Metal oxide or metal nitride coated steel fasteners

Abstract

A fastener assembly and a method for forming a coated fastener are provided. The fastener assembly includes a fastener configured to connect at least two metal components, and at least one of a metal oxide dielectric coating or a metal nitride dielectric coating disposed on an outer surface of the fastener. The fastener is formed of steel. The at least one of the metal oxide dielectric coating or the metal nitride dielectric coating includes an aluminum-based material and is configured to provide a dielectric barrier between the steel of the fastener and at least one second metal, the second metal being different from the steel of the fastener.

Description

Technical Field

[0001] This invention relates to a coated fastener, and more specifically, to a coated steel fastener for connecting different metal parts. Background Technology

[0002] Magnesium, as a lightweight structural metal, possesses high specific strength, stiffness, and excellent damping properties, making it an ideal material for body-in-white (BIW) applications in motor vehicles. However, when magnesium is bonded to another metal (such as aluminum or steel), it is prone to galvanic corrosion due to its low (negative) electrochemical potential. Magnesium is susceptible to corrosion in salt spray or humid environments (where it acts as an electrolyte). This is because magnesium, being more electronegative, acts as the anode and loses electrons to the more electronegative metal, such as aluminum or steel (which acts as the cathode). This electrochemical reaction leads to the oxidation of magnesium and subsequent corrosion.

[0003] In automotive applications, magnesium and aluminum sheets are typically connected using bolts and flow drill screws. These bolts are usually made of carbon steel because of its high strength and shear resistance, which are crucial for safety in high-stress applications. However, the introduction of steel bolts can form galvanic couples with magnesium and aluminum, exacerbating corrosion problems. To mitigate this issue, steel bolts are often coated with a zinc-based alloy because zinc is closer to magnesium in the galvanic series and provides better corrosion protection. Alternatively, aluminum bolts can be used, which are less corrosive to magnesium than steel bolts.

[0004] While existing fasteners have achieved their intended purpose, there is still a need for new and improved fasteners that offer enhanced performance and stability. Summary of the Invention

[0005] According to several aspects of the present invention, a fastener assembly is provided. The fastener assembly includes: a fastener configured for connecting at least two metal parts, and at least one of a metal oxide dielectric coating or a metal nitride dielectric coating disposed on the outer surface of the fastener. The fastener is formed of steel. At least one of the metal oxide dielectric coating or the metal nitride dielectric coating comprises an aluminum-based material and is configured to provide a dielectric barrier between the steel of the fastener and at least one second metal, the second metal being different from the steel of the fastener.

[0006] According to another aspect of the invention, the fastener is a bolt.

[0007] According to another aspect of the invention, the steel is carbon steel.

[0008] According to another aspect of the invention, at least one second metal includes at least one of magnesium (Mg), iron (Fe), or aluminum (Al).

[0009] According to another aspect of the invention, the first metal component is magnesium and the second metal component is aluminum.

[0010] According to another aspect of the invention, at least one of the metal oxide dielectric coating or the metal nitride dielectric coating comprises an inner layer of aluminum (Al) and an outer dielectric layer of Al2O3.

[0011] According to another aspect of the invention, at least one of the metal oxide dielectric coating or the metal nitride dielectric coating comprises an inner layer of aluminum (Al) and an outer dielectric layer of aluminum nitride (AlN).

[0012] According to various aspects of the present invention, a method for forming a coated fastener is provided. The method includes: applying an aluminum-based metal to the fastener using at least one of electroplating or hot-dip aluminizing; and converting the aluminum-based metal into at least one of a metal oxide dielectric coating or a metal nitride dielectric coating to form a coated fastener having a dielectric coating. The fastener is formed of steel. The dielectric coating is configured to provide a dielectric barrier between the steel of the fastener and at least one second metal, the second metal being different from the steel of the fastener.

[0013] According to another aspect of the invention, at least one of the metal oxide dielectric coating or the metal nitride dielectric coating includes aluminum oxide (Al2O3).

[0014] According to another aspect of the invention, the aluminum-based metal includes anodizing the aluminum-based metal coating.

[0015] According to another aspect of the invention, the aluminum-based metal includes an anodizing time between 30 minutes and 120 minutes.

[0016] According to another aspect of the invention, the aluminum-based metal is anodized at a voltage between 10 volts and 40 volts of direct current (DC).

[0017] According to another aspect of the invention, the aluminum-based metal is anodized at an electrolyte temperature between 5°C and 20°C.

[0018] According to another aspect of the invention, aluminum-based metals include anodizing using an H2SO4 solution concentration between 10 and 20% by weight.

[0019] According to another aspect of the invention, the aluminum-based metal includes plasma nitriding of the aluminum-based metal.

[0020] According to another aspect of the present invention, plasma nitriding of aluminum-based metals includes: subjecting the aluminum-based metal to argon (Ar) gas at a temperature between 250°C and 450°C. + Pre-sputtering removes the naturally formed oxide layer.

[0021] According to another aspect of the invention, plasma nitriding of aluminum-based metals is carried out at a temperature between 400°C and 500°C.

[0022] According to another aspect of the invention, converting aluminum-based metals includes forming aluminum oxide (Al2O3) on fasteners using a sol-gel process.

[0023] According to another aspect of the invention, at least one second metal comprises a first magnesium sheet and a second aluminum sheet.

[0024] According to several aspects of the present invention, a method for forming a coated fastener is provided. The method includes: applying an aluminum-based metal to the fastener using electroplating; and converting the aluminum-based metal into a metal oxide dielectric coating comprising aluminum oxide (Al₂O₃) using a hard coating anodizing process, thereby forming a coated fastener having the dielectric coating. The fastener is formed of steel. The aluminum-based metal comprises a first aluminum coating and a second aluminum oxide coating, and the total thickness of the first aluminum coating and the second aluminum oxide coating is between 10 micrometers and 30 micrometers (μm). The dielectric coating is configured to provide a dielectric barrier between the steel of the fastener and at least one of a first magnesium sheet and / or a second aluminum sheet.

[0025] The above-described features and advantages, as well as other features and advantages, of the system and method of the present invention will become apparent when taken in conjunction with the accompanying drawings and from the detailed embodiments, including the claims and examples. Attached Figure Description

[0026] This disclosure will be more fully understood in light of the specific embodiments and accompanying drawings, wherein:

[0027] Figure 1 This is a cross-sectional side view illustrating an example of a fastener assembly according to the invention, wherein the fastener assembly includes fasteners having a dielectric coating.

[0028] Figure 2 This illustrates the method for forming according to the invention. Figure 1 The diagram illustrates a method for fastening fasteners with dielectric coatings, which includes electroplating and anodizing or hot-dip plating.

[0029] Figure 3 This illustrates the use of the present invention in... Figure 1 The diagram illustrates the steps of a method for forming an alumina layer on a fastener, wherein the steps include plasma nitriding.

[0030] Figure 4 This illustrates the use of the present invention in... Figure 1 The diagram illustrates the steps of a method for forming an alumina layer on a fastener, wherein the steps include a sol-gel process. Detailed Implementation

[0031] The following description is merely exemplary in nature and is not intended to limit this disclosure, its application, or its uses. Furthermore, it is not intended to be bound by any express or implied theory presented in the foregoing background, summary of the invention, or the following detailed description. It should be understood that throughout the drawings, corresponding reference numerals denote similar or corresponding parts and features.

[0032] Several examples of the invention will now be described in detail with reference to the accompanying drawings. Where possible, the same or similar reference numerals are used in the drawings and description to denote the same or similar parts or steps. The drawings are presented in a simplified form and are not drawn to scale. The following description is merely exemplary in nature and is not intended to limit this disclosure, its application, or its uses.

[0033] This paper discloses a method for coating fasteners and forming such fasteners. Several challenges exist in using zinc (Zn) coated steel bolts and aluminum (Al) bolts in mixed-metal joints. First, for galvanic corrosion to occur, an electrical connection must exist between the different metal components. Ideally, the coating on the fastener metal should provide a dielectric barrier to prevent electron transfer from the anode to the cathode. However, metal coatings such as zinc (Zn) still allow electrical connections, increasing the likelihood of galvanic corrosion. Furthermore, zinc is a soft metal, making the zinc coating susceptible to wear or abrasion, especially in the threaded area of ​​the bolt. High friction and mechanical deformation during installation can cause coating delamination, leaving the threads unprotected. On the other hand, Al bolts are softer than steel, requiring redesign of the joint, for example, using larger diameters and modifying bolt spacing, to make them more suitable for low-stress applications to prevent bolt damage or failure due to overtightening.

[0034] To address these issues, an ideal steel bolt coating should act as a dielectric barrier to prevent electron flow between Mg (anode) and Fe / Al (cathode). The coating must also possess high strength and wear resistance to prevent wear in the head and threaded areas during bolt insertion. This paper discloses a method for forming a hard dielectric coating on steel fasteners (e.g., bolts or flow drill screws) to prevent corrosion in mixed-metal joints. The process involves electroplating or applying a pure Al layer to the bolt, followed by partial conversion of the Al layer into alumina (Al₂O₃) or aluminum nitride (AlN). This conversion forms a dielectric barrier between the metal components, effectively preventing galvanic corrosion.

[0035] Figure 1 A fastener assembly 10 is shown, which includes a fastener 12 and a metal oxide dielectric coating or a metal nitride dielectric coating 14 disposed on the outer surface of the fastener 12. Figure 1In the view, the coated fastener 12 is shown as connecting the first metal 16 and the second metal 18. The first metal 16 and / or the second metal 18 may be in the form of a sheet.

[0036] Fastener 12 may comprise carbon steel bolts or screws designed for high-stress applications requiring superior strength and durability. Fastener 12 is characterized by its high tensile strength and shear resistance, making it suitable for use as fastening components in the automotive, aerospace, and construction industries. Fastener 12 may feature a threaded shank and a hexagonal head, for example, facilitating installation and removal using standard tools. It is understood that fastener 12 may comprise, and may be formed from, other suitable metals and materials configured to be used as fasteners.

[0037] A dielectric coating 14 is applied to the outer surface of the fastener 12. To enhance corrosion resistance, the fastener 12 is coated with the dielectric coating 14, which provides a barrier protecting the underlying steel of the fastener 12 from oxidative degradation. Furthermore, the dielectric coating 14 ensures uniform coverage, including threaded areas, to prevent galvanic corrosion when used in conjunction with dissimilar metals such as aluminum or magnesium. This embodiment provides a reliable and cost-effective solution for connecting metal components while maintaining structural integrity and service life in harsh environments.

[0038] The dielectric coating 14 comprises an aluminum-based material (e.g., alumina (such as Al2O3)) configured to provide a dielectric barrier between the steel and magnesium or aluminum sheets of the fastener (e.g., two metal parts, a first metal 16 and / or a second metal 18) in an electrolyte environment. In addition to providing a dielectric barrier, the dielectric coating 14 is relatively harder than other conventional coatings or materials (such as zinc coatings and aluminum parts (e.g., bolts)). For example, zinc coatings are particularly susceptible to wear or abrasion. This can manifest as scratches in the fastener head during operation / installation. In particular, in the threaded region of the bolt, the zinc coating can delaminate due to high friction and mechanical deformation during bolt installation. This delamination leaves the threads vulnerable to corrosion. When using Al bolts, redesign of the joint may be necessary, such as using a larger diameter and / or modifying the bolt spacing. Therefore, Al bolts are better suited for low-stress applications where sufficiently low stress is required to prevent bolt damage or failure due to overtightening.

[0039] In this example, the dielectric coating 14 is aluminum oxide (Al2O3). The dielectric coating 14 can be formed by forming an aluminized coating (e.g., aluminum or an aluminum alloy) on the fastener 12, followed by anodizing the aluminized coating on the surface of the fastener 12 to at least partially convert it into an Al2O3 layer. The Al2O3 layer is advantageous because Al2O3 is electrically insulating and has a high dielectric strength ranging from 14 to 16 kV / mm. Furthermore, the Al2O3 layer has high hardness (~350 Vickers hardness (HV)), providing abrasion resistance to the dielectric coating 14.

[0040] In another example, the dielectric coating 14 comprises multiple layers. For instance, the dielectric coating 14 may comprise a first layer 20 (or inner layer) of aluminum or an aluminum alloy and a second layer 22 (or outer layer) of Al2O3. In another example, the dielectric coating 14 may comprise a first layer 20 (or inner layer) of aluminum or an alloy thereof and a second layer 22 (or outer layer) of aluminum nitride (AlN) (e.g. Figure 2 (As shown). It should be understood that the dielectric coating 14 may include other suitable materials that are dielectric and provide suitable hardness for fastening different metals.

[0041] like Figure 1 As shown, the electrolyte environment may include a first metal 16 (e.g., a magnesium sheet or other metal component) and a second metal 18 (e.g., an aluminum sheet or other metal component). When magnesium (Mg) is electrochemically coupled to steel (Fe), magnesium (Mg), acting as the anode, will oxidize and readily lose its electrons to the iron side of the couple (which acts as the cathode), resulting in magnesium (Mg) corrosion.

[0042] Figure 2 A method 100 for forming a coated fastener 12 used in the fastener assembly 10 described above is shown. Method 100 begins at step 102.

[0043] Step 102 describes applying an aluminum-based metal (aluminum layer 24) to the fastener 12 using at least one of electroplating or hot-dip aluminizing. In this step, the fastener 12 may be cleaned using, for example, sandblasting and / or chemical cleaning. In some cases, a zincate treatment may be applied to the fastener 12 to facilitate better adhesion to subsequent layers. Furthermore, in some cases, electroless nickel plating may be applied to the fastener 12 as a barrier and as a smooth surface for the aluminizing.

[0044] In the case of electroplating, as shown in step 102A, the fastener 12 can then be immersed in an electrolyte solution containing aluminum ions. An aluminum layer 24 is deposited onto the fastener 12 via an electrolytic process. In this example, the thickness of the aluminum layer 24 can be between 5 and 50 micrometers. Using an anodizing process, the metallic Al or Al alloy dielectric coating 14 is partially converted into an aluminum oxide (Al2O3) layer. When the dielectric coating 14 comprises two layers (e.g., Al and Al2O3), the thickness of the aluminum layer can range from 5 to 20 micrometers, and the thickness of the Al2O3 layer can be between 10 and 30 micrometers. After the anodizing treatment, the pores in the anodized layer are sealed using organic dyes and polymer sealants.

[0045] In the case of hot-dip aluminizing, as shown in step 102B, the fastener 12 is immersed in a bath of molten aluminum (Al) or aluminum-silicon alloy (AlSi) (e.g., 90 wt% aluminum / 10 wt% silicon, 93 wt% aluminum / 7 wt% silicon) or Al-Mg-Si alloy (containing about 8-10 wt% silicon and 5-15 wt% magnesium). The temperature of the aluminum bath can be about 700°C (1292°F). After the immersion step, the fastener 12 can be cooled to allow the resulting aluminum-based metal layer to solidify and form a strong metallurgical bond with the steel of the fastener 12.

[0046] Next, step 104 describes the conversion of the aluminum-based metal into a dielectric coating 14 to form a coated fastener 12. As described above, the dielectric coating 14 is configured to provide a dielectric barrier between the steel of the fastener and an electrolyte environment, the electrolyte environment comprising at least one second metal (different from the steel of the fastener), such as a first metal 16 and / or a second metal 18.

[0047] In an example, as shown in step 104A, converting the aluminum-based metal into a dielectric coating 14 includes anodizing the aluminum-based metal. Anodizing may include an electrolytic passivation process. During the anodizing of the fastener 12, the fastener 12 with the aluminum metal coating can be immersed and suspended as the anode in an electrolyte solution (e.g., sulfuric acid), using stainless steel as the cathode. In a specific example, converting the aluminum-based metal using anodizing may include using a voltage between 10 and 40 volts of direct current (VDC), an electrolyte temperature between 5 and 20°C, sulfuric acid (H₂SO₄) at a concentration between 10 and 20 wt%, and / or anodizing time between 30 and 120 minutes. In an example, the resulting anodized layer of alumina may be sealed with a sealant and / or nickel acetate, which may or may not contain organic dyes.

[0048] In another instance, such as Figure 3As shown, converting an aluminum-based metal into a dielectric coating 14 includes a method 300 of plasma nitriding the aluminum-based metal. Plasma nitriding (also known as ion nitriding) includes a thermochemical surface treatment. Typically, as shown in step 302, a naturally formed oxide layer 26 (e.g., Al2O3) can be formed on the surface of an aluminum metal layer deposited on the fastener 12. As shown in step 304, the naturally formed oxide layer 26 can be removed using, for example, argon pre-sputtering of the surface of the aluminum metal layer. In an example, argon pre-sputtering can be performed at a temperature between 250 and 450°C. Subsequently, as shown in step 306, plasma nitriding can be performed on the fastener 12 and the pre-sputtered aluminum-based metal 24. Plasma nitriding may include exposing the fastener 12 to nitrogen ions (N2O3). + Plasma nitriding forms a hard aluminum nitride (AlN) layer 28, as shown in step 308. In this example, plasma nitriding can be performed at a temperature between 400 and 500°C.

[0049] like Figure 4 As shown, converting an aluminum-based metal into a dielectric coating 14 can include a method 400 of forming alumina (Al2O3) on a fastener 12 using a sol-gel process. The sol-gel process includes converting a solution into a gel-like network, which can then be dried and heat-treated to form solid alumina (Al2O3). Typically, a metal alkoxide (e.g., an aluminum alkoxide) is used as a precursor and dissolved in a solvent to form a solution, as shown in step 402. Next, the fastener 12 is immersed in the solution, where it undergoes hydrolysis (step 404) and condensation reactions (step 406), resulting in the formation of a colloidal suspension or sol. The sol transforms into a gel of a network of interconnected particles. The fastener 12 is then removed and dried to remove the solvent, forming a porous solid (step 408). The fastener 12 is then heat-treated to densify and crystallize the porous solid, forming an alumina (Al2O3) coating.

[0050] The fastener 12 and method of the present invention offer advantages and benefits over the prior art. The fastener 12 disclosed herein proposes a method for forming a hard dielectric coating on steel fasteners (e.g., bolts or flow drill screws) to prevent corrosion in mixed metal joints. The process involves electroplating or applying a pure Al layer to the bolt, followed by partially converting the Al layer into aluminum oxide (Al₂O₃) or aluminum nitride (AlN). This conversion forms a dielectric barrier between the metal components, effectively preventing galvanic corrosion, while providing a robust coating that prevents delamination at the head or threads during fastener installation.

[0051] The description of this invention is merely exemplary in nature and is in no way intended to limit this disclosure, its application, or its uses. The broad teachings of this invention can be implemented in various forms. Therefore, although the invention includes specific examples, its true scope should not be so limited, as other modifications will become apparent upon examination of the drawings, description, and appended claims.

Claims

1. A fastener assembly, comprising: Fasteners configured for connecting at least two metal parts, wherein the fasteners are formed of steel; as well as At least one of a metal oxide dielectric coating or a metal nitride dielectric coating is disposed on the outer surface of the fastener, wherein the at least one of the metal oxide dielectric coating or the metal nitride dielectric coating comprises an aluminum-based material, and wherein the at least one of the metal oxide dielectric coating or the metal nitride dielectric coating is configured to provide a dielectric barrier between the steel of the fastener and at least one second metal, the second metal being different from the steel of the fastener.

2. The fastener assembly according to claim 1, wherein, The fastener is a bolt.

3. The fastener assembly according to claim 1, wherein, The steel in question is carbon steel.

4. The fastener assembly according to claim 1, wherein, The at least one second metal includes at least one of magnesium (Mg), iron (Fe), or aluminum (Al).

5. The fastener assembly according to claim 1, wherein, The first metal component is made of magnesium, and the second metal component is made of aluminum.

6. The fastener assembly according to claim 1, wherein, At least one of the metal oxide dielectric coatings or metal nitride dielectric coatings includes an inner layer of aluminum (Al) and an outer dielectric layer of Al2O3.

7. The fastener assembly according to claim 1, wherein, At least one of the metal oxide dielectric coatings or metal nitride dielectric coatings comprises an inner layer of aluminum (Al) and an outer dielectric layer of aluminum nitride (AlN).

8. A method for forming coated fasteners, comprising: An aluminum-based metal is applied to a fastener using at least one of electroplating or hot-dip aluminizing, wherein the fastener is formed of steel; as well as An aluminum-based metal is converted into at least one of a metal oxide dielectric coating or a metal nitride dielectric coating to form a coated fastener having a dielectric coating, wherein the dielectric coating is configured to provide a dielectric barrier between the steel of the fastener and at least one second metal, the second metal being different from the steel of the fastener.

9. The method according to claim 8, wherein, At least one of the metal oxide dielectric coatings or metal nitride dielectric coatings includes aluminum oxide, i.e., Al2O3.

10. The method according to claim 8, wherein, The conversion of aluminum-based metals includes anodizing aluminum-based metal coatings.

Images