Powder metallurgy process for a special plane type piston for a refrigerator compressor

Abstract

This invention discloses a powder metallurgy process for a special-shaped piston used in refrigerator compressors, belonging to the field of powder metallurgy technology. It aims to solve the problems of uneven density around the pin hole and difficulty in ensuring dimensional accuracy of valve slots and inner grooves when pressing pistons with complex shapes using existing processes. These problems lead to insufficient piston wear resistance and sealing performance, as well as high subsequent machining costs. The invention includes the following steps: S1, mixing: weighing iron-based powder, copper powder, graphite powder, and lubricant according to weight and mixing them evenly; S2, pressing: pressing the piston into shape using a powder metallurgy mold under a pressing pressure of 600-800 MPa; S3, sintering: sintering at 1100-1200℃ for 20-40 minutes under a protective atmosphere; S4, post-processing: sequentially performing dimensional finishing, deburring, and steam treatment to obtain the final product. This invention effectively improves the dimensional accuracy and density uniformity of complex-shaped pistons by optimizing the pressing pressure and mold design, especially the precise matching of the mandrel with the pin hole and inner groove.

Description

Technical Field

[0001] This invention belongs to the field of powder metallurgy technology, specifically, it relates to a powder metallurgy process for a special-shaped piston for a refrigerator compressor. Background Technology

[0002] The refrigerator compressor piston is the core reciprocating component of the compressor, and its working environment requires the piston to have high wear resistance, high dimensional accuracy, and good fatigue resistance. As compressors develop towards miniaturization and high efficiency, the piston structure is becoming increasingly complex. For example, piston designs with special profiles such as toothed structures, valve grooves, inner grooves, bosses, and pin holes have emerged, which poses a severe challenge to the piston manufacturing process.

[0003] Traditionally, these pistons are mostly produced by casting blanks and then performing extensive machining. This not only results in low material utilization, long processing cycles, and high costs, but also makes it easy for stress concentration and burrs to occur in thin-walled or irregularly shaped parts such as valve plate grooves and inner grooves during machining, affecting the piston's fatigue life and sealing performance.

[0004] In recent years, powder metallurgy has been introduced into the piston manufacturing field due to its advantages such as near-net-shape forming, high material utilization, and suitability for mass production. For example, Chinese patent CN106424703A discloses a method for preparing a compressor piston, including steps such as mixing, forming, machining, sintering, finishing, and steam treatment. However, the technical solution described in this patent is mainly aimed at pistons with conventional shapes. When applied to pistons with complex surfaces such as toothed structures, valve grooves, inner grooves, bosses, and pin holes, the following prominent problems still exist: First, during the pressing process, the presence of the mandrel obstructs powder flow around the pin hole, resulting in a significantly lower pressing density than the piston body. After sintering, micropores or density gradients are prone to appear near the pin hole, reducing the piston's compressive strength and wear resistance. In severe cases, this can lead to cracking of the pin hole during operation. CN106424703A only discloses the forming density requirements but does not reveal how to strengthen the density of the pin hole area.

[0005] Secondly, complex surfaces such as valve plate grooves, inner grooves, and bosses are prone to roughening, chipping, or dimensional springback during demolding, making it difficult to control the valve plate groove contour, inner groove position, and boss position within the design tolerance range. CN106424703A also does not disclose a mold structure and pressing process that can simultaneously guarantee the accuracy of these complex surfaces.

[0006] Third, the tooth profile, gauge bar pitch, and tooth runout have a significant impact on the airtightness of the compressor. Traditional powder metallurgy processes cannot simultaneously meet the requirements of tooth profile accuracy and overall density.

[0007] Therefore, how to provide a powder metallurgy process that can simultaneously ensure the dimensional accuracy of complex surfaces of special-shaped pistons, the uniformity of density around pin holes, and the overall mechanical properties is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] In order to overcome the above-mentioned technical problems, the purpose of this invention is to provide a powder metallurgy process for a special-shaped piston for a refrigerator compressor.

[0009] The objective of this invention can be achieved through the following technical solutions: A powder metallurgical process for a specially shaped piston for a refrigerator compressor, the piston having a toothed structure, valve groove, inner groove, boss, and pin hole, includes the following steps: S1. Mixing: Weigh the iron-based powder, copper powder, graphite powder and lubricant according to the weight parts, mix them evenly to obtain a mixed powder; S2. Pressing: The mixed powder is filled into a powder metallurgy mold and pressed into shape under a pressing pressure of 600-800MPa to obtain a green blank; the powder metallurgy mold includes an upper die punch, a lower die punch, a mandrel and a female die, the inner cavity of the female die matches the outer surface of the piston, and the shape of the mandrel matches the inner groove and pin hole of the piston. S3. Sintering: The green blank is sintered at 1100-1200℃ for 20-40 minutes under a protective atmosphere to obtain a sintered blank; S4. Post-processing: The sintered blank is subjected to dimensional finishing, deburring and steam treatment in sequence to obtain the final product.

[0010] As a preferred embodiment of the present invention, the toothed structure has a gauge distance of 133.75-0.15mm and a tooth runout of ≤0.1mm; the valve plate groove has a contour of ≤0.25mm; the inner groove has a position of ≤0.15mm; the boss has a position of ≤0.25mm; and the pin hole has a position of ≤0.1mm.

[0011] As a preferred embodiment of the present invention, step S2 specifically includes: S21. The mixed powder is first pre-pressed by the lower die punch; S22, The upper die punch then performs the main pressing; S23. Finally, the piston pin hole inner wall is pressurized in the reverse direction by the mandrel.

[0012] As a preferred embodiment of the present invention, the female mold is provided with a floating insert structure at the position of the valve plate groove and the inner groove corresponding to the piston.

[0013] As a preferred embodiment of the present invention, the weight ratio of each component in the mixed powder is as follows: 85-95 parts of iron-based powder, 1-5 parts of copper powder, 0.5-2 parts of graphite powder, and 0.5-1.5 parts of lubricant.

[0014] As a preferred embodiment of the present invention, the particle size distribution of the mixed powder is 60-200 mesh.

[0015] As a preferred embodiment of the present invention, the protective atmosphere in step S3 is a nitrogen-hydrogen mixed atmosphere.

[0016] As a preferred embodiment of the present invention, the density of the final product is greater than 6.8 g / cm³, and the hardness is HRB50-85.

[0017] As a preferred embodiment of the present invention, the dimensional finishing is molding finishing.

[0018] As a preferred embodiment of the present invention, the mixing operation in step S1 is carried out using a V-type mixer.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. Significantly Improved Density Uniformity Around the Pin Hole: In the pressing process of this invention, the lower die punch performs pre-pressing, followed by the upper die punch performing main pressing, and finally, the mandrel applies reverse pressure to the inner wall of the piston pin hole. This three-step pressing process, especially the mandrel reverse pressure, causes secondary densification of the powder on the inner wall of the pin hole, effectively eliminating the low-density area around the pin hole in traditional processes. The piston produced using this invention has a pin hole periphery hardness that is essentially the same as the piston body, with no microcracks, greatly improving the piston's impact fatigue resistance.

[0020] 2. Precision forming of complex surfaces, eliminating extensive machining: This invention designs the inner cavity of the female mold to match the outer surface of the piston, and incorporates floating insert structures for the valve plate groove and inner groove positions. Combined with optimized pressing pressure, this allows the toothed structure, valve plate groove, inner groove, and boss to achieve near-final dimensional accuracy during the pressing stage. Testing shows that the toothed structure's gauge rod pitch, tooth runout, valve plate groove contour, inner groove position, boss position, and pin hole position are all controlled within the high-precision range required by the design. Subsequent molding finishing is sufficient to meet the finished product requirements, eliminating the need for secondary machining of complex parts such as the valve plate groove and inner groove, significantly reducing manufacturing costs and processing time.

[0021] 3. Excellent overall product performance: After appropriate sintering temperature and time, and steam treatment, the product exhibits high density, moderate and uniform hardness, and a dense iron oxide film on the surface, significantly improving wear resistance and corrosion resistance. Furthermore, the use of a nitrogen-hydrogen mixed atmosphere during sintering avoids oxidation and decarburization, ensuring the uniformity of the piston's microstructure. Experimental results show that the piston's performance indicators fully meet the high-load, long-life requirements of refrigerator compressors.

[0022] 4. Wide process window, suitable for industrial mass production: The entire process does not involve complex equipment or special post-processing, and it is easy to achieve continuous automated production on existing powder metallurgy production lines. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a simplified flowchart of the overall process modules of the present invention; Figure 2 This is a simplified flowchart of the pressing step of the present invention. Detailed Implementation

[0025] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention: Please see Figure 1-2 According to an embodiment of the present invention, a powder metallurgy process is used for a special-shaped piston for a refrigerator compressor. The piston has complex surface features such as a toothed structure, valve groove, inner groove, boss, and pin hole. The powder raw materials, by weight, include: 85-95 parts iron-based powder, 1-5 parts copper powder, 0.5-2 parts graphite powder, and 0.5-1.5 parts lubricant. The iron-based powder can be water-atomized iron powder or reduced iron powder, with a particle size of 60-200 mesh; the lubricant is preferably zinc stearate or amide wax.

[0026] The process steps of this invention are as follows: S1. Mixing: Weigh each raw material according to the ratio, put them into a V-type mixer and mix for 30-60 minutes to obtain a uniform mixed powder.

[0027] S2. Pressing: The mixed powder is filled into a powder metallurgy mold. The mold includes an upper die punch, a lower die punch, a mandrel, and a female mold. The inner cavity of the female mold matches the outer surface of the piston; the mandrel matches the shape of the piston's inner groove and pin hole; floating insert structures are provided in the female mold at the positions corresponding to the piston valve plate groove and inner groove. During pressing, the mixed powder is first pre-pressed by the lower die punch (pressure 200-400MPa), then the upper die punch applies the main pressure to a total pressure of 600-800MPa, and finally the mandrel applies reverse pressure to the inner wall of the piston's pin hole (pressure 100-300MPa). After pressing, a green blank is obtained.

[0028] S3. Sintering: The green blank is placed in a protective atmosphere furnace with a nitrogen-hydrogen mixed atmosphere (e.g., 90% N2 + 10% H2) and sintered at 1100-1200℃ for 20-40 minutes to obtain the sintered blank.

[0029] S4. Post-treatment: The sintered blank is sequentially subjected to molding finishing (finishing pressure 500-700MPa), deburring (vibration grinding or sandblasting can be used), and steam treatment. The steam treatment conditions are: temperature 500-600℃, steam pressure 0.5-0.8MPa, and treatment time 30-90 minutes, to form a dense iron oxide film on the piston surface, improving wear resistance and corrosion resistance. The final product density should be greater than 6.8g / cm³, and the hardness should be HRB50-85. Example 1

[0030] 1. Raw material ratio: Iron-based powder (water-atomized iron powder, 100 mesh): 90 parts; Copper powder (electrolytic copper powder, 200 mesh): 3 parts; Graphite powder (natural flake graphite, 325 mesh): 1.2 parts; Lubricant (zinc stearate): 0.8 parts.

[0031] 2. Process parameters: Mixing: Mix for 45 minutes using a V-type mixer; Compression: Pre-compression pressure 300MPa, main pressure 700MPa, mandrel reverse pressure 200MPa. Floating inserts are provided in the valve slot and inner groove of the mold. Sintering: Nitrogen-hydrogen atmosphere (95% N2 + 5% H2), sintering temperature 1150℃, holding time 30 minutes; Post-processing: Molding finishing pressure 600MPa; Steam treatment temperature 550℃, pressure 0.6MPa, time 60 minutes.

[0032] 3. Performance Testing: The piston manufactured according to the above process was tested and found to have the following characteristics: density 7.05 g / cm³, hardness HRB78; tooth profile spacing 133.70 mm (tolerance -0.05 mm), tooth runout 0.08 mm; valve plate groove contour 0.20 mm; inner groove position 0.12 mm; boss position 0.20 mm; pin hole position 0.08 mm; and no defects such as cracks or burrs.

[0033] In this embodiment, under moderate proportions and process conditions, high-density, high-precision special-shaped pistons can be stably obtained. The combination of reverse pressurization and floating inserts enables complex surfaces to be formed in one step without subsequent machining. Example 2

[0034] 1. Raw material ratio: Iron-based powder (reduced iron powder, 80 mesh): 93 parts; Copper powder (200 mesh): 5 parts; Graphite powder (325 mesh): 1.8 parts; Lubricant (amide wax): 1.2 parts.

[0035] 2. Process parameters: Mixing: Mix for 60 minutes using a V-type mixer; Compression: Pre-compression pressure 400MPa, main pressure 800MPa, mandrel reverse pressure 300MPa; Sintering: Nitrogen-hydrogen atmosphere (90% N2 + 10% H2), sintering temperature 1180℃, holding time 25 minutes; Post-treatment: Molding finishing pressure 650MPa; Steam treatment temperature 600℃, pressure 0.7MPa, time 45 minutes.

[0036] 3. Performance Testing: Density 7.12 g / cm³, hardness HRB82; gauge bar spacing 133.68 mm (-0.07 mm), tooth runout 0.06 mm; valve plate groove profile 0.18 mm; inner groove position 0.10 mm; boss position 0.18 mm; pin hole position 0.06 mm. All precision parameters exceed design requirements.

[0037] In this embodiment, the higher copper content and pressing pressure help to increase density and strength, and the reverse pressure further strengthens the area around the pin hole, resulting in better wear resistance and making it suitable for high-load refrigerator compressors. Example 3

[0038] 1. Raw material ratio: Iron-based powder (water-atomized iron powder, 120 mesh): 87 parts; Copper powder (200 mesh): 2 parts; Graphite powder (325 mesh): 0.7 parts; Lubricant (zinc stearate): 0.6 parts.

[0039] 2. Process parameters: Mixing: Mix for 35 minutes using a V-type mixer; Compression: Pre-compression pressure 250MPa, main pressure 610MPa, mandrel reverse pressure 150MPa; Sintering: Nitrogen-hydrogen atmosphere (92% N2 + 8% H2), sintering temperature 1120℃, holding time 38 minutes; Post-processing: Molding finishing pressure 550MPa; Steam treatment temperature 520℃, pressure 0.5MPa, time 80 minutes.

[0040] 3. Performance Testing: The density is 6.92 g / cm³, and the hardness is HRB68; the gauge bar spacing is 133.72 mm (-0.03 mm), the tooth runout is 0.09 mm, the valve plate groove profile is 0.23 mm, the inner groove position is 0.14 mm, the boss position is 0.24 mm, and the pin hole position is 0.10 mm. The density and hardness are slightly lower than those of Examples 1 and 2, but still meet the usage requirements.

[0041] In this embodiment, even with lower copper and graphite content and lower pressing pressure, as long as the three-step pressing (especially reverse pressing) is still performed and appropriate sintering parameters are used, qualified products can still be obtained.

[0042] Comparative Example 1 1. Raw material ratio: Same as in Example 1 (90 parts iron-based, 3 parts copper, 1.2 parts graphite, 0.8 parts lubricant).

[0043] 2. Process parameters: The mixing, sintering, and post-treatment processes are the same as in Example 1.

[0044] Difference in pressing steps: Only pre-pressing (300MPa) and main pressing (700MPa) are performed, without reverse pressing by the mandrel. That is, demolding is performed directly after the main pressing is completed, and the inner wall of the pin hole is not subjected to secondary pressure reinforcement.

[0045] 3. Performance Testing: The density is 6.89 g / cm³, and the hardness is HRB62. The pin hole position tolerance is 0.22 mm (exceeding the 0.1 mm requirement), and slight density unevenness is visible around the pin hole. Some samples show micro-cracks at the pin hole. The valve plate groove contour tolerance is 0.32 mm, and the inner groove position tolerance is 0.25 mm, both exceeding the tolerance to varying degrees. The tooth profile gauge rod pitch is 133.82 mm (out of tolerance).

[0046] In this comparative example, the lack of a mandrel reverse pressurization step resulted in insufficient powder compaction near the pin hole, leading to lower density and strength. Furthermore, uneven local stress release caused overall dimensional deformation, failing to meet accuracy requirements. This demonstrates that reverse pressurization is a crucial step in ensuring the positional accuracy of pistons with unique surface profiles, particularly the pin hole.

[0047] Comparative Example 2 1. Raw material ratio: Same as in Example 1.

[0048] 2. Process parameters: Compression: Pre-compression 200MPa, main pressure 500MPa (lower than the 600-800MPa required by this invention), mandrel reverse pressure only 80MPa. The remaining steps are the same as in Example 1.

[0049] 3. Performance Testing: The green blank lacks sufficient strength, resulting in chipped corners and burrs on the valve plate groove and toothed edges during demolding; after sintering, the density is only 6.55 g / cm³, and the hardness is HRB45; all dimensional accuracy indicators fail to meet standards, with a gauge bar pitch deviation exceeding -0.20 mm, tooth runout of 0.18 mm, and pin hole position accuracy of 0.35 mm. The product is unusable.

[0050] In this comparative example, the pressing pressure was too low, the mixed powder was not sufficiently densified, and the green body strength was insufficient to maintain the complex shape, resulting in severe deformation after sintering. This demonstrates that a pressing pressure of 600-800 MPa is a necessary condition to ensure the complete formation of the unique shape.

[0051] Table 1: Summary Table of Examples and Comparative Examples

[0052] Summarize: 1. The powder metallurgy process of this invention, through a three-step pressing process (pre-pressing, main pressing, and mandrel reverse pressing) combined with a mold with floating inserts, can stably manufacture refrigerator compressor pistons with complex profiles such as toothed structures, valve plate grooves, inner grooves, bosses, and pin holes. All three embodiments yielded products with a density > 6.8 g / cm³, a hardness of HRB50-85, and meeting stringent tolerance requirements.

[0053] 2. Comparative Example 1 shows that even when using the same raw materials and sintering parameters, the positional accuracy of the pin hole and the overall dimensional accuracy will be severely deteriorated if the "reverse pressure of the mandrel on the inner wall of the pin hole" step is missing. This proves that reverse pressure is a key feature for solving local density inhomogeneity and ensuring the accuracy of special surface features.

[0054] 3. Comparative Example 2 shows that if the pressing pressure is lower than 600 MPa, the green body strength is insufficient, the complex shape cannot be maintained, and the density and precision after sintering are not up to standard, verifying the necessity of pressing pressure of 600-800 MPa.

[0055] 4. Although the three embodiments differ in formulation and process parameters, all of them yield qualified products, demonstrating that the technical solution of the present invention is repeatable and feasible.

[0056] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A powder metallurgy process for a special-shaped piston used in a refrigerator compressor, wherein the piston has a toothed structure, a valve plate groove, an inner groove, a boss, and a pin hole; characterized in that, Includes the following steps: S1. Mixing: Weigh the iron-based powder, copper powder, graphite powder and lubricant according to the weight parts, mix them evenly to obtain a mixed powder; S2. Pressing: The mixed powder is filled into a powder metallurgy mold and pressed into shape under a pressing pressure of 600-800MPa to obtain a green blank; the powder metallurgy mold includes an upper die punch, a lower die punch, a mandrel and a female die, the inner cavity of the female die matches the outer surface of the piston, and the shape of the mandrel matches the inner groove and pin hole of the piston. S3. Sintering: The green blank is sintered at 1100-1200℃ for 20-40 minutes under a protective atmosphere to obtain a sintered blank; S4. Post-processing: The sintered blank is subjected to dimensional finishing, deburring and steam treatment in sequence to obtain the final product.

2. The powder metallurgy process for a special-face piston for a refrigerator compressor according to claim 1, characterized in that, The toothed structure has a gauge bar pitch of 133.75-0.15mm and a tooth runout of ≤0.1mm; the valve plate groove has a contour of ≤0.25mm; the inner groove has a position of ≤0.15mm; the boss has a position of ≤0.25mm; and the pin hole has a position of ≤0.1mm.

3. The powder metallurgy process for a special-face piston for a refrigerator compressor according to claim 1, characterized in that, Step S2 specifically includes: S21. The mixed powder is first pre-pressed by the lower die punch; S22, The upper die punch then performs the main pressing; S23. Finally, the piston pin hole inner wall is pressurized in the reverse direction by the mandrel.

4. The powder metallurgy process for a special-face piston for a refrigerator compressor according to claim 1, characterized in that, The female mold has a floating insert structure at the position of the valve plate groove and inner groove corresponding to the piston.

5. The powder metallurgy process for a special-face piston for a refrigerator compressor according to claim 1, characterized in that, The weight ratio of each component in the mixed powder is as follows: 85-95 parts iron-based powder, 1-5 parts copper powder, 0.5-2 parts graphite powder, and 0.5-1.5 parts lubricant.

6. The powder metallurgy process for a special-face piston for a refrigerator compressor according to claim 1, characterized in that, The particle size distribution of the mixed powder is 60-200 mesh.

7. The powder metallurgy process for a special-face piston for a refrigerator compressor according to claim 1, characterized in that, The protective atmosphere in step S3 is a nitrogen-hydrogen mixed atmosphere.

8. The powder metallurgy process for a special-face piston for a refrigerator compressor according to claim 1, characterized in that, The final product has a density greater than 6.8 g / cm³ and a hardness of HRB50-85.

9. The powder metallurgy process for a special-face piston for a refrigerator compressor according to claim 1, characterized in that, The dimensional finishing is molding finishing.

10. The powder metallurgy process for a special-face piston for a refrigerator compressor according to claim 1, characterized in that, In step S1, the mixing operation is carried out using a V-type mixer.

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