machined to the final dimensions.

2. A process for integrated cold extrusion and laser cladding of a piston according to claim 1, characterized in that, In step S1, the parameters of the cold spraying process are: the carrier gas is nitrogen or helium, the gas temperature is 300-600℃, the gas pressure is 2.0-4.0 MPa, the spraying distance is 10-30 mm, and the composition of the activation layer matches the composition of the piston substrate aluminum alloy but the oxygen content is less than 1.0 wt%.

3. A process for integrated cold extrusion and laser cladding of a piston as claimed in claim 1, wherein, In step S3, laser cladding is performed when the surface temperature of the piston substrate blank is between 80°C and 250°C after it is removed from the mold; the laser cladding uses a oscillating laser head with an oscillation frequency of 50-500 Hz and an oscillation width of 1.5-3 times the spot diameter.

4. The integrated cold extrusion - laser cladding process of claim 1, wherein, In step S3, the metal-based composite powder is an Al-Si alloy powder as the binder phase, wherein 20-50 vol% of ceramic reinforcing phase is uniformly dispersed. The ceramic reinforcing phase includes, but is not limited to, one or more of SiC, Al2O3, TiB2, and TiC, and its particle size is 5-50 μm.

5. A process for integrated cold extrusion and laser cladding of a piston according to claim 4, characterized in that, The metal-based composite powder adopts a core-shell structure design: its core is the ceramic reinforcing phase, and its shell is a eutectic alloy layer with the same composition as the Al-Si alloy powder but with a lower melting point.

6. A process for integrated cold extrusion and laser cladding of a piston as claimed in claim 1 wherein, In step S4, the heat treatment in one pass is a graded aging treatment. The specific process is as follows: first, hold at 150-180℃ for 2-4 hours, and then raise the temperature to 190-220℃ and hold for 1-3 hours. This process can effectively reduce the thermal residual tensile stress in the cladding layer while ensuring that the substrate reaches the peak hardness of T6 or T7, and induce compressive stress.

7. A process for integrated cold extrusion and laser cladding of a piston as claimed in claim 4 wherein, In the laser cladding process in step S3, a gradient cladding strategy of dividing regions and powders is adopted: first, a layer of metal-based composite powder with a reinforcing phase volume fraction of 20-30 vol% is clad on the activation layer, and then a layer of metal-based composite powder with a reinforcing phase volume fraction of 30-50 vol% is clad on it.

8. A process for integrated cold extrusion and laser cladding of a piston according to claim 7, characterized in that, In step S3, by controlling the laser power and scanning speed, the depth of the molten pool of laser cladding is ensured to be at least 1.2 to 1.8 times the original thickness of the activation layer, so as to achieve full metallurgical bonding of the cladding material, the activation layer and the substrate.

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