Method for producing microchannelled aluminium flat tubes with a gradient distribution of the corrosion potential in the layers

Abstract

The application discloses a production method of a micro-channel aluminum flat tube with a layer distribution of corrosion potential gradient, and steps are as follows: (1) a micro-channel aluminum flat tube is made of a deformed aluminum alloy round ingot, the deformed aluminum alloy round ingot comprises 1.0-1.5 wt% of Mn and 0.1-0.15 wt% of Ti; (2) a zinc layer is sprayed on the surface of the micro-channel aluminum flat tube by a zinc spraying system under the condition that the temperature is greater than or equal to 500 DEG C, and a zinc-sprayed aluminum flat tube is formed; (3) the zinc-sprayed aluminum flat tube is subjected to heat treatment, the zinc layer on the surface of the zinc-sprayed aluminum flat tube diffuses to the inside, and a layer gradient diffusion layer is spontaneously formed. According to the application, the layer gradient distribution of gradually increasing corrosion potential from the surface of the micro-channel aluminum flat tube to the inside is achieved, and the corrosion potential of different regions of the surface of the micro-channel aluminum flat tube is close to consistent when extending to the inside by the same distance, so that the corrosion of the micro-channel aluminum flat tube is converted from point corrosion to layer corrosion in the use process, the corrosion resistance is doubled, and the leakage time of SWAAT salt spray test is increased from 2000 hours to 5000 hours.

Description

Technical Field

[0001] This invention relates to a method for producing microchannel aluminum flat tubes with a layered distribution of corrosion potential gradient. Background Technology

[0002] With the increasing prominence of global energy and environmental issues, the mandatory use of aluminum microchannel heat exchangers in automotive air conditioning systems has made the replacement of these systems in residential and commercial air conditioning systems inevitable. Although aluminum microchannel heat exchangers have begun to be used in residential and commercial air conditioning, copper materials still dominate. This is mainly because copper has better corrosion resistance than aluminum, resulting in a longer service life for copper-based heat exchange systems. This, to some extent, limits the use of aluminum materials in residential and commercial air conditioning. Therefore, improving the corrosion resistance of microchannel aluminum flat tubes is a necessary requirement for expanding their application in the air conditioning field. Summary of the Invention

[0003] To address the above problems, this invention proposes a method for producing microchannel aluminum flat tubes with a layered corrosion potential gradient, comprising the following steps:

[0004] (1) A microchannel aluminum flat tube is made from a deformed aluminum alloy round ingot using a hot extrusion molding process. The deformed aluminum alloy round ingot contains 1.0-1.5 wt% Mn and 0.1-0.15 wt% Ti. In addition to the above-mentioned 1.0-1.5 wt% Mn and 0.1-0.15 wt% Ti, the remaining part of the deformed aluminum alloy round ingot is Al and unavoidable impurities.

[0005] (2) At a temperature ≥ 500℃, a zinc layer is sprayed onto the surface of the microchannel aluminum flat tube using a zinc spraying system to form a zinc-sprayed aluminum flat tube.

[0006] (3) The zinc-sprayed aluminum flat tube is heat-treated, and the zinc layer on the surface of the zinc-sprayed aluminum flat tube diffuses inward and spontaneously forms a layered gradient diffusion layer. After heat treatment, the zinc content in the layered gradient diffusion layer area gradually decreases from the outside to the inside, and the corrosion potential in the layered gradient diffusion layer area shows a layered gradient distribution that gradually increases from the outside to the inside. The layered gradient diffusion layer area refers to the area with a layered gradient diffusion layer.

[0007] Specifically, to ensure the inward diffusion depth of the zinc layer, the heat treatment temperature is 620±3℃ for 6 minutes. If the temperature is too low, the diffusion depth of zinc will be insufficient; if the temperature is too high, it will cause the remelting of the second phase in the aluminum.

[0008] Specifically, in order to control the diffusion depth of zinc in the microchannel aluminum flat tube, the zinc layer thickness on the surface of the zinc-sprayed aluminum flat tube is 1±0.1μm.

[0009] Specifically, to ensure the uniformity of the zinc layer thickness, a nonlinear arc spraying method is used for zinc spraying. This nonlinear arc spraying method includes the following steps:

[0010] (1) The zinc spraying speed determines the amount of zinc sprayed per unit time. The zinc spraying current is used to characterize the zinc spraying speed. The control system of the zinc spraying equipment converts the speed signal into a current signal, according to the formula AY = BX. 2 +CX+D determines the zinc spraying current and the extrusion speed of the microchannel aluminum flat tube, where Y represents the zinc spraying current in amperes, X represents the aluminum product extrusion speed in m / min, and A, B, C, and D are constants, with B and C not being 0 at the same time, and A not being 0.

[0011] (2) During the zinc spraying process, the speed sensor is used to collect the extrusion speed X of the microchannel aluminum flat tube and transmit it to the control system of the zinc spraying equipment. The control system of the zinc spraying equipment calculates the zinc spraying current corresponding to the aluminum product extrusion speed X according to the formula in step (1), which is recorded as Y2. At the same time, the control system automatically collects the current actual zinc spraying current Y1 and determines whether it satisfies Y2-σ≤Y1≥Y2+σ, where σ is the set deviation value and σ>0;

[0012] (2.1) If Y2-σ≤Y1≥Y2+σ is not satisfied, and the time exceeds T, where T is the set delay time and T>0, then the control system automatically adjusts the zinc spraying ratio so that the actual zinc spraying current Y1 becomes Y1', and satisfies Y2-σ≤Y1'≥Y2+σ.

[0013] (2.2) If Y2-σ≤Y1≥Y2+σ is satisfied, continue zinc spraying according to the actual zinc spraying current Y1 value.

[0014] Specifically, in step (2), σ is (1~3) amperes and T is (1~2) seconds.

[0015] Preferably, the formula AY = BX 2 In the equation +CX+D, A = 160, B = -1, C = 176, and D = 740.

[0016] In commonly used aluminum alloys for microchannel aluminum flat tubes, the Ti content is mostly between 0.005% and 0.02%, primarily serving to refine the grain size. Through long-term research, the inventors of this application discovered that when the Ti content reaches 0.1% or higher, the distribution of Ti in the aluminum alloy is no longer diffuse, but rather exhibits a layered, banded distribution, which can balance the potential difference at different locations within the aluminum alloy. Therefore, this application controls the Ti content in the alloy used for microchannel aluminum flat tubes to be between 0.1% and 0.15%, laying the foundation for precise control of the layered potential gradient.

[0017] Microchannel aluminum flat tubes are formed by hot extrusion of deformed aluminum alloy round ingots. Before extrusion, the ingot temperature is heated to 500℃. The ingot is then extruded under high pressure to form microchannel aluminum flat tubes. A portion of the work done by the extrusion pressure is converted into heat energy for the flat tube, causing its temperature to exceed 500℃. At this high temperature, a layer of zinc is sprayed onto the upper and lower surfaces of the microchannel aluminum flat tube using a zinc spraying system. Subsequently, the microchannel aluminum flat tube is rapidly cooled, achieving good adhesion. In subsequent finishing processes, the zinc is less likely to detach from the surface of the microchannel aluminum flat tube, ensuring the consistency of the zinc layer thickness on the surface of the microchannel aluminum flat tube.

[0018] The spraying current and the microchannel aluminum flat tube extrusion speed are set to have a non-linear relationship. The current deviation value and delay time are set. When the actual value of the spraying current is detected to be different from the set value, the spraying current is automatically corrected to keep the spraying current within a certain range of the set value. This ensures that the zinc layer thickness on the surface is maintained at 1±0.1μm regardless of how the microchannel aluminum flat tube extrusion speed changes.

[0019] During heat treatment, the temperature in different areas of the heat treatment furnace is maintained within ±3℃ of the set value. The transport speed of the microchannel aluminum flat tube is controlled during the brazing heat treatment process to keep the heat treatment time of the microchannel aluminum flat tube at a fixed time. By precisely controlling the heat treatment temperature and time, the degree of zinc diffusion into the interior of different areas on the surface of the microchannel aluminum flat tube is made consistent, so that the Zn content is nearly uniform when different areas on the surface of the microchannel aluminum flat tube extend inward by the same distance.

[0020] Through the above technical means, a layered gradient distribution of corrosion potential that gradually increases from the surface of the microchannel aluminum flat tube inward is finally achieved. Moreover, the corrosion potential is nearly consistent when extending inward over the same distance in different areas of the microchannel aluminum flat tube surface. This enables the corrosion of the microchannel aluminum flat tube to be transformed from pitting corrosion to layered corrosion during the later use of the microchannel aluminum flat tube, doubling the corrosion resistance of the microchannel aluminum flat tube and increasing the leakage time in the SWAAT salt spray test from 2000 hours to 5000 hours. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a microchannel flat tube.

[0022] Figure 2 This is a photo of zinc diffusion energy dispersive spectroscopy analysis after heat treatment.

[0023] Figure 3 This is a micrograph of the pitting corrosion observed during a flat-hole leak after a salt spray test.

[0024] Figure 4 This is a micrograph of the layered corrosion structure during a flat-hole leak after a salt spray test.

[0025] Figure 5 This is a graph showing the trend of zinc diffusion data on samples in sample group A.

[0026] Figure 6 This is a graph showing the trend of zinc diffusion data on samples in sample group B.

[0027] Figure 7 This is a graph showing the trend of zinc diffusion data on the samples in sample group C.

[0028] Figure 8 This is a graph showing the trend of zinc diffusion data on samples in sample group D.

[0029] Figure 9 This is a graph showing the trend of zinc diffusion data on the samples in sample group E.

[0030] Figure 10 This is a trend diagram of corrosion potential on the sample in sample group A.

[0031] Figure 11 This is a trend diagram of corrosion potential on the sample in sample group B.

[0032] Figure 12 This is a trend diagram of corrosion potential on the sample in sample group C.

[0033] Figure 13 This is a trend diagram of corrosion potential on the sample in sample group D.

[0034] Figure 14 This is a trend diagram of corrosion potential on the sample in sample group E. Detailed Implementation

[0035] The technical solution of the present invention will be further described in detail below through embodiments.

[0036] (1) A wrought aluminum alloy round ingot with a diameter of 203 mm was produced. The measured chemical composition was Mn 1.224 wt%, Ti 0.138 wt%, and the remainder was Al and unavoidable impurities. The wrought aluminum alloy round ingot was formed into microchannel aluminum flat tubes by hot extrusion molding process.

[0037] (2) Five sample groups were made using the above-mentioned microchannel aluminum flat tubes. Each sample group included 10 microchannel aluminum flat tubes. The five sample groups were labeled as A, B, C, D and E respectively. The 10 microchannel aluminum flat tubes in each sample group were numbered as tube 1#, tube 2#, tube 3#, tube 4#, tube 5#, tube 6#, tube 7#, tube 8#, tube 9# and tube 10# respectively.

[0038] Sample groups A, B, and C were zinc-sprayed using a nonlinear arc spraying method, with zinc layer thicknesses of 1 μm, 1.2 μm, and 0.8 μm, respectively. Sample groups D and E were zinc-sprayed using a linear arc spraying method, with zinc layer thicknesses of 1 μm and 1.2 μm, respectively. The zinc-sprayed microchannel aluminum flat tubes are called zinc-sprayed aluminum flat tubes.

[0039] The zinc spraying current and the extrusion rate of the microchannel aluminum flat tube used in each sample group are calculated using the following formulas:

[0040] Sample group A uses the formula 160Y = -X 2 +176X+740;

[0041] Sample group B uses the formula 400Y = -3X 2 +520X+2300;

[0042] Sample group C uses the formula 400Y = -X 2 +240X+4900;

[0043] Sample group D uses the formula 15Y = 8X + 240;

[0044] Sample group E uses the formula 20Y = 11X + 430.

[0045] The zinc layer thickness on the surface of the zinc-sprayed aluminum flat tube was measured using a Shimadzu zinc layer thickness gauge from Japan. The test data are shown in Table 1.

[0046] Table 1 Zinc Coating Thickness Measurement Data

[0047]

[0048] As can be seen from Table 1, when spraying using the nonlinear arc spraying method, the zinc layer thickness on the surface of the flat tube is relatively uniform; when spraying using the linear arc spraying method, the zinc layer thickness on the surfaces of different parts within the same part group varies considerably.

[0049] (3) The zinc-aluminum flat tubes of the five sample groups were heat-treated in the brazing furnace. Each sample group took tubes 1# to 5#. The heat treatment temperature was 620℃ and the time was 6min.

[0050] After heat treatment, zinc diffusion data were analyzed using scanning electron microscopy (SEM) energy dispersive spectroscopy. The detection method involved extending the sample from the surface of the flat tube inwards, analyzing the zinc content (wt%) every 20 μm until a depth of 200 μm from the surface. The detection data are shown in Table 2.

[0051] As shown in Table 2, the zinc content gradually decreases from the surface of the aluminum flat tube inwards. The zinc content in sample group A drops to 0 at 180 μm, in sample group B it drops to 0 at 200 μm, in sample group C it drops to 0 at 160 μm, in sample group D it drops to 0 at 160-200 μm, and in sample group E it drops to 0 at 180-200 μm.

[0052] Table 2 Zinc diffusion detection data

[0053]

[0054]

[0055] Plot the zinc diffusion data trend diagrams for each zinc-aluminum flat tube in the five sample groups, and list them separately. Figures 5-9 ,Depend on Figures 5-9 It can be seen that the zinc diffusion data change trends on the samples in sample groups A, B, and C are relatively consistent, while the zinc diffusion data change trends on the samples in sample groups D and E are relatively inconsistent.

[0056] (4) The corrosion potential was tested using an electrochemical workstation:

[0057] The test solution was prepared by adding 58.6 g NaCl, 9 ml H2O2 and deionized water to a volume of 1 L. The reference electrode was a saturated calomel electrode and the counter electrode was a platinum electrode.

[0058] The detection method was as follows: extending from the surface of the zinc-aluminum sprayed flat tube inwards, the corrosion potential (unit: mV) was measured every 20 μm until a distance of 200 μm from the surface of the zinc-aluminum sprayed flat tube. For each sample group, tube #1 was used, and five detection points were taken on each zinc-aluminum sprayed flat tube. These five detection points were designated as point #1, point #2, point #3, point #4, and point #5. The detection data are shown in Table 3. It can be seen that extending from the surface of the flat tube inwards, the corrosion potential gradually increases with decreasing zinc content until it reaches a stable state of approximately -718 mV. Sample group A reached a stable state at 180 μm, sample group B at 200 μm, sample group C at 160 μm, sample group D at 160-200 μm, and sample group E at 180-200 μm.

[0059] Table 3 Corrosion Potential Detection Data

[0060]

[0061]

[0062] Trend charts were created using the corrosion potential data of the zinc-coated aluminum flat tubes from five sample groups, as follows: Figures 10-14 ,Depend on Figures 10-14 It can be seen that the corrosion potential of the zinc-aluminum sprayed flat tubes in sample groups A, B, and C has a relatively good consistency, while the corrosion potential of the zinc-aluminum sprayed flat tubes in sample groups D and E has a relatively poor consistency.

[0063] (5) The corrosion resistance was tested by the SWAAT salt spray test, specifically the neutral salt spray test, according to the test standard GB6458-86. Each sample was assembled into three cores, which were assembled from flat tubes, fins and manifolds and then brazed.

[0064] Every 100 hours, the core was removed from the salt spray test chamber for verification and to check for leaks. If a leak was found, the test for that core was terminated; otherwise, the core continued the salt spray test until the salt spray test time reached 5000 hours, at which point the test was terminated. The test data are shown in Table 4.

[0065] Table 4 SWAAT Salt Spray Test Data

[0066]

[0067]

[0068] According to Table 4, the core corrosion resistance performance is as follows: A > C > B > E > D.

[0069] The average zinc layer thickness of sample group A is 1 μm, with good consistency and the best corrosion resistance. The microstructure of the core made from sample group A after corrosion shows layered corrosion.

[0070] The average zinc layer thickness of sample group B is 1.2 μm, which makes it prone to leakage at the weld joint of the flat tube manifold.

[0071] The average zinc layer thickness of sample group C is 0.8 μm, with good consistency and corrosion resistance second only to A.

[0072] The zinc layer thickness of samples D and E is relatively inconsistent, making them prone to pitting corrosion and exhibiting the worst corrosion resistance. The microstructure of the core made from sample E after corrosion shows pitting corrosion.

Claims

1. A method for producing microchannel aluminum flat tubes with a layered distribution of corrosion potential gradient, characterized in that, Includes the following steps: (1) Microchannel aluminum flat tubes are made from deformed aluminum alloy round ingots using hot extrusion molding process. The deformed aluminum alloy round ingots contain 1.0-1.5 wt% Mn and 0.1-0.15 wt% Ti. (2) At a temperature ≥ 500℃, a zinc layer is sprayed onto the surface of the microchannel aluminum flat tube using a zinc spraying system to form a zinc-sprayed aluminum flat tube; (3) The zinc-sprayed aluminum flat tube is heat-treated, and the zinc layer on the surface of the zinc-sprayed aluminum flat tube diffuses inward and spontaneously forms a layered gradient diffusion layer. During heat treatment, the temperature is 620±3℃ and the time is 6min; The zinc layer thickness on the surface of the zinc-sprayed aluminum flat tube is 1±0.1μm; Zinc spraying is performed using a nonlinear arc spraying method, which includes the following steps: (1) The zinc spraying speed determines the amount of zinc sprayed per unit time. The zinc spraying current is used to characterize the zinc spraying speed. The control system of the zinc spraying equipment converts the speed signal into a current signal. According to the formula AY = BX 2 +CX+D determines the zinc spraying current and the extrusion speed of the microchannel aluminum flat tube, where Y represents the zinc spraying current in amperes, X represents the aluminum product extrusion speed in m / min, and A, B, C, and D are constants, with B and C not being 0 at the same time, and A not being 0. (2) During the zinc spraying process, the speed sensor is used to collect the extrusion speed X of the microchannel aluminum flat tube and transmit it to the control system of the zinc spraying equipment. The control system of the zinc spraying equipment calculates the zinc spraying current corresponding to the aluminum product extrusion speed X according to the formula in step (1), which is recorded as Y2. At the same time, the control system automatically collects the current actual zinc spraying current Y1 and judges whether it satisfies Y2-σ≤Y1≥Y2+σ, where σ is the set deviation value and σ>0; (2.1) If Y2-σ≤Y1≥Y2+σ is not satisfied, and the time exceeds T, where T is the set delay time and T>0, then the control system automatically adjusts the zinc spraying ratio so that the actual zinc spraying current Y1 becomes Y1', and satisfies Y2-σ≤Y1'≥Y2+σ; (2.2) If Y2-σ≤Y1≥Y2+σ is satisfied, continue zinc spraying according to the actual zinc spraying current Y1 value; Formula AY=BX 2 In the equation +CX+D, A=160, B=-1, C=176, and D=740.

2. The production method according to claim 1, characterized in that, After heat treatment, the zinc content in the layered gradient diffusion layer region gradually decreases from the outside to the inside, and the corrosion potential in the layered gradient diffusion layer region shows a layered gradient distribution that gradually increases from the outside to the inside. The layered gradient diffusion layer region refers to the region with a layered gradient diffusion layer.

3. The production method according to claim 1, characterized in that, In step (2), σ is 1 to 3 amperes and T is 1 to 2 seconds.

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