CN113388832A - Copper-based composite material with high-hardness conductive surface and laser additive manufacturing method thereof - Google Patents

Abstract

The invention discloses a laser additive manufacturing method of a copper-based composite material with a high-hardness conductive surface, wherein the copper-based composite material with the high-hardness conductive surface is a Cu/Fe/Cr composite coating, namely copper is used as a substrate, and an Fe-based coating and a Cr-based coating are sequentially arranged on the surface of the copper substrate. According to the invention, by adopting a laser cladding device and a high-speed laser cladding method, through optimizing various processes in the laser cladding process, such as the processes that the Fe-based coating is prepared by adopting positive defocusing laser, the chromium-based coating is prepared by adopting negative defocusing laser and the like, the Fe-based coating with high dilution rate and the Cr-based coating with low dilution rate are sequentially prepared on the surface of a copper matrix, and the Cu/Fe/Cr composite coating with specific components, structural distribution, high hardness and high conductivity is obtained, wherein the highest conductivity of the Cu/Fe/Cr composite coating is 16.8% IACS, and the micro Vickers hardness of the Cu/Fe/Cr composite coating reaches 420 HV.

Description

Copper-based composite material with high-hardness conductive surface and laser additive manufacturing method thereof

Technical Field

The invention particularly relates to a copper-based composite material with a high-hardness conductive surface and a laser additive manufacturing method thereof.

Background

Copper and copper alloy have high electrical conductivity and high thermal conductivity, so that the copper and copper alloy are widely applied to machinery manufacturing, electrical and electronic, aerospace, marine industry, automobile industry, military industry and the like, are important basic materials for national economy and scientific and technological development, but the problems of low hardness, poor wear resistance, no high temperature resistance, no arc ablation resistance and the like of copper are prominent, the application range of the copper and copper alloy is greatly influenced, and the development of the copper and copper alloy in the fields of lead frames, electromagnetic guide rails, electric contacts and the like is restricted.

The iron-chromium alloy is a common stainless steel model alloy material, has high hardness and ductility, is not easy to rust and resist corrosion and high-temperature corrosion, and is commonly used in heavily polluted industrial areas and marine industry. Meanwhile, the iron and chromium also has good conductivity and arc ablation resistance, so that the iron and chromium composite material has a wide application prospect in the fields of electrical contacts, electromagnetic rails and the like.

Laser cladding is an important means for material surface modification. However, due to the excellent thermal conductivity and high reflectivity of copper, cladding on the copper-based surface is difficult to realize by the conventional laser cladding technology.

Disclosure of Invention

The invention discloses a material design and a preparation method of a high-hardness chromium-based coating by using iron as a transition and strengthening medium, aiming at overcoming the technical problem that the conventional laser cladding is difficult to prepare the high-hardness chromium-based coating on the copper surface.

In the invention, the Fe-based coating is additionally coated before the chromium coating is prepared on the copper surface, the reason that the solid solubility of chromium in copper is extremely low (the maximum solid solubility is only 0.56 wt.%) is considered, the chromium coating on the copper surface usually obtains a mixed structure of a copper phase and a chromium phase, and the hardness of the coating is difficult to improve due to the existence of the copper phase. If a method similar to chromium electroplating is directly adopted, although a hard chromium layer with the thickness of only tens of microns can be obtained on the surface of copper, the difference between the physical properties of the hard chromium layer and the copper matrix is very large, the elastic modulus and the thermal conductivity are suddenly changed at an interface and are subjected to limited force and heat exchange service, namely, microcracks are induced at the interface to cause the hard chromium layer to be damaged and fall off. If a transition layer can be designed between copper and chromium, the difference of physical properties of copper and chromium can be remarkably relieved, the elastic modulus and the thermal conductivity are gradually changed in a gradient manner, and the reliability of the coating in an alternating force thermal load service environment is improved; meanwhile, if the transition layer can also strengthen the chromium layer, the hardness of the chromium on the surface layer can be further improved, and the wear resistance of the copper workpiece can be improved. Therefore, the transition layer should have a large solid solubility with copper to form a two-phase structure on the one hand and should form a single-phase structure with chromium on the other hand. While iron has a maximum solid solubility of 4.6 wt.% in copper, iron and chromium are miscible to form a solid solution. Therefore, the applicant selects iron as a transition coating, on one hand, the iron is used as an intermediate transition layer, the great physical property difference of chromium and copper materials can be overcome, meanwhile, part of iron is dissolved in chromium in a solid mode, the hardness of a chromium layer is obviously improved, and the bonding strength of each layer is guaranteed by designing a copper/iron/chromium composite system.

However, even if the Fe transition layer is designed, it is known to those skilled in the art that if the conventional process is adopted, as in the preparation method of making a single Fe/Cu bonding structure or a single Fe/Cr solid solution, since the difference between the physical properties of the hard chromium layer and the copper matrix is very large, the high hardness chromium-based coating layer formed on the Fe transition layer and the copper matrix and on the Fe transition layer may have a fatal influence, and thus a high hardness conductive coating layer of a theoretical Cu/Fe/Cr structure cannot be obtained.

Based on the problems of the prior art, the invention explores a high-speed laser cladding preparation method and designs of various process parameters in the preparation process, and finally realizes the high-hardness conductive coating of Cu/Fe/Cr with specific components and structural distribution.

In order to achieve the purpose, the applicant of the invention selects high-speed laser cladding to prepare the copper/iron/chromium composite coating, the problem of high thermal conductivity and high reflectivity of copper can be solved due to the high-speed laser cladding, and meanwhile, the applicant prepares an iron layer with specific components and structures on the surface of copper by optimizing a cladding process so as to prepare a chromium layer. The technical scheme provided by the invention is as follows.

A laser additive manufacturing method of a copper-based composite material with a high-hardness conductive surface is provided, wherein the copper-based composite material with the high-hardness conductive surface is a composite coating with a Cu/Fe/Cr structure, namely, copper is used as a substrate, and an Fe-based coating and a Cr-based coating are sequentially arranged on the surface of the copper substrate, and a laser cladding device is adopted to carry out high-speed laser cladding, and the laser additive manufacturing method comprises the following steps:

(1) removing the oxide layer on the surface of the copper and cleaning the surface;

(2) fixing the copper workpiece at a laser cladding position of a laser cladding device;

(3) starting a high-speed laser cladding fiber laser of the laser cladding device, and simultaneously synchronously feeding iron powder to a cladding surface to prepare an iron-based cladding layer with high dilution rate;

in the step, due to limited solid solution of iron and copper during preparation of the iron layer, the applicant adopts a positive defocusing 0.8-1 mm mode to obtain the iron layer with high dilution rate to ensure good metallurgical bonding, namely, laser is focused at a position 0.8-1 mm right above the surface to be clad. The laser adopts the positive defocusing laser beam to ensure that the laser energy density is high in the positive defocusing state, and the depth of a molten pool is obtained, so that an iron layer with high dilution rate can be obtained, namely, copper with higher concentration is contained in the iron coating layer, the iron coating layer is ensured to have good plasticity and conductivity, and a good physical buffer transition layer is provided for the subsequent preparation of the high-hardness conductive chromium-based coating layer. Meanwhile, the power of the used laser is gradually changed according to the following principle: and (4) adopting 3200W power within the first 10s of cladding, then continuously reducing the power at the rate of 20W/s until the power is 2700W, and keeping the power until the cladding is finished. The laser power adopts a gradual change mode, because copper has excellent heat conductivity, high-energy-density laser is needed in the initial stage of cladding an iron layer, and the energy density of the laser is gradually reduced in the middle and later stages, so that the generation of pores caused by overheating and violent evaporation of a molten pool is avoided. Because the copper workpiece has high reflectivity and quick heat conduction to laser, high energy density is needed to ensure that the iron powder and the copper surface layer are melted at the early stage of cladding to form a copper-iron molten pool; along with the proceeding of cladding, the region of the workpiece to be clad is fully preheated under the action of heat transfer of the molten pool in the previous process, so that the laser power is required to be gradually reduced, copper evaporation caused by overhigh temperature of the molten pool is avoided, and the dynamic balance of heat input and heat output of the workpiece is realized until the laser power is reduced to a stable value.

(4) Removing an oxide layer and residues on the surface of the iron-based coating by using a laser rust removal mode; the laser rust removal mode is adopted, an oxide layer and slag on the surface of the cladding layer can be removed in situ, a good-quality surface is provided for a subsequent cladding chromium layer, the in-situ laser rust removal operation is convenient, the preheating is provided for the surface of a workpiece, and the laser input energy requirement for the subsequent cladding chromium layer is favorably reduced.

(5) Starting a high-speed laser cladding fiber laser of the laser cladding device, and synchronously feeding chromium powder to a cladding surface to obtain a chromium layer with a low dilution rate;

in the step, because iron and chromium are infinitely mutually dissolved when the chromium layer is prepared on the iron layer, the applicant uses a laser negative defocusing 0.8-1 mm mode to avoid the phenomenon of copper return caused by excessive remelting of the iron layer; the negative defocusing light beam has uniform energy, and is beneficial to preparing a uniform coating with larger thickness. Meanwhile, the power of the used laser is gradually changed according to the following principle: adopting 2800W power within the first 10s of cladding, and then continuously reducing the power at the rate of 20W/s until the power is 2400W, and keeping the power until the cladding is finished. The laser power which is obviously lower than that in the step (3) is adopted, because the laser rust removal preheating is carried out in the step (4), an iron-based coating is already arranged on the surface, the reflection of iron to laser is weaker than that of copper to laser, and the heat conduction capability of the iron-based coating is lower than that of copper, so that the iron layer and the chromium powder can be melted by relatively lower laser power. Because the wettability of chromium and iron is good, the solid solubility is high, and an iron-chromium solid solution with good combination is easy to form; and the dynamic balance of heat input and heat output of the cladding piece is realized by gradually reducing the laser power.

(6) And after cladding, naturally cooling the workpiece after preserving heat at 300 +/-10 ℃ for not less than 2 hours. Because even though the iron transition layer is designed, the great physical property difference between copper and chromium still causes great thermal stress in the workpiece, and the internal stress needs to be released through heat preservation, so that the chromium-based coating is prevented from generating cracks in the rapid cooling process.

Further, the laser scanning speed in the step (3) is 60-120 mm/s, the scanning mode is lap-joint scanning, and the lap-joint rate is 60% -80%; the powder feeding speed is 1.25-3.75 g/min, and the shielding gas is 11-12L/min;

the positive defocusing of the laser used in the step (4) is 0.2-0.5 mm, the scanning speed is 100-300 mm/s, the scanning mode is lap-joint scanning, and the lap-joint rate is less than or equal to 10%; no powder was discharged and the laser power was 1000W.

The laser scanning speed used in the step 5) is 60-120 mm/s, the scanning mode is lap-joint scanning, and the lap-joint rate is 60-80%; the powder feeding speed is 1.25-3.75 g/min, and the shielding gas is 11-12L/min;

compared with the prior art, the invention has the following advantages:

the method overcomes the great physical property difference between copper and chromium, solves the problem of preparing a chromium layer with high bonding force on the surface of the copper workpiece, ensures that the surface of the copper workpiece has high-hardness conductive property, and effectively improves the wear resistance of the copper workpiece under the condition of keeping sufficient electrical property. Compared with common 304 stainless steel, the conductivity is improved by more than 5 times, the highest conductivity can reach 16.8 percent IACS, and the micro Vickers hardness can reach 420 HV.

Drawings

FIG. 1 is a metallographic structure of a coating cross section in example 1

FIG. 2 is a scanning electron micrograph of an iron transition layer near a chromium-based cladding layer of example 1

FIG. 3 EDS Spectroscopy test lines and quantitative calculated values corresponding to FIG. 2

FIG. 4 is a metallographic structure of a coating cross section of comparative example 3

FIG. 5 is a metallographic structure of a coating layer of comparative example 4

FIG. 6 is the metallographic structure of the coating of comparative example 5.

FIG. 7 is the metallographic structure of the coating of comparative example 7.

FIG. 8 is the metallographic structure of the coating of comparative example 8.

Detailed Description

The technical solution of the present invention is further illustrated by the following specific examples, but the scope of the present invention is not limited thereto:

example 1:

(1) polishing the surface of the copper workpiece by a polisher and abrasive paper to remove a surface oxide layer; cleaning the surface of the copper workpiece by using absolute ethyl alcohol or acetone to obtain a bright surface with a clean surface;

(2) clamping and fixing the copper workpiece by adopting a clamp consisting of a common M8T type screw nut and a pressure plate;

(3) spherical iron powder with the particle size of 200 meshes is filled in a powder cylinder (the powder is dried for 0.5 h at 80 ℃ in advance), the powder is delivered to the surface of a workpiece through a carrier gas powder delivery system, the iron powder and the surface of the workpiece are synchronously melted through the laser of a high-speed laser cladding fiber laser, and a Fe-based coating with high dilution rate is obtained by cladding on the surface of the workpiece; the positive defocusing of the laser is 0.8 mm, the scanning speed is 100 mm/s, the scanning mode is lap-joint scanning, and the lap-joint rate is 60 percent; the powder feeding speed is 1.25 g/min, and the protective gas flow is 11L/min; the laser power used is gradually changed according to the following principle: adopting 3200W power within the first 10s of cladding, then continuously reducing the power according to the rate of 20W/s until the power is 2700W, and keeping the power until the cladding is finished;

(4) controlling the cladding head to walk along the surface of the iron layer once, and removing an oxide layer and residues on the surface by using a laser rust removal mode; the positive defocusing of the laser is 0.2-0.5 mm, the scanning speed is 100-300 mm/s, the scanning mode is lap-joint scanning, and the lap-joint rate is less than or equal to 10%; no powder is sent out, and the laser power is 1000W;

(5) spherical chromium powder with the particle size of 200 meshes is filled in the other powder cylinder (the powder is dried at 80 ℃ for 0.5 h in advance), the powder is delivered to the surface of a workpiece through a carrier gas powder delivery system, and simultaneously the chromium powder and the surface of the workpiece are synchronously melted through the laser of a high-speed laser cladding fiber laser, and a chromium-based coating is obtained by cladding on the surface of the workpiece; the negative defocusing of the laser is 1 mm, the scanning speed is 120 mm/s, the scanning mode is lap-joint scanning, and the lap-joint rate is 80%; the powder feeding speed is 3.75 g/min, and the shielding gas is 12L/min; the laser power used is gradually changed according to the following principle: adopting 2800W power within the first 10s of cladding, then continuously reducing the power according to the rate of 20W/s until the power is 2400W, and keeping the power until the cladding is finished;

(6) after laser cladding is finished, the workpiece is kept at the temperature of 300 +/-10 ℃ for 2 hours and then is naturally cooled.

And observing the cross section structure of the coating by adopting a metallographic microscope, testing the hardness of the surface layer by adopting a micro Vickers hardness meter, and testing the conductivity of the surface layer by adopting a four-probe method.

Example 2:

the laser used in the step (3) is out of focus by 1 mm, the scanning speed is 120 mm/s, and the lap joint rate is 80%; the powder feeding speed is 2.5 g/min, and the protective gas flow is 12L/min;

the negative defocusing of the laser used in the step (5) is 0.8 mm, and the lap joint rate is 60 percent; the powder feeding speed is 2.5 g/min, and the shielding gas is 22L/min;

the remaining steps and parameters were the same as in example 1.

Comparative example 3:

steps (3) and (4) were eliminated, and the remaining steps and parameters were the same as in example 1.

Comparative example 4:

and (6) is omitted, namely the workpiece is naturally cooled after the laser cladding is finished. The remaining steps and parameters were the same as in example 1.

Comparative example 5:

the step (3) adopts negative defocusing of 0.8 mm, and the rest steps and parameters are the same as those of the embodiment 1.

Comparative example 6:

the step (5) adopts a positive defocusing 1 mm, and the rest steps and parameters are the same as those of the embodiment 1.

Comparative example 7:

step (4) was eliminated and the remaining steps and parameters were the same as in example 1.

Comparative example 8:

the laser power was kept at 3200W in step (3) and 2800W in step (5), and the rest of the steps and parameters were the same as those in example 1.

TABLE 1 summary of test results for each of the examples and comparative examples

The metallographic structure of the cross section of example 1 is shown in fig. 1, and it can be clearly seen that an iron-based coating and a chromium-based coating are sequentially present above a copper substrate, the metallurgical bonding between the layers is good, the structure quality is excellent, and pores and cracks are substantially free. FIG. 2 is a scanning electron micrograph of the iron transition layer of example 1 near the chromium-based cladding layer, with fine iron, chromium and copper phases coexisting. FIG. 3 is the EDS spectrum test lines and the quantitative calculations corresponding to FIG. 2 demonstrating the presence of iron, chromium and copper elements.

By comparing the analysis of example 1 and example 2, it can be seen that, according to the protocol of the present application, within the parameters disclosed in the present application, a chromium-based coating with high hard conductivity can be obtained.

By comparing and analyzing the example 1 and the comparative example 3, although the chromium layer can be cladded if the iron transition layer is eliminated, the physical difference between copper and chromium is large, the bonding force of the cladding is poor, and the interface cracking is serious. FIG. 4 is a metallographic structure of the cross section of the coating of comparative example 3, and it can be observed that the coating/matrix interface is almost completely cracked.

As can be seen from comparative analysis of example 1 and comparative example 4, if step (6) is omitted, that is, the workpiece is left to cool naturally after the laser cladding is completed, the cracking of the clad layer is severe because the thermal stress between the clad layer and the copper matrix is not sufficiently released, resulting in cracking. FIG. 5 is the metallographic structure of the coating of comparative example 4, and it can be observed that cracks have a plurality of cracks.

By comparing and analyzing the example 1 and the comparative example 5, if the negative defocusing 0.8 mm is adopted in the step (3), the cracking of the coating is serious because the dilution rate is lower in the negative defocusing mode, the Cu content of the Fe base layer is less, the physical transition of the two materials is not smooth enough, and when the high-hardness chromium-based coating is further coated, the interface cracking is easily caused by the drastic physical change. FIG. 6 is the metallographic structure of the coating of comparative example 5, and it can be observed that cracks have a plurality of cracks.

As can be seen from a comparison of the analysis of example 1 and comparative example 6, if a positive defocus of 1 mm is used in step (5), a better quality coating can be obtained, except that the hardness and conductivity of the coating are relatively low, because the dilution rate in the positive defocus mode is higher and more copper and iron enter the chromium-based coating, resulting in a decrease in hardness and an increase in conductivity.

As can be seen from a comparison between example 1 and comparative example 7, if step (4) is eliminated, i.e., the oxide layer and slag on the surface are not removed, a large amount of pores and inclusion defects are easily generated in the chromium-based coating layer. FIG. 7 is the metallographic structure of the coating of comparative example 7, and it can be observed that the chromium-based coating has more air inclusions.

It can be seen from the comparison between example 1 and comparative example 8 that if the laser power is kept constant rather than being decreased slowly during the laser cladding process, the quality of the cladding layer is deteriorated, and the number of pores is increased, so that the electrical conductivity of the cladding layer is obviously decreased. The reason is that the molten pool is overheated in the middle and later stages of cladding, the base material is evaporated violently, and the air holes are increased. FIG. 8 is a photograph of the metallographic structure of the coating of comparative example 8, and it can be seen that there are many defects.

Claims (7)

1. The laser additive manufacturing method of the copper-based composite material with the high-hardness conductive surface is characterized in that the copper-based composite material with the high-hardness conductive surface is a Cu/Fe/Cr composite coating, namely, copper is used as a substrate, and an Fe-based coating and a Cr-based coating are sequentially arranged on the surface of the copper substrate;

the laser additive manufacturing method adopts a laser cladding device and adopts a high-speed laser cladding method, and comprises the following steps:

1) removing the oxide layer on the surface of the copper and cleaning the surface;

2) fixing the copper workpiece at a laser cladding position of a laser cladding device;

3) starting laser of a laser cladding device, and simultaneously synchronously feeding iron powder to a cladding surface to prepare an iron-based cladding layer with high dilution rate; wherein the laser adopts a positive defocusing 0.8-1 mm mode; meanwhile, the laser power is 3200W within the first 10s of cladding, then the laser power is continuously reduced according to the speed of 20W/s until the power is 2700W, and the power is maintained until the cladding of the iron-based cladding is finished;

4) removing an oxide layer and residues on the surface of the iron-based coating in a laser rust removal mode;

5) starting laser of a laser cladding device, and synchronously feeding chromium powder to a cladding surface to obtain a chromium layer with low dilution rate; wherein the laser adopts a negative defocusing 0.8-1 mm mode; adopting 2800W power within the first 10s of the laser power cladding, then continuously reducing the power according to the rate of 20W/s until the power is 2400W, and keeping the power until the chromium-based cladding is finished; 6) and after cladding, naturally cooling the workpiece after preserving heat at 300 +/-10 ℃ for not less than 2 hours.

2. The laser additive manufacturing method of the copper-based composite material with the high-hardness conductive surface according to claim 1, characterized in that: and 3) the laser scanning speed is 60-120 mm/s, the scanning mode is lap-joint scanning, and the lap-joint rate is 60-80%.

3. The laser additive manufacturing method of the copper-based composite material with the high-hardness conductive surface according to claim 1, characterized in that: and step 3), the powder feeding speed is 1.25-3.75 g/min, and the powder feeding protective gas is 11-12L/min.

4. The laser additive manufacturing method of the copper-based composite material with the high-hardness conductive surface according to claim 1, characterized in that: step 4), defocusing the laser used in the laser derusting mode by 0.2-0.5 mm, scanning speed is 100-300 mm/s, the scanning mode is lap scanning, and the lap ratio is less than or equal to 10%; the laser power was 1000W.

5. The laser additive manufacturing method of the copper-based composite material with the high-hardness conductive surface according to claim 1, characterized in that: the laser scanning speed used in the step 5) is 60-120 mm/s, the scanning mode is lap-joint scanning, and the lap-joint rate is 60% -80%.

6. The laser additive manufacturing method of the copper-based composite material with the high-hardness conductive surface according to claim 1, characterized in that: and 5) feeding powder at a speed of 1.25-3.75 g/min and feeding powder shielding gas at a speed of 11-12L/min.

7. A copper-based composite material having a high hard conductive surface, characterized by: the copper-based composite material with the high-hardness conductive surface refers to a Cu/Fe/Cr composite coating, namely copper is used as a substrate, and a Fe-based coating and a Cr-based coating are sequentially arranged on the surface of the copper substrate; the conductivity of the Cu/Fe/Cr composite coating can reach 16.8% IACS at most, and the micro Vickers hardness reaches 420 HV.

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