Online reworking process applied to unqualified six-series aluminum alloy extruded profile

Abstract

The invention belongs to the technical field of aluminum alloy, and particularly relates to an online reworking process applied to an unqualified six-series aluminum alloy extruded profile. According to the method, efficient repair of complex surface defects of the unqualified six-series aluminum alloy extruded profile is achieved, a composite acid system with an accurate proportion is adopted as an etching solution, and a surface layer defect area can be selectively dissolved without damaging a base body; the corrosion inhibitor is prepared by compounding urotropine, benzotriazole and lauryl sodium sulfate according to an optimized ratio and can effectively inhibit excessive corrosion, and the complexing agent is prepared from etidronic acid, hydrogen peroxide, sulfamic acid and ammonium bifluoride in a synergistic manner and can efficiently chelate and dissolve aluminum ions released in the process and prevent secondary deposition. In the subsequent dust removal step, a mixed solution of low-concentration sulfuric acid and hydrogen peroxide is adopted, etching residues are thoroughly removed under the mild condition, and the phenomenon that a dust film interferes with the subsequent passivation effect is avoided.

Description

Technical Field

[0001] This invention belongs to the field of aluminum alloy technology, specifically relating to an online rework process for substandard 6-series aluminum alloy extruded profiles. Background Technology

[0002] While the current online rework process for defective 6-series aluminum alloy automotive structural components can address common surface defects such as flow marks, watermarks, spots, oil stains, corrosion, and impurities to some extent, it still reveals many shortcomings in terms of technology and operation in actual production applications, which seriously restricts rework efficiency and product quality consistency.

[0003] First, existing rework processes typically follow the standard pre-treatment procedures of normal production: "pre-degreasing—main degreasing—water washing—passivation—hot water washing—drying." This approach is not optimized for the specific contamination state of the rework parts. Especially for stubborn defects such as deep corrosion or embedded impurities, repeatedly using strong alkaline cleaning processes not only fails to completely remove contaminants but may also exacerbate the corrosion of the aluminum substrate due to repeated alkaline washing, and even induce new microcracks, further weakening the mechanical properties and corrosion resistance of the structural components.

[0004] Secondly, in the degreasing process, the pre-degreasing and main degreasing stages generally use alkaline degreasing agents with the same composition and similar concentrations, lacking a reasonable gradient design. This non-differentiated treatment method cannot effectively achieve the step-by-step removal of contaminants of different types and adhesion strengths. In particular, when dealing with highly viscous lubricating oil, high-temperature oxidation residues, or complex stains, the cleaning efficiency drops significantly, resulting in problems such as localized oil film residue or uneven cleaning after rework.

[0005] Furthermore, the passivation solutions used in the passivation process often contain highly corrosive components such as hydrofluoric acid and hexafluorotitanic acid, and their mechanism of action is extremely sensitive to process parameters. During rework, improper control—such as excessively long immersion times, large fluctuations in bath temperature, or excessively high aluminum ion concentration due to repeated rework—can easily lead to excessive corrosion in localized areas or uneven passivation film thickness, thereby inducing new surface defects such as spots, flow marks, or even pitting. These problems not only affect the appearance quality but may also interfere with the adhesion of subsequent coating or bonding processes, ultimately reducing the reliability of the entire vehicle assembly. Summary of the Invention

[0006] To address the aforementioned problems in the prior art, this invention presents an online rework process for substandard six-series aluminum alloy extruded profiles.

[0007] The objective of this invention can be achieved through the following technical solutions: An online rework process for defective 6-series aluminum alloy extruded profiles, the process comprising the following steps: S1. The aluminum alloy profile is etched in an etching solution and then removed from the ash in a ash removal solution. S2. Then place it in a passivation solution for passivation treatment, and finally dry it.

[0008] In the above-mentioned online rework process for substandard six-series aluminum alloy extruded profiles, the etching solvent raw materials are as follows by mass percentage: 5-15% sulfuric acid, 1-5% nitric acid, 0.3-0.8% corrosion inhibitor, 0.3-0.8% complexing agent, and the balance is water.

[0009] In the aforementioned online rework process for substandard six-series aluminum alloy extruded profiles, the corrosion inhibitor is a mixture of hexamethylenetetramine, benzotriazole, and sodium dodecyl sulfate in a mass ratio of 100:(10-20):(1-5). The corrosion inhibitor of this invention is a mixture of hexamethylenetetramine, benzotriazole (BTA), and sodium dodecyl sulfate (SDS) in a specific ratio, which effectively inhibits the anode and cathode processes of the corrosion cell and forms a composite multidimensional protective film. Following descaling, the profile is placed in a passivation solution to enhance its corrosion resistance, and finally dried. Specifically, hexamethylenetetramine and BTA work synergistically at the electrochemical level, acting as a cathode-type corrosion inhibitor to slow down the hydrogen evolution reaction and forming a dense chemical adsorption film on the metal surface to directly prevent metal dissolution. Simultaneously, SDS, as a surfactant, optimizes the transport process by significantly reducing the surface tension of the solution, ensuring that the corrosion inhibitor can uniformly cover the entire metal surface, thereby maximizing utilization efficiency. This comprehensive treatment method not only improves the corrosion resistance of the material but also significantly extends its service life, providing an effective technical solution to the quality problems of 6-series aluminum alloy extruded profiles.

[0010] In the above-mentioned online rework process for substandard six-series aluminum alloy extruded profiles, the complexing agent is hydroxyethylidene diphosphonic acid, hydrogen peroxide, aminosulfonic acid, and ammonium bifluoride in a mass ratio of (15-25):(3-8):(5-15):(1-5). In this invention, hydroxyethylidene diphosphonic acid acts as a powerful chelating agent, rapidly complexing and stabilizing aluminum and other metal ions in the solution, preventing their redeposition; hydrogen peroxide exerts an oxidizing effect, effectively decomposing organic pollutants and destroying the surface oxide film; aminosulfonic acid provides a mild and stable acidic environment, promoting the continuous reaction without excessively corroding the substrate; and ammonium bifluoride exhibits high specificity, efficiently dissolving common silicide impurities and stubborn passivation layers on the aluminum alloy surface. The four components work together to construct a cascade reaction mechanism of "oxidation film breaking - acid hydrolysis activation - complexation solubilization", which simultaneously removes complex dirt and bad oxide layers from the surface of the profile from multiple levels. This not only significantly improves cleaning efficiency, but also lays a good foundation for subsequent passivation and drying treatment, thereby ensuring the surface quality and performance consistency of the profile after rework.

[0011] In the above-mentioned online rework process for substandard six-series aluminum alloy extruded profiles, the etching temperature is 20-25℃ and the time is 150-200s.

[0012] In the above-mentioned online rework process for substandard six-series aluminum alloy extruded profiles, the descaling solution is a mixture of sulfuric acid solution and hydrogen peroxide solution with a mass ratio of 1:(3-5), wherein the concentration of sulfuric acid solution is 6-10 g / L and the concentration of hydrogen peroxide solution is 20-40 g / L.

[0013] In the above-mentioned online rework process for substandard 6-series aluminum alloy extruded profiles, the ash removal temperature is 22-26℃ and the time is 100-150s.

[0014] In the aforementioned online rework process for substandard six-series aluminum alloy extruded profiles, the passivation solution raw materials, by mass percentage, are: 10-20% sulfuric acid, 5-15% hexafluorotitanic acid, 0.05-0.07% two-dimensional transition metal carbide material, 5-15% ammonium sulfate, 1-5% hydrofluoric acid, and the balance being water. This invention's passivation system is based on a strong acid environment, effectively activating the aluminum alloy surface and moderately dissolving the impurity layer. Hexafluorotitanic acid hydrolyzes under acidic conditions, generating highly active nano-TiO2 nuclei in situ. The two-dimensional transition metal carbide material not only provides abundant heterogeneous nucleation sites but also constructs a conductive network, promoting electron transfer and uniform nucleation of the film. Simultaneously, the synergistic effect of ammonium sulfate and hydrofluoric acid precisely controls the solution pH and fluoride ion concentration, both inhibiting excessive corrosion and accelerating the densification growth of the titanium-based passivation film. The resulting titanium-carbide composite passivation film exhibits excellent adhesion, density, and chemical stability. Its corrosion resistance is significantly superior to that of traditional single titanium salt conversion films, providing long-term protection for reworked 6-series aluminum alloy profiles while meeting the stringent surface conditions requirements of subsequent coating or anodizing processes.

[0015] As a preferred option, the two-dimensional transition metal carbide material is MXene-Ti3C2T. x .

[0016] The two-dimensional transition metal carbide material used in the passivation solution of this invention is MXene-Ti3C2T. x Its unique layered structure, high specific surface area, excellent conductivity, and abundant oxygen-containing functional groups (such as –OH, –F, =O, etc.) enable it to play a dual crucial role as a conductive framework and nucleation template during passivation. Specifically, MXene-Ti3C2T x It can effectively adsorb Ti generated by the hydrolysis of hexafluorotitanic acid 4⁺ ions are oriented and guided to deposit in situ on the surface of the aluminum alloy to form nano-sized TiO2, thereby promoting the orderly and dense growth of the titanium-based passivation film. Simultaneously, under the synergistic regulation of ammonium sulfate and hydrofluoric acid, the pH value and fluoride ion concentration of the solution are optimized, further regulating the nucleation rate and crystal orientation of the film and inhibiting defect formation. Ultimately, a titanium-based composite passivation film with high density, good conductivity, and excellent mechanical strength and toughness is constructed on the aluminum alloy surface. This film not only significantly improves the corrosion resistance of the material but also enhances the interfacial bonding with subsequent coatings or processing techniques. Its comprehensive performance far exceeds that of traditional titanium salt conversion films, providing technical support for the efficient online rework of substandard 6-series aluminum alloy profiles.

[0017] In the above-mentioned online rework process for substandard six-series aluminum alloy extruded profiles, the passivation treatment temperature is 35-45℃ and the time is 90-180s.

[0018] In the above-mentioned online rework process for substandard six-series aluminum alloy extruded profiles, after etching, dust removal and passivation, a hot water washing process is also included, wherein the hot water washing temperature is 58-62℃ and the time is 55-65s.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention is applied to the online rework process of defective six-series aluminum alloy extruded profiles. Compared with traditional rework methods, it has significant technical advantages and many innovations, which not only improve rework efficiency and surface treatment quality, but also greatly reduce costs and resource waste.

[0020] 2. This invention abandons the traditional, crude rework method that relies on repeated alkaline washing and manual polishing. Through a scientifically designed three-step core process—etching → descaling → passivation—it achieves highly efficient repair of complex surface defects such as flow marks, watermarks, spots, oil stains, corrosion, and embedded impurities. The etching solution uses a precisely proportioned composite acid system, acting at 20-25°C for 150-200 seconds to selectively dissolve surface defect areas without damaging the substrate. The corrosion inhibitor is a compound of hexamethylenetetramine, benzotriazole, and sodium dodecyl sulfate in an optimized ratio, effectively inhibiting excessive corrosion. The complexing agent is composed of hydroxyethylidene diphosphonic acid, hydrogen peroxide, aminosulfonic acid, and ammonium bifluoride, which efficiently chelates aluminum ions released during dissolution, preventing secondary deposition. The subsequent descaling step uses a mixed solution of low-concentration sulfuric acid and hydrogen peroxide to thoroughly remove etching residues under gentle conditions, preventing the ash film from interfering with the subsequent passivation effect.

[0021] 3. This invention innovatively introduces a two-dimensional transition metal carbide material, MXene-Ti3C2T, into the passivation solution. x(Addition amount is only 0.05-0.07%), its high specific surface area and excellent chemical activity significantly enhance the density, uniformity, and corrosion resistance of the passivation film. Combined with the synergistic effect of hexafluorotitanic acid, hydrofluoric acid, and ammonium sulfate, a high-quality conversion film can be formed within 90-180 seconds at 41-42℃. Each stage of the process is followed by a precisely temperature-controlled hot water wash (58-62℃, 55-65 seconds) to ensure a clean, residue-free surface. More importantly, this process is fully automated online, requiring no scrapping or manual intervention. This not only saves significant manpower and time costs but also greatly shortens the rework cycle, allowing defective products to be quickly returned to the normal production line, effectively reducing inventory stagnation and significantly lowering the defect rate. In summary, this rework process combines high efficiency, environmental friendliness, and economy, providing an advanced and reliable solution for the quality control of 6-series aluminum alloy structural components. Attached Figure Description

[0022] Figure 1 The image shows the substandard aluminum alloy before rework in Example 1.

[0023] Figure 2 The image shows the aluminum alloy after rework in Example 1. Detailed Implementation

[0024] The technical solution of the present invention will be further described below with reference to specific embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0025] The substandard aluminum alloy in the following examples has the following composition: 0.40 wt% silicon, 0.12 wt% iron, 0.06 wt% copper, 0.02 wt% manganese, and 0.55 wt% magnesium, with the balance being aluminum and unavoidable impurities. Products that are defective due to watermarks, oil stains, flow marks, or substandard film weight require rework.

[0026] The MXene-Ti3C2T used in the following examples x MXene-Ti3C2T was prepared using analytical grade Ti3AlC2(MAX) purchased from Xi'an Qiyue Biotechnology Co., Ltd., and prepared by the following method. x First, the Al layer of the precursor Ti3AlC2(MAX) is selectively etched to obtain MXene-Ti3C2T. x Nanosheets. The specific steps are as follows: 20 mL of 9 M HCl was added to 2.0 g of LiF, stirred for 30 min, and then ultrasonically stirred for 5 min. Subsequently, 5 g of Ti3AlC2 was added to the above mixture, and the mixture was stirred in a water bath at 50 °C for 24 h. The mixture was then washed several times with water until the pH reached 6. Finally, MXene-Ti3C2T... xThe suspension needs to be diluted; long-term use requires freeze-drying. Note: LiF analytical grade was purchased from Beijing Yili Fine Chemicals Co., Ltd.

[0027] Example 1: S1. Place the aluminum alloy profile in the etching solution at 22°C for 180 seconds, and then wash it with water at 60°C for 60 seconds. The etching solvent raw materials, by mass percentage, are: 10% sulfuric acid, 3% nitric acid, 0.5% corrosion inhibitor, 0.5% complexing agent, and the balance being water; The corrosion inhibitor is hexamethylenetetramine, benzotriazole and sodium dodecyl sulfate in a mass ratio of 100:15:5; The complexing agent is hydroxyethylidene diphosphonic acid, hydrogen peroxide, aminosulfonic acid and ammonium hydrogen fluoride in a mass ratio of 20:5:10:3; S2. Then place it in the ash removal solution for 25℃ ash removal treatment for 120s, and then wash it with water at 60℃ for 60s. The ash removal solution is a mixture of sulfuric acid solution and hydrogen peroxide solution in a mass ratio of 1:4, wherein the concentration of sulfuric acid solution is 8 g / L and the concentration of hydrogen peroxide solution is 35 g / L. S3. Then place it in the passivation solution for passivation treatment at 42℃ for 150s, and then wash it with water at 60℃ for 60s. The passivation solution raw materials, by mass percentage, are: 15% sulfuric acid, 10% hexafluorotitanic acid, and 0.06% MXene-Ti3C2T. x 10% ammonium sulfate, 3% hydrofluoric acid, and the remainder is water.

[0028] S4. Finally, dry in an 80℃ oven for 600 seconds.

[0029] All washes are performed with pure water, all have overflow, conductivity <100, and pH=6.9.

[0030] Figure 1 The image shows the defective aluminum alloy before rework in Example 1. Figure 2 The image shows the aluminum alloy after rework in Example 1. As can be seen from the image, the surface of the non-conforming 6-series aluminum alloy passivated product has residual oil stains and watermarks, severely affecting its appearance and welding performance in subsequent processes. After the rework process of this invention, the product appearance is free of corrosion, oil stains, watermarks, and other appearance defects, and the passivation film thickness is within the standard range.

[0031] Example 2: S1. Place the aluminum alloy profile in the etching solution at 22°C for 180 seconds, and then wash it with water at 60°C for 60 seconds. The etching solvent raw materials, by mass percentage, are: 5% sulfuric acid, 1% nitric acid, 0.3% corrosion inhibitor, 0.3% complexing agent, and the balance is water; The corrosion inhibitor is hexamethylenetetramine, benzotriazole and sodium dodecyl sulfate in a mass ratio of 100:10:1; The complexing agent is hydroxyethylidene diphosphonic acid, hydrogen peroxide, aminosulfonic acid and ammonium hydrogen fluoride in a mass ratio of 20:3:5:1; S2. Then place it in the ash removal solution for 25℃ ash removal treatment for 120s, and then wash it with water at 60℃ for 60s. The ash removal solution is a mixture of sulfuric acid solution and hydrogen peroxide solution in a mass ratio of 1:3, wherein the concentration of sulfuric acid solution is 8 g / L and the concentration of hydrogen peroxide solution is 35 g / L. S3. Then place it in the passivation solution for passivation treatment at 42℃ for 150s, and then wash it with water at 60℃ for 60s. The passivation solution raw materials, by mass percentage, are: 10% sulfuric acid, 5% hexafluorotitanic acid, and 0.05% MXene-Ti3C2T. x 5% ammonium sulfate, 1% hydrofluoric acid, and the remainder is water.

[0032] S4. Finally, dry in an 80℃ oven for 600 seconds.

[0033] All washes are performed with pure water, all have overflow, conductivity <100, and pH=6.9.

[0034] Example 3: S1. Place the aluminum alloy profile in the etching solution at 22°C for 180 seconds, and then wash it with water at 60°C for 60 seconds. The etching solvent raw materials, by mass percentage, are: 15% sulfuric acid, 5% nitric acid, 0.8% corrosion inhibitor, 0.8% complexing agent, and the balance being water; The corrosion inhibitor is hexamethylenetetramine, benzotriazole and sodium dodecyl sulfate in a mass ratio of 100:20:5; The complexing agent is hydroxyethylidene diphosphonic acid, hydrogen peroxide, aminosulfonic acid and ammonium hydrogen fluoride in a mass ratio of 20:8:15:5; S2. Then place it in the ash removal solution for 25℃ ash removal treatment for 120s, and then wash it with water at 60℃ for 60s. The ash removal solution is a mixture of sulfuric acid solution and hydrogen peroxide solution in a mass ratio of 1:5, wherein the concentration of sulfuric acid solution is 8 g / L and the concentration of hydrogen peroxide solution is 35 g / L. S3. Then place it in the passivation solution for passivation treatment at 42℃ for 150s, and then wash it with water at 60℃ for 60s. The passivation solution raw materials, by mass percentage, are: 20% sulfuric acid, 15% hexafluorotitanic acid, and 0.07% MXene-Ti3C2T. x 15% ammonium sulfate, 5% hydrofluoric acid, and the remainder is water.

[0035] S4. Finally, dry in an 80℃ oven for 600 seconds.

[0036] All washes are performed with pure water, all have overflow, conductivity <100, and pH=6.97.

[0037] Example 4: The only difference from Example 1 is that no corrosion inhibitor was added to the etching solvent raw material.

[0038] Example 5: The only difference from Example 1 is that hexamethylenetetramine was not added to the corrosion inhibitor raw material.

[0039] Example 6: The only difference from Example 1 is that benzotriazole was not added to the corrosion inhibitor raw material.

[0040] Example 7: The only difference from Example 1 is that no complexing agent was added to the etching solvent raw material.

[0041] Example 8: The only difference from Example 1 is that hydroxyethylidene diphosphonic acid was not added to the complexing agent raw material.

[0042] Example 9: The only difference from Example 1 is that sulfamic acid was not added to the complexing agent raw material.

[0043] Example 10: The only difference from Example 1 is that ammonium bifluoride was not added to the complexing agent raw material.

[0044] Example 11: The only difference from Example 1 is that the ash removal solution is only a sulfuric acid solution.

[0045] Example 12: The only difference from Example 1 is that the ash removal solution is only a hydrogen peroxide solution.

[0046] Example 13: The only difference from Example 1 is that MXene-Ti3C2T was not added to the passivation solution raw material. x .

[0047] Comparative Example 1: The only difference from Example 1 is that no etching process was performed.

[0048] Comparative Example 2: The only difference from Example 1 is that no ash removal treatment was performed.

[0049] Comparative Example 3: The only difference from Example 1 is that no passivation treatment was performed.

[0050] The aluminum alloys from Examples 1-13 and Comparative Examples 1-3, after online rework, underwent the following quality testing: A ThermoNition XL3t handheld spectrometer was used for film weight testing. Three points (front, middle, and back) were selected as test locations, and the measured values ​​were recorded using the instrument readings. The results were expressed in mg / m³. 2 The membrane weight is expressed in units of 1, and the results are shown in Table 1.

[0051] Table 1: Results of online rework performance testing of aluminum alloys The results show that the process of this invention abandons the traditional, extensive rework method that relies on repeated alkaline washing and manual polishing. Through a scientifically designed three-step core process—etching → descaling → passivation—it achieves efficient repair of complex surface defects such as flow marks, watermarks, spots, oil stains, corrosion, and embedded impurities. The etching solution uses a precisely proportioned composite acid system, acting at 20-25°C for 150-200 seconds, selectively dissolving surface defect areas without damaging the substrate. The corrosion inhibitor is a compound of hexamethylenetetramine, benzotriazole, and sodium dodecyl sulfate in an optimized ratio, effectively inhibiting excessive corrosion. The complexing agent is composed of hydroxyethylidene diphosphonic acid, hydrogen peroxide, aminosulfonic acid, and ammonium bifluoride, which efficiently chelates aluminum ions released during dissolution, preventing secondary deposition. The subsequent descaling step uses a mixed solution of low-concentration sulfuric acid and hydrogen peroxide to thoroughly remove etching residues under gentle conditions, avoiding interference from the subsequent passivation effect.

[0052] All aspects, embodiments, and features of this invention should be considered illustrative in all respects and not limiting of the invention, the scope of which is defined only by the claims. Other embodiments, modifications, and uses will become apparent to those skilled in the art without departing from the spirit and scope of the invention as claimed. Finally, it should be noted that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the implementation of the invention.

Claims

1. An online rework process for defective 6-series aluminum alloy extruded profiles, characterized in that, The process includes the following steps: S1. The aluminum alloy profile is etched in an etching solution and then removed from the ash in a ash removal solution. S2. Then place it in a passivation solution for passivation treatment, and finally dry it.

2. The online rework process for defective 6-series aluminum alloy extruded profiles according to claim 1, characterized in that, The etching solvent raw materials, by mass percentage, are: 5-15% sulfuric acid, 1-5% nitric acid, 0.3-0.8% corrosion inhibitor, 0.3-0.8% complexing agent, and the balance is water.

3. The online rework process for defective 6-series aluminum alloy extruded profiles according to claim 2, characterized in that, The corrosion inhibitor is hexamethylenetetramine, benzotriazole and sodium dodecyl sulfate in a mass ratio of 100:(10-20):(1-5).

4. The online rework process for substandard 6-series aluminum alloy extruded profiles according to claim 2, characterized in that, The complexing agent is hydroxyethylidene diphosphonic acid, hydrogen peroxide, aminosulfonic acid and ammonium fluoride in a mass ratio of (15-25):(3-8):(5-15):(1-5).

5. The online rework process for substandard 6-series aluminum alloy extruded profiles according to claim 2, characterized in that, The etching temperature is 20-25℃ and the time is 150-200s.

6. The online rework process for defective 6-series aluminum alloy extruded profiles according to claim 1, characterized in that, The ash removal solution is a mixture of sulfuric acid solution and hydrogen peroxide solution in a mass ratio of 1:(3-5), wherein the concentration of sulfuric acid solution is 6-10 g / L and the concentration of hydrogen peroxide solution is 20-40 g / L.

7. The online rework process for defective 6-series aluminum alloy extruded profiles according to claim 1, characterized in that, The ash removal process is carried out at a temperature of 22-26℃ for 100-150 seconds.

8. The online rework process for defective 6-series aluminum alloy extruded profiles according to claim 1, characterized in that, The passivation solution raw materials, by mass percentage, are: 10-20% sulfuric acid, 5-15% hexafluorotitanic acid, 0.05-0.07% two-dimensional transition metal carbide material, 5-15% ammonium sulfate, 1-5% hydrofluoric acid, and the balance is water.

9. The online rework process for defective 6-series aluminum alloy extruded profiles according to claim 1, characterized in that, The passivation treatment temperature is 35-45℃, and the time is 90-180s.

10. The online rework process for defective 6-series aluminum alloy extruded profiles according to claim 1, characterized in that, After etching, descaling, and passivation, a hot water washing process is also included, with a temperature of 58-62℃ and a duration of 55-65 seconds.

Images