An anodizing electrolytic coloring method for an aluminum alloy work

Abstract

This invention belongs to the field of surface treatment technology, specifically relating to an anodizing electrolytic coloring method for aluminum alloy workpieces. The invention involves anodizing the aluminum alloy workpiece to obtain an oxide film on its surface; placing the anodized workpiece in an acidic pretreatment solution and applying AC voltage or pulsed DC voltage for pre-adjusted barrier layer treatment; then placing the pre-adjusted barrier layer workpiece in a metal salt coloring electrolyte and applying an asymmetric AC voltage for segmented coloring treatment; the segmented coloring treatment includes a low-voltage nucleation stage, a gradual deepening stage, and a constant-voltage color fixing stage. The anodizing electrolytic coloring method provided by this invention can effectively reduce the risk of blooming and burns, and improve coloring uniformity and batch consistency; it has significant engineering value for improving product quality and expanding the application range of the process.

Description

Technical Field

[0001] This invention belongs to the field of surface treatment technology, and specifically relates to an anodizing electrolytic coloring method for aluminum alloy workpieces. Background Technology

[0002] Aluminum alloys, as lightweight and highly formable structural materials, are widely used in building profiles, transportation vehicles, 3C product exterior components, and home appliance housings. Anodizing treatment can generate a porous anodic oxide film with strong adhesion and high chemical stability on the surface of aluminum alloys. This film consists of a dense, non-porous barrier layer close to the metal substrate and a porous layer on the outer side. Before sealing, the pores of the oxide film possess excellent electrochemical deposition activity, and an integrated decorative and functional surface can be achieved through electrolytic coloring.

[0003] Electrolytic coloring, also known as two-step electrolytic coloring, involves placing anodized workpieces in an electrolyte system containing metal salts such as nickel, tin, and copper. An alternating current (AC) or AC-DC superimposed electric field is applied, causing metal ions to undergo a reduction reaction at the bottom of the oxide film pores. The metal ions are deposited within the pores as elemental metals or metal compounds, resulting in stable hues ranging from champagne and bronze to brown and black through the absorption, scattering, and interference of visible light. Compared to organic dyeing, the inorganic metal deposits formed by electrolytic coloring exhibit superior weather resistance, lightfastness, heat resistance, and UV resistance. They are less prone to fading and discoloration, making them particularly suitable for high-durability applications such as outdoor architectural aluminum materials, automotive parts, and weather-resistant exterior components. This has become the mainstream technology for high-end surface treatment of aluminum alloys.

[0004] Currently, existing electrolytic coloring technologies mainly focus on optimizing metal salt system formulations, designing coloring voltage and current regimes, and controlling AC waveforms (such as symmetrical / asymmetrical AC, AC / DC superposition, and pulsed AC). Some existing technologies improve coloring uniformity and color controllability through asymmetrical AC or controllable waveforms, but in industrial continuous production, it is difficult to simultaneously achieve stable coloring with low burns, high uniformity, and high batch consistency. Summary of the Invention

[0005] The purpose of this invention is to provide an anodizing electrolytic coloring method for aluminum alloy workpieces. The anodizing electrolytic coloring method provided by this invention can effectively reduce the risk of coloring defects and burns, and improve coloring uniformity and batch consistency; it has important engineering value for improving product quality and expanding the application range of the process.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for anodizing and electrolytic coloring of aluminum alloy workpieces, comprising the following steps: (1) Anodize the aluminum alloy workpiece to obtain an oxide film on the surface of the aluminum alloy workpiece, and obtain anodized workpiece. The oxide film includes a bottom hole barrier layer and a porous oxide layer stacked together. The bottom hole barrier layer is in contact with the surface of the aluminum alloy workpiece. (2) The anodized workpiece is placed in an acidic pretreatment solution, and an AC voltage or a pulsed DC voltage is applied to perform a pre-adjusted barrier layer treatment to obtain a pre-adjusted barrier layer treated workpiece, wherein the pre-adjusted barrier layer treatment voltage is ≤5 V; (3) The pre-adjusted barrier layer treated workpiece is placed in a metal salt coloring electrolyte, and an asymmetric AC voltage is applied for segmented coloring treatment; the segmented coloring treatment includes a low-voltage nucleation stage, a gradual slope deepening stage, and a constant voltage color fixing stage in sequence; the effective voltage value (Vrms) of the low-voltage nucleation stage is 6~12 V, and the treatment time is 30~180 s; the effective voltage value of the gradual slope deepening stage increases from the effective voltage value of the low-voltage nucleation stage to 12~20 V at a rate of 0.2~1.5 V / min, and the treatment time is 60~600 s; the effective voltage value of the constant voltage color fixing stage is 12~22 V, and the treatment time is 30~300 s.

[0007] Preferably, the anodic oxidation treatment is carried out in a sulfuric acid-based electrolyte, wherein the concentration of H2SO4 in the sulfuric acid-based electrolyte is 150~220 g / L; the anodic oxidation treatment is direct current anodic oxidation, and the conditions for the anodic oxidation treatment include: a temperature of 15~25 °C and a current density of 1~2.5 A / dm³. 2 .

[0008] Preferably, the thickness of the oxide film is 8~25 μm.

[0009] Preferably, in step (2), the pulse frequency of the pulsed DC voltage is 50~1000 Hz and the duty cycle is 10~60%.

[0010] Preferably, in step (2), the conditions for the pre-adjusted barrier layer treatment include: a voltage of 0.5~5 V and a treatment time of 10~180 s; and a pH value of 2~6 for the acidic pretreatment solution.

[0011] Preferably, in step (3), the waveform of the asymmetric AC voltage includes an asymmetric rectangular wave, an asymmetric trapezoidal wave or other asymmetric equivalent controllable waveforms, and the amplitude, conduction angle or pulse width of the positive half-cycle and the negative half-cycle of the waveform of the asymmetric AC voltage are different in at least one of the following: the ratio of the effective values ​​of the positive and negative half-cycles of the asymmetric AC voltage is 1.05 to 1.50.

[0012] Preferably, when the aluminum alloy workpiece is a high-silicon aluminum alloy workpiece, before the pre-conditioning barrier layer treatment after the anodizing treatment, a decontamination treatment or an activation water washing treatment is further performed; the Si content of the high-silicon aluminum alloy workpiece is ≥3 wt%; the decontamination treatment is performed using an acidic decontamination solution, and the decontamination treatment time is 5~30 s; the activation water washing treatment is performed using water or an acidic water washing solution, and the activation water washing treatment time is 10~60 s.

[0013] Preferably, before performing the anodizing process, the aluminum alloy workpiece is further subjected to a pretreatment, which includes sequentially performing degreasing, water washing, alkaline etching or surface conditioning, neutralization and brightening, and water washing.

[0014] Preferably, the alkaline etching is performed using a NaOH solution with a NaOH concentration of 20-60 g / L. The alkaline etching conditions include: a treatment temperature of 35-60 °C and a treatment time of 10-120 s; the surface conditioning is performed using a surface conditioning agent with a treatment time of 10-60 s; and the neutralization and brightening treatment time is 10-90 s.

[0015] Preferably, after the segmented coloring process is completed, the colored workpiece is obtained, and the colored workpiece is further subjected to pre-washing, sealing, washing and drying in sequence. The sealing is carried out by hot water sealing or nickel salt sealing. The conditions for hot water sealing include: hot water temperature of 92~100 ℃ and immersion time of 10~10 min. The nickel salt sealing is performed using a nickel salt system with a nickel salt concentration of 5-15 g / L. The nickel salt sealing conditions include: a sealing temperature of 70-90 ℃, a sealing time of 5-20 min, and a pH value of 5.5-6.5.

[0016] This invention provides an anodizing electrolytic coloring method for aluminum alloy workpieces, comprising the following steps: (1) anodizing the aluminum alloy workpiece to obtain an oxide film on the surface of the aluminum alloy workpiece, thereby obtaining an anodized workpiece, wherein the oxide film comprises a bottom-blocking layer and a porous oxide layer stacked together, the bottom-blocking layer being in contact with the surface of the aluminum alloy workpiece; (2) placing the anodized workpiece in an acidic pretreatment solution and applying an AC voltage or a pulsed DC voltage to pre-adjust the blocking layer, thereby obtaining a pre-adjusted blocking layer treated workpiece, wherein the pre-adjusted blocking layer treatment voltage is ≤5 V; (3) placing the pre-adjusted blocking layer treated workpiece in a metal salt coloring electrolyte and applying an asymmetric AC voltage to perform segmented coloring treatment; wherein the segmented coloring treatment includes sequentially performing a low-voltage nucleation stage, a gradual slope deepening stage, and a constant voltage color fixing stage; wherein the effective voltage value (Vrms) of the low-voltage nucleation stage is 6~12 V, and the processing time is 30~180 s; wherein the effective voltage value of the gradual slope deepening stage is 0.2~1.5 times the effective voltage value of the low-voltage nucleation stage. The voltage increases at a rate of 12-20 V / min, with a processing time of 60-600 s; the effective voltage value of the constant voltage color fixing stage is 12-22 V, with a processing time of 30-300 s. This invention has found that the hole-bottom barrier layer formed in situ during anodizing is affected by the composition, microstructure, current distribution, and temperature fluctuations of the aluminum alloy workpiece matrix, resulting in uneven thickness, discrete resistivity distribution, and localized overcurrent at the moment of coloring energization. This easily leads to defects such as workpiece edge burns, surface mottled appearance, streaks, and uneven color distribution. To address these problems, before electrolytic coloring, this invention applies a low-voltage AC or pulsed DC to an acidic pretreatment solution to gently electrochemically modify the hole-bottom barrier layer: the low voltage does not break down or dissolve the hole-bottom barrier layer, only activating and reorganizing the interfacial micro-regions; the AC or pulsed DC causes bidirectional oscillation of the electric field, moderately activating high-resistance areas and moderately passivating low-resistance areas, ultimately making the resistance of the hole-bottom barrier layer across the entire surface more uniform, eliminating the basis for current concentration at its source. Meanwhile, this invention employs a segmented coloring process. Under the influence of an asymmetric alternating electric field, metal ions in the metal salt coloring electrolyte undergo reduction deposition at the bottom of the pores. Three-stage voltage program control enables precise control of the entire deposition process. The low-voltage nucleation stage features a gentle voltage, coupled with a pore-bottom barrier layer with uniform resistance distribution, ensuring synchronous nucleation of metal ions at all pore bottoms and preventing localized preferential deposition. The gradual deepening stage uses a slow voltage increase to steadily raise the deposition rate, allowing metal grains to grow uniformly along the pores and avoiding voltage surges that could cause current surges and localized over-deposition. The constant-voltage color-fixing stage uses a stable voltage to ensure the deposition depth and light absorption reach the target hue, achieving precise color locking. Furthermore, this invention uses an asymmetric alternating voltage for electrolytic coloring and optimizes the asymmetric alternating parameters in all three stages, further reducing burns / blooming and improving color repeatability.

[0017] In summary, the anodizing electrolytic coloring method for aluminum alloy workpieces provided by this invention adds a pre-adjusted barrier layer treatment step before electrolytic coloring and adopts an asymmetric AC segmented program during the coloring stage, making the resistance distribution of the barrier layer at the bottom of the hole more uniform, thereby reducing local overcurrent in the early stage of coloring. Simultaneously, through program control of the "low-voltage nucleation stage, gradual slope deepening stage, and constant voltage color fixing stage," the risks of blooming, burning, and batch color difference are reduced, improving color uniformity, color repeatability, and process window width. Compared with the prior art, this invention has the following beneficial effects: This invention eliminates local overcurrent by pre-adjusting the uniform interface resistance of the barrier layer, combined with segmented asymmetric AC control, significantly reducing the risks of blooming and burning; this invention achieves synchronous nucleation and uniform growth of the metal, resulting in small surface color difference and high batch stability; this invention uses a three-segment voltage program combined with asymmetric AC voltage, allowing for repeatable and precisely controllable color; this invention is adaptable to conventional aluminum alloys and high-silicon die-cast aluminum alloys, solving the problems of difficult coloring and low yield of complex substrates, and is suitable for industrial continuous production. Detailed Implementation

[0018] This invention provides a method for anodizing and electrolytic coloring of aluminum alloy workpieces, comprising the following steps: (1) Anodize the aluminum alloy workpiece to obtain an oxide film on the surface of the aluminum alloy workpiece, and obtain anodized workpiece. The oxide film includes a bottom hole barrier layer and a porous oxide layer stacked together. The bottom hole barrier layer is in contact with the surface of the aluminum alloy workpiece. (2) The anodized workpiece is placed in an acidic pretreatment solution, and an AC voltage or a pulsed DC voltage is applied to perform a pre-adjusted barrier layer treatment to obtain a pre-adjusted barrier layer treated workpiece, wherein the pre-adjusted barrier layer treatment voltage is ≤5 V; (3) The pre-adjusted barrier layer treated workpiece is placed in a metal salt coloring electrolyte, and an asymmetric AC voltage is applied for segmented coloring treatment; the segmented coloring treatment includes a low-voltage nucleation stage, a gradual slope deepening stage, and a constant voltage color fixing stage in sequence; the effective voltage value (Vrms) of the low-voltage nucleation stage is 6~12 V, and the treatment time is 30~180 s; the effective voltage value of the gradual slope deepening stage increases from the effective voltage value of the low-voltage nucleation stage to 12~20 V at a rate of 0.2~1.5 V / min, and the treatment time is 60~600 s; the effective voltage value of the constant voltage color fixing stage is 12~22 V, and the treatment time is 30~300 s.

[0019] In this invention, unless otherwise specified, all raw materials / components used in preparation are commercially available products well-known to those skilled in the art. Unless otherwise specified, all percentages in this invention refer to mass percentages. Unless otherwise specified, all solutions in this invention are aqueous solutions with water as the solvent; for example, NaOH solution is an aqueous solution of NaOH. Room temperature in this invention generally refers to a temperature between 15°C and 30°C, and is generally defined as 25°C.

[0020] The present invention performs anodizing treatment on aluminum alloy workpieces to obtain an oxide film on the surface of the aluminum alloy workpieces, thereby obtaining anodized workpieces. The oxide film includes a bottom hole barrier layer and a porous oxide layer stacked together, and the bottom hole barrier layer is in contact with the surface of the aluminum alloy workpieces.

[0021] In this invention, the aluminum alloy workpiece includes cast aluminum alloy or die-cast aluminum alloy. The aluminum alloy workpiece can be a high-silicon aluminum alloy workpiece, specifically a high-silicon die-cast aluminum alloy workpiece. In this invention, the Si content of the high-silicon aluminum alloy workpiece is preferably ≥3 wt%, and can be 3~12 wt.%.

[0022] In this invention, the aluminum alloy workpiece can be a 6060 aluminum alloy workpiece. The high-silicon die-cast aluminum alloy workpiece can be A380 or A360.

[0023] Before performing the anodizing treatment, the present invention preferably includes pretreatment of the aluminum alloy workpiece, which preferably includes degreasing, water washing, alkaline etching or surface conditioning, neutralization and brightening, and water washing in sequence. The present invention does not have specific requirements for the specific implementation of the degreasing and water washing. In the present invention, the alkaline etching is preferably performed using a NaOH solution. The concentration of NaOH in the NaOH solution is preferably 20~60 g / L, more preferably 30~50 g / L. The alkaline etching conditions preferably include: a processing temperature of preferably 35~60 ℃, more preferably 40~50 ℃, and in the embodiment, 45 ℃; and a processing time of preferably 10~120 s, more preferably 20~80 s, and in the embodiment, 60 s. The workpiece is washed with water before the neutralization and brightening after the alkaline etching. The present invention removes the natural oxide film and slight mechanical damage layer from the surface of the aluminum alloy workpiece through alkaline etching.

[0024] This invention replaces alkaline etching with surface conditioning for workpieces requiring high appearance or dimensional stability. In this invention, the surface conditioning is preferably performed using a surface conditioning agent, which can be a weakly alkaline surface conditioning agent. This invention does not have special requirements for the surface conditioning agent; commercially available products well-known to those skilled in the art can be used. In this invention, the surface conditioning temperature is preferably room temperature to 40°C. The surface conditioning treatment time is preferably 10 to 60 seconds, more preferably 20 to 40 seconds. This invention achieves a more uniform surface state and reduces roughness through the surface conditioning process.

[0025] In this invention, the reagent used for neutralization and brightening is preferably a nitric acid solution, and the volume content of HNO3 in the nitric acid solution is preferably 10-40 vol.%, which can be 30 vol.% in the example. The reagent used for neutralization and brightening may contain additives, and this invention does not have special requirements on the type and amount of the additives. The temperature of the neutralization and brightening treatment is room temperature. The treatment time of the neutralization and brightening is preferably 10-90 s, preferably 20-60 s, which can be 30 s in the example. This invention preferably removes alkaline corrosion residue and improves surface brightness through the neutralization and brightening treatment. In this invention, after the neutralization and brightening is completed, a water wash is performed until there is no obvious acid residue on the surface. A small amount of fluorine-containing components can be added to the water used for the wash.

[0026] In this invention, the anodizing treatment is preferably carried out in a sulfuric acid-based electrolyte. The sulfuric acid-based electrolyte is a sulfuric acid solution. The concentration of H₂SO₄ in the sulfuric acid-based electrolyte is preferably 150-220 g / L, more preferably 180-200 g / L, and in the examples, it can be 200 g / L. The anodizing treatment is preferably direct current anodizing. The conditions for the anodizing treatment preferably include: a temperature preferably 15-25 °C, and a current density preferably 1-2.5 A / dm³. 2 More preferably 1.5~2 A / dm 2 This invention optimizes the conditions of the anodic oxidation process to obtain a pore structure suitable for electrolytic coloring.

[0027] In this invention, the thickness of the oxide film is preferably 8-25 μm, more preferably 10-16 μm, and in the embodiment, it can be 20 μm. The anodizing treatment time is based on the thickness of the oxide film.

[0028] After obtaining the anodized workpiece, the present invention places the anodized workpiece in an acidic pretreatment solution and applies an AC voltage or a pulsed DC voltage to perform a pre-adjusted barrier layer treatment to obtain a pre-adjusted barrier layer treated workpiece, wherein the pre-adjusted barrier layer treatment voltage is ≤5 V.

[0029] In this invention, the pH value of the acidic pretreatment solution is preferably 2-6, more preferably 2-5. The acidic pretreatment solution can be a sulfuric acid solution, and the concentration of H2SO4 in the sulfuric acid solution is preferably 1-5 g / L, and in the example, it can be 3 g / L. The pre-adjusted barrier layer treatment is preferably performed using a pulsed DC voltage. The pulse frequency of the pulsed DC voltage is preferably 50-1000 Hz, and in the example, it can be 500 Hz; the duty cycle is preferably 10-60%, and in the example, it can be 40%. By using a pulsed DC voltage for the pre-adjusted barrier layer treatment and optimizing the pulse frequency and duty cycle of the pulsed DC voltage, this invention can effectively reduce local overcurrent in the initial stage of coloring.

[0030] In this invention, the conditions for the pre-adjustment barrier layer treatment preferably include: a voltage of 0.5~5 V, more preferably 1~4 V, and even more preferably 2~4 V. The treatment time is preferably 10~180 s, more preferably 20~150 s, and even more preferably 30~100 s; in the embodiments, it can be 100 s or 160 s. This invention, through the pre-adjustment barrier layer treatment, enables the resistance of the barrier layer at the bottom of the hole to tend to be uniform.

[0031] In this invention, when the aluminum alloy workpiece is a die-cast aluminum alloy or a high-silicon aluminum alloy workpiece, the processing time of the pre-adjusted barrier layer treatment is preferably 20~180 s, more preferably 30~170 s, and in the embodiment it can be 160 s.

[0032] In this invention, when the aluminum alloy workpiece is a cast aluminum alloy or a low-silicon aluminum alloy workpiece, the processing time of the pre-adjusted barrier layer treatment is preferably 10~120 s, more preferably 20~100 s, and in the embodiment it can be 100 s.

[0033] After obtaining the pre-adjusted barrier layer treated workpiece, the present invention places the pre-adjusted barrier layer treated workpiece in a metal salt coloring electrolyte and applies an asymmetric AC voltage for segmented coloring treatment; the segmented coloring treatment includes a low-voltage nucleation stage, a gradual slope deepening stage, and a constant voltage color fixing stage in sequence; the effective voltage value (Vrms) of the low-voltage nucleation stage is 6~12 V, preferably 8~10 V, and the treatment time is 30~180 s, preferably 50~150 s, and can be 100 s in the embodiment; the effective voltage value of the gradual slope deepening stage increases from the effective voltage value of the low-voltage nucleation stage to 12~20 V, preferably 15~20 V, and can be 20 V in the embodiment, at a rate of 0.2~1.5 V / min (preferably 0.5~1.2 V / min, and can be 1.0 V / min in the embodiment), and the treatment time is 60~600 s, preferably 100~500 s, and more preferably 200~400 s. The effective voltage value of the constant voltage color fixing stage is 12~22 V, preferably 15~20 V, and can be 20 V in the embodiment. The processing time is 30~300 s, preferably 100~300 s, and can be 300 s in the embodiment.

[0034] In this invention, the waveform of the asymmetric AC voltage preferably includes an asymmetric rectangular wave, an asymmetric trapezoidal wave, or other asymmetric equivalent controllable waveforms. Preferably, the amplitude, conduction angle, or pulse width of the positive and negative half-cycles of the asymmetric AC voltage waveform are different. The ratio of the effective values ​​of the positive and negative half-cycles of the asymmetric AC voltage is preferably 1.05 to 1.50.

[0035] In this invention, during the segmented coloring process, it is preferable to collect current-time curves and use the characteristic of "peak current decreasing to plateau current" as the criterion for the end of segment A or the start of segment B in the segmented coloring process. The peak current is the maximum current value occurring within 1-30 seconds during the initial power-on period. The plateau current is the stable average current value with fluctuations less than a preset threshold within 5-30 seconds after the peak value.

[0036] In this invention, when the aluminum alloy workpiece is a high-silicon aluminum alloy workpiece, before performing the pre-conditioning barrier layer treatment after the anodizing treatment, the invention preferably further includes a decontamination treatment or an activation water washing treatment. In this invention, the high-silicon aluminum alloy workpiece can be a high-silicon die-cast aluminum alloy workpiece. The Si content of the high-silicon aluminum alloy workpiece is preferably ≥3 wt%, and can be 3~12 wt.%. The decontamination treatment is preferably performed using an acidic decontamination solution, and the decontamination treatment time is preferably 5~30 s. The activation water washing treatment is preferably performed using water or an acidic water washing solution, and the water can be deionized water. The activation water washing treatment time is preferably 10~60 s.

[0037] In this invention, after the segmented coloring process, a color-treated workpiece is obtained. Preferably, the invention further includes sequentially pre-washing, sealing, washing, and drying the color-treated workpiece. In this invention, the sealing is preferably performed using hot water sealing or nickel salt sealing. The conditions for hot water sealing are preferably: a hot water temperature of 92-100 °C and an immersion time of 10-10 min. The nickel salt sealing is preferably performed using a nickel salt system. The concentration of nickel salt in the nickel salt system is preferably 5-15 g / L. The conditions for nickel salt sealing are preferably: a sealing temperature of 70-90 °C, a sealing time of 5-20 min, and a pH value of 5.5-6.5.

[0038] This invention, by adding a pre-adjustment barrier layer step and applying segmented asymmetrical AC voltage during the coloring process, can make the bottom resistance distribution of the holes more consistent in the early stage of coloring, reduce local overcurrent, and thus reduce the risk of burns, mottling, and color difference.

[0039] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0040] Example 1 This embodiment provides an anodizing electrolytic coloring method for aluminum alloy workpieces, as detailed below: S1 Pretreatment: The aluminum alloy workpiece (6060 aluminum alloy) is degreased, washed, alkali-etched, neutralized and brightened, and then washed again. The alkali etching is performed using a NaOH solution with a concentration of 50 g / L, at 45 ℃ for 60 s, to remove the natural oxide film and minor mechanical damage layer on the surface. After alkali etching, the surface is washed. Neutralization and brightening are performed using a nitric acid solution with a HNO3 volume content of 30 vol.%, treated at room temperature for 30 s to remove residual ash from the alkali etching and improve surface brightness. After treatment, the surface is washed until no obvious acid residue remains.

[0041] S2 anodizing: DC anodizing was performed in a sulfuric acid-based electrolyte at a concentration of 200 g / L H2SO4 and a temperature of 25°C, with a current density of 2.0 A / dm³. 2 To obtain a pore structure suitable for electrolytic coloring, an oxide film with a thickness of 20 μm was obtained; S3 Pre-adjusting the barrier layer: Immerse the workpiece obtained in step S2 in a weakly acidic pretreatment solution (3 g / L sulfuric acid solution), apply a 4 V pulsed DC voltage (pulse frequency of 500 Hz, duty cycle of 40%), and process for 100 s to make the resistance of the barrier layer at the bottom of the hole tend to be uniform. S4 Electrolytic Coloring: The workpiece obtained in step S3 is placed in a metal salt coloring electrolyte, and an asymmetric AC voltage is applied (the amplitude or conduction angle of the positive and negative half-cycles of the asymmetric AC voltage is controllable, so that the ratio of the effective values ​​of the positive and negative half-cycles is 1.50) for segmented coloring, including: a low-voltage nucleation segment (Vrms is 10 V, processing time is 100 s); a gradual deepening segment (Vrms increases to 20 V at 1 V / min, processing time is 500 s); and a constant-voltage color fixing segment (Vrms is 20 V, processing time is 300 s). During the S4 electrolytic coloring process, the current-time curve is collected, and the characteristic of "the current dropping to the plateau after the peak current" is used as the criterion for the end of segment A or the start of segment B.

[0042] S5 Washing and Sealing: After washing, perform hot water sealing. Hot water sealing uses deionized water at 92~100℃, and the immersion time is 30 minutes. After removal, rinse with deionized water and dry.

[0043] Example 2 This embodiment provides an anodizing electrolytic coloring method for aluminum alloy workpieces, as detailed below: S1 Pretreatment: The high-silicon die-cast aluminum alloy (A380) was degreased, washed, alkali-etched, neutralized and brightened, and then washed again. The alkali etching was performed using a NaOH solution with a concentration of 50 g / L, at 45 ℃ for 60 s, to remove the natural oxide film and minor mechanical damage layer. After alkali etching, the surface was washed with water. Neutralization and brightening were performed using a nitric acid solution with a HNO3 volume content of 30 vol.%, treated at room temperature for 30 s to remove residual ash from the alkali etching and improve surface brightness. After treatment, the surface was washed until no obvious acid residue remained.

[0044] S2 anodizing: DC anodizing was performed in a sulfuric acid-based electrolyte at a concentration of 200 g / L H2SO4 and a temperature of 25°C, with a current density of 2.0 A / dm³. 2 To obtain a pore structure suitable for electrolytic coloring, an oxide film with a thickness of 20 μm was obtained; The anodized workpiece is treated with a weak acidic cleaning solution at room temperature for 30 seconds to remove loose contaminants or uneven surface deposits remaining after anodizing. S3 Pre-adjusting the barrier layer: Immerse the workpiece obtained in step S2 in a weakly acidic pretreatment solution, apply a 4 V pulsed DC voltage (pulse frequency of 500 Hz, duty cycle of 40%), and process for 160 s to make the resistance of the barrier layer at the bottom of the hole tend to be uniform. S4 Electrolytic Coloring: The workpiece obtained in step S3 is placed in a metal salt coloring electrolyte, and an asymmetric AC voltage is applied (the amplitude or conduction angle of the positive and negative half-cycles of the asymmetric AC voltage is controllable, so that the ratio of the effective values ​​of the positive and negative half-cycles is 1.50) for segmented coloring, including: a low-voltage nucleation segment (Vrms is 10 V, processing time is 100 s); a gradual deepening segment (Vrms increases to 20 V at 1 V / min, processing time is 500 s); and a constant-voltage color fixing segment (Vrms is 20 V, processing time is 300 s). During the S4 electrolytic coloring process, the current-time curve is collected, and the characteristic of "the current dropping to the plateau after the peak current" is used as the criterion for the end of segment A or the start of segment B.

[0045] S5 Washing and Sealing: After washing, perform hot water sealing. Hot water sealing uses deionized water at 92~100℃, and the immersion time is 30 minutes. After removal, rinse with deionized water and dry.

[0046] Example 3 Similar to Example 2, except that the step of “treating the anodized workpiece with a weak acidic cleaning solution at room temperature for 30 seconds to remove loose contaminants or uneven surface deposits remaining after anodizing” in step 2 is omitted.

[0047] Example 4 It is basically the same as Example 2, except that the processing time in step S3 is 100 s.

[0048] Example 5 It is basically the same as Example 2, except that the silicon die-cast aluminum alloy is A360.

[0049] Comparative Example 1 This comparative example provides an anodizing electrolytic coloring method for aluminum alloy workpieces, as detailed below: S1 Pretreatment: The aluminum alloy workpiece (6060 aluminum alloy) is degreased, washed, alkali-etched, neutralized and brightened, and then washed again. The alkali etching is performed using a NaOH solution with a concentration of 50 g / L, at 45 ℃ for 60 s, to remove the natural oxide film and minor mechanical damage layer on the surface. After alkali etching, the surface is washed. Neutralization and brightening are performed using a nitric acid solution with a HNO3 volume content of 30 vol.%, treated at room temperature for 30 s to remove residual ash from the alkali etching and improve surface brightness. After treatment, the surface is washed until no obvious acid residue remains.

[0050] S2 anodizing: DC anodizing was performed in a sulfuric acid-based electrolyte at a concentration of 200 g / L H2SO4 and a temperature of 25°C, with a current density of 2.0 A / dm³. 2To obtain a pore structure suitable for electrolytic coloring, an oxide film with a thickness of 20 μm was obtained; S3 Electrolytic Coloring: The workpiece obtained in step S2 is placed in a metal salt coloring electrolyte for conventional AC coloring. The specific method of conventional AC coloring is to place the anodized workpiece directly in the metal salt coloring electrolyte and treat it at a constant voltage of 20 V for 15 min under AC conditions of 50 Hz to achieve the target color depth.

[0051] Comparative Example 2 This comparative example provides an anodizing electrolytic coloring method for aluminum alloy workpieces, as detailed below: S1 Pretreatment: The aluminum alloy workpiece (6060 aluminum alloy) is degreased, washed, alkali-etched, neutralized and brightened, and then washed again. The alkali etching is performed using a NaOH solution with a concentration of 50 g / L, at 45 ℃ for 60 s, to remove the natural oxide film and minor mechanical damage layer on the surface. After alkali etching, the surface is washed. Neutralization and brightening are performed using a nitric acid solution with a HNO3 volume content of 30 vol.%, treated at room temperature for 30 s to remove residual ash from the alkali etching and improve surface brightness. After treatment, the surface is washed until no obvious acid residue remains.

[0052] S2 anodizing: DC anodizing was performed in a sulfuric acid-based electrolyte at a concentration of 200 g / L H2SO4 and a temperature of 25°C, with a current density of 2.0 A / dm³. 2 To obtain a pore structure suitable for electrolytic coloring, an oxide film with a thickness of 20 μm was obtained; S3 Pre-adjusting the barrier layer: Immerse the workpiece obtained in step S2 in a weakly acidic pretreatment solution, apply a 4 V pulsed DC voltage (pulse frequency of 500 Hz, duty cycle of 40%), and process for 100 s to make the resistance of the barrier layer at the bottom of the hole tend to be uniform. S4 Electrolytic Coloring: The workpiece obtained in step S3 is placed in a metal salt coloring electrolyte and subjected to single-stage constant voltage AC. The specific method of single-stage constant voltage AC is to treat at a constant voltage of 20 V for 15 min at 50 Hz. This corresponds to the segmented asymmetric AC in Example 1.

[0053] Example 1, Comparative Example 1, and Comparative Example 2 were compared, and the results are shown in Table 1.

[0054] Table 1. Comparison results of Example 1, Comparative Example 1, and Comparative Example 2

[0055] The results of comparing Examples 2, 3, and 4 are shown in Table 2.

[0056] Table 2 Comparison results of Examples 2, 3 and 4

[0057] As can be seen from the above embodiments, the present invention provides a method for electrolytic coloring of aluminum alloy workpieces after anodizing. The method includes pretreatment, anodizing in a sulfuric acid system to form a porous oxide film, pre-adjusting a barrier layer, electrolytic coloring, and sealing steps. The pre-adjusting barrier layer is prepared by applying a low-voltage AC or pulsed DC voltage in a weakly acidic medium to make the resistance of the barrier layer at the bottom of the holes more uniform. The electrolytic coloring stage employs an asymmetric AC segmented procedure, including at least a low-voltage nucleation stage, a gradual deepening stage, and a constant-voltage color-fixing stage, to reduce the risk of blooming and burning caused by localized overcurrent, and to improve coloring uniformity and batch consistency.

[0058] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. Other embodiments can be obtained based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for anodizing and electrolytic coloring of aluminum alloy workpieces, characterized in that, Includes the following steps: (1) Anodize the aluminum alloy workpiece to obtain an oxide film on the surface of the aluminum alloy workpiece, and obtain anodized workpiece. The oxide film includes a bottom hole barrier layer and a porous oxide layer stacked together. The bottom hole barrier layer is in contact with the surface of the aluminum alloy workpiece. (2) The anodized workpiece is placed in an acidic pretreatment solution, and an AC voltage or a pulsed DC voltage is applied to perform a pre-adjusted barrier layer treatment to obtain a pre-adjusted barrier layer treated workpiece, wherein the pre-adjusted barrier layer treatment voltage is ≤5 V; (3) The pre-adjusted barrier layer treated workpiece is placed in a metal salt coloring electrolyte, and an asymmetric AC voltage is applied for segmented coloring treatment; the segmented coloring treatment includes a low-voltage nucleation stage, a gradual slope deepening stage, and a constant voltage color fixing stage in sequence; the effective voltage value of the low-voltage nucleation stage is 6~12 V, and the treatment time is 30~180 s; the effective voltage value of the gradual slope deepening stage increases from the effective voltage value of the low-voltage nucleation stage to 12~20 V at a rate of 0.2~1.5 V / min, and the treatment time is 60~600 s; the effective voltage value of the constant voltage color fixing stage is 12~22 V, and the treatment time is 30~300 s.

2. The anodizing electrolytic coloring method for aluminum alloy workpieces according to claim 1, characterized in that, The anodic oxidation process is carried out in a sulfuric acid-based electrolyte with an H₂SO₄ concentration of 150–220 g / L. The anodic oxidation process is direct current anodic oxidation, and the conditions include: a temperature of 15–25 °C and a current density of 1–2.5 A / dm³. 2 .

3. The anodizing electrolytic coloring method for aluminum alloy workpieces according to claim 1 or 2, characterized in that, The thickness of the oxide film is 8~25 μm.

4. The anodizing electrolytic coloring method for aluminum alloy workpieces according to claim 1, characterized in that, In step (2), the pulse frequency of the pulsed DC voltage is 50~1000 Hz and the duty cycle is 10~60%.

5. The anodizing electrolytic coloring method for aluminum alloy workpieces according to claim 1 or 4, characterized in that, In step (2), the conditions for the pre-adjusted barrier layer treatment include: a voltage of 0.5~5 V and a treatment time of 10~180 s; and a pH value of 2~6 for the acidic pretreatment solution.

6. The anodizing electrolytic coloring method for aluminum alloy workpieces according to claim 1, characterized in that, In step (3), the waveform of the asymmetric AC voltage includes an asymmetric rectangular wave, an asymmetric trapezoidal wave, or other asymmetric equivalent controllable waveforms. The amplitude, conduction angle, or pulse width of the positive half-cycle and the negative half-cycle of the waveform of the asymmetric AC voltage are different in at least one of the following: the ratio of the effective values ​​of the positive and negative half-cycles of the asymmetric AC voltage is 1.05 to 1.

50.

7. The anodizing electrolytic coloring method for aluminum alloy workpieces according to claim 1, characterized in that, When the aluminum alloy workpiece is a high-silicon aluminum alloy workpiece, before the pre-conditioning barrier layer treatment after the anodizing treatment, a decontamination treatment or an activation water washing treatment is also performed; the Si content of the high-silicon aluminum alloy workpiece is ≥3 wt%; the decontamination treatment is performed using an acidic decontamination solution, and the decontamination treatment time is 5~30 s; the activation water washing treatment is performed using water or an acidic water washing solution, and the activation water washing treatment time is 10~60 s.

8. The anodizing electrolytic coloring method for aluminum alloy workpieces according to claim 1, characterized in that, Before performing the anodizing process, the aluminum alloy workpiece is further subjected to a pretreatment, which includes degreasing, water washing, alkaline etching or surface conditioning, neutralization and brightening, and water washing in sequence.

9. The anodizing electrolytic coloring method for aluminum alloy workpieces according to claim 8, characterized in that, The alkaline etching is performed using a NaOH solution with a NaOH concentration of 20-60 g / L. The alkaline etching conditions include: a treatment temperature of 35-60 °C and a treatment time of 10-120 s. The surface conditioning is performed using a surface conditioning agent with a treatment time of 10-60 s. The neutralization and brightening treatment time is 10-90 s.

10. The anodizing electrolytic coloring method for aluminum alloy workpieces according to claim 1, characterized in that, After the segmented coloring process is completed, the colored workpiece is obtained. The process also includes pre-washing, sealing, washing and drying the colored workpiece in sequence. The sealing is done with hot water or nickel salt. The conditions for hot water sealing include: hot water temperature of 92~100 ℃ and immersion time of 10~10 min. The nickel salt sealing is performed using a nickel salt system with a nickel salt concentration of 5-15 g / L. The nickel salt sealing conditions include: a sealing temperature of 70-90 ℃, a sealing time of 5-20 min, and a pH value of 5.5-6.5.

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