Bi-Metal Screws: The Highest-Performing Self-Tapping Screws: A Complete Guide to Applications and Selection

Screws play an indispensable role as essential fasteners in numerous fields, including industrial manufacturing, building renovation, and home appliance production. Bi-metal self-tapping screws, with their unique structural design and excellent overall performance, are gradually becoming a preferred alternative to traditional single-material screws. This article will comprehensively analyze the core characteristics, application scenarios, key purchasing considerations, and maintenance techniques of bi-metal screws, helping industry practitioners and procurement personnel accurately grasp the core value of this new type of fastener.

I. Bi-Metal Screws & Self-Tapping Screws: Definition and Core Structure

To understand the advantages of bi-metal self-tapping screws, it's first necessary to understand their basic definition and structural components. Self-tapping screws are fasteners that, without pre-drilling a hole in the workpiece, can be screwed into the material using their own threads to form a mating thread. They widely address the inefficiency of traditional screw installation. Bi-metal screws are an upgraded version of self-tapping screws, created using bimetal composite technology.

Their core construction features a composite structure combining an anti-corrosion outer layer and a high-strength inner layer. The outer layer is typically made of stainless steel, such as 304 or 316, leveraging its excellent corrosion and weather resistance to protect against environmental damage. The inner layer, made of a high-strength base material like 45 steel or alloy steel, ensures the screw maintains its mechanical properties during screwing and load-bearing, preventing breakage or deformation. This "rigid-flexible" design addresses the high cost and insufficient strength of all-stainless steel screws, while also addressing the rust and short lifespan of ordinary carbon steel self-tapping screws, achieving a perfect balance between performance and cost.

In terms of production, bi-metal screws undergo multiple steps, including base material molding, surface lamination, thread processing, and heat treatment. Bi-metal composite technology is the core. The currently mainstream "metallurgical bonding method" creates a strong atomic bond between the outer stainless steel layer and the inner base material, ensuring that delamination and detachment will not occur under the application of installation torque. This is the core difference between high-quality bi-metal screws and inferior composite screws.

II. Core Advantages of Bi-Metal Screws over Traditional Screws

Compared with ordinary carbon steel self-tapping screws and all-stainless steel screws, bi-metal screws offer significant advantages in performance, cost, and applicability. These advantages can be summarized in the following four aspects:

1. A Double Breakthrough in Corrosion Resistance and Strength

Although ordinary carbon steel self-tapping screws are inexpensive, they are prone to rusting in harsh environments such as humidity, acidity, and alkalinity, leading to loosening, failure, and even equipment failure. While all-stainless steel screws are corrosion-resistant, they are relatively weak, making them prone to thread stripping and shank breakage when installing thick plates or high-strength materials. The stainless steel outer layer of a bi-metal screw effectively protects against corrosion from water, oxygen, and chemical media, while the high-strength inner base material can withstand tensile strengths exceeding 500 MPa. Testing has shown that in humid outdoor environments, its service life is 3-5 times longer than that of carbon steel self-tapping screws. In high-stress installation scenarios, its load-bearing capacity is over 40% higher than that of all-stainless steel screws.

2. A precise balance of cost and performance

Due to the high cost of raw materials, all-stainless steel screws typically cost 3-4 times more than carbon steel screws, placing significant pressure on large-scale production. Bi-metal screws utilize only stainless steel for the outer layer, while using a lower-cost carbon steel base material for the inner layer. This reduces overall costs by over 50% compared to all-stainless steel screws, while also significantly outperforming carbon steel self-tapping screws. For example, a home appliance company with an annual production of 1 million units could save 200,000 to 300,000 yuan in fastener procurement costs by switching to bi-metal screws, and the after-sales repair rate due to screw failure would drop from 8% to below 1.5%.

3. Comprehensive Improvement in Installation Efficiency and Adaptability

As an upgraded version of self-tapping screws, bi-metal screws inherit the advantage of self-tapping screws that require no pre-drilling. Furthermore, due to the higher hardness of the inner base material, they offer a 20% increase in screw insertion speed compared to all-stainless steel self-tapping screws. This ensures a "one-tight, one-tight" installation, especially in common materials like aluminum alloy and galvanized steel. Furthermore, their optimized thread design offers a variety of options, including coarse and fine pitch, and self-drilling and self-tapping screws. They are compatible with workpieces made of diverse materials, including wood, metal, and plastic, and cover a wide range of applications, from light appliances to heavy machinery.

4. Significant Environmental Adaptability and Environmental Protection

The advantages of bi-metal screws are even more pronounced in applications involving extreme environments. In coastal areas with high salt fog, the screws can withstand salt spray tests for over 500 hours, far exceeding the 200-hour standard for carbon steel screws. Even in cold chain equipment at temperatures as low as -40°C, their impact toughness remains stable, preventing the risk of brittle fracture. Furthermore, the bi-metal screw production process reduces stainless steel consumption, aligning with the industry trend of lightweighting and energy conservation. Furthermore, the bi-metal screws can be recycled through metal separation technology after disposal, making them more environmentally friendly than single-material screws.

III. Analysis of Key Application Scenarios for Bi-Metal Screws

Leveraging the aforementioned advantages, bi-metal screws have achieved large-scale adoption across multiple industries, becoming a preferred alternative to self-tapping screws in diverse scenarios. The following is a detailed explanation of key application areas:

1. Curtain Wall and Decoration Engineering

During curtain wall installation, screws must withstand both wind loads and corrosive environments. Traditional carbon steel self-tapping screws are susceptible to rusting due to rainwater erosion, causing curtain wall panels to loosen. Stainless steel screws, however, are insufficiently strong and prone to failure when installing thick aluminum alloy profiles. Bi-metal screws address this conflict and are currently widely used in applications such as connecting aluminum studs in glass curtain walls and securing hanging fixtures in stone curtain walls. After adopting bi-metal self-tapping screws in a large commercial complex project, the inspection pass rate for curtain wall connectors increased from 92% to 100%, and the service life is expected to exceed 25 years, far exceeding the original design standard of 15 years.

2. Home Appliance and Electronic Equipment Manufacturing

Home appliances place extremely high demands on screws for rust resistance and lightweighting. Components like washing machine drums and refrigerator condensers are exposed to humid environments for extended periods. Rusting not only affects the appearance but can also contaminate food or cause electrical failures. Bi-metal screws are widely used in sheet metal connections in appliances like washing machines, air conditioners, and refrigerators. Data from a well-known home appliance company shows that after switching to bi-metal self-tapping screws, complaints about rust in washing machine drums decreased by 90%, and the product qualification rate increased to 99.8%. In the electronic equipment sector, miniaturized bi-metal screws are also used to secure laptop and smartphone casings and internal components, ensuring both rust resistance and assembly precision.

3. Automotive and New Energy Industries

In automotive manufacturing, screws in chassis, doors, engine compartments, and other areas must withstand multiple challenges, including vibration, high temperatures, and corrosion. Traditional carbon steel self-tapping screws require additional galvanizing for rust prevention, but the coating easily wears during installation. Fully stainless steel screws struggle to meet the high-strength connection requirements. Bi-metal screws excel in applications such as automotive seat rails, exhaust pipe fastenings, and battery pack frame connections. They are particularly well-suited to the harsh environment of the battery compartment in new energy vehicles, where their resistance to electrolyte corrosion makes them ideal. After adopting bi-metal screws, a new energy vehicle manufacturer achieved IP68-rated sealing and rust resistance for its battery packs, while also reducing fastener costs per vehicle by 15%.

4. Machinery and Equipment Assembly

In heavy equipment such as machine tools, construction machinery, and agricultural machinery, the strength and durability of screws directly impact operational stability. The high-strength inner base material of bi-metal screws is suitable for joining hard materials such as thick steel plates and cast iron. Its wear resistance prevents threads from loosening under long-term vibration. After a machine tool manufacturer replaced traditional carbon steel self-tapping screws with bi-metal screws, the failure rate of spindle connections on their equipment dropped from 5% to 0.3%, and the equipment maintenance cycle was doubled.

IV. How to Accurately Select Bi-Metal Self-Tapping Screws?

With a wide variety of bi-metal screws on the market, careful selection requires careful consideration of key factors such as material, specifications, and craftsmanship. This can help prevent installation failures and costly losses due to the wrong product. Key points for accurate selection are as follows:

1. Clarify the Material Combination and Applicable Environment

The performance of a bi-metal screw hinges on the material combination of its outer and inner layers, which should be tailored to the intended use. For humid environments (such as interior decoration), a "304 stainless steel + 45 steel" combination is recommended. For high-salt spray environments (such as coastal construction and ships), a "316 stainless steel + alloy steel" combination is recommended. For low-temperature environments (such as cold chain equipment), a "304L stainless steel + low-carbon alloy steel" combination is recommended for low-temperature resistance. When purchasing, request a material test report from the supplier confirming that the bimetallic bond strength is ≥200 MPa to avoid purchasing inferior products with "electroplated imitation composite" finishes.

2. Matching Specifications with Installation Requirements

Specifications should be selected based on the workpiece material, thickness, and installation torque. For soft materials like wood and plastic, choose fine-thread bi-metal self-tapping screws with a diameter of 3-5mm and a pitch of 1.25mm. For medium-hard materials like aluminum alloy and galvanized steel, choose standard-thread products with a diameter of 4-8mm and a pitch of 1.5mm. For hard materials like carbon steel and cast iron, choose coarse-thread products with a diameter of 6-12mm and a pitch of 2.0mm. A self-drilling tip is recommended for improved installation efficiency. Also, choose a product with a yield strength suitable for the torque range of the installation tool. Generally, hand tools are suitable for screws with a yield strength of 400MPa or less, while power tools are suitable for screws with a yield strength of 600MPa or less.

3. Verify Workmanship Details and Quality Certification

High-quality bi-metal screws should have a smooth, burr-free surface, clear, and complete threads, and no visible gaps or bubbles at the bimetallic joint. Quality can be assessed through simple tests: scratch the surface with a hard object; the stainless steel outer layer should not peel off. Soak the screw in a 5% saline solution for 24 hours; no rust should appear. Additionally, prioritize products that are ISO 9001 certified and RoHS certified. For export, they must also comply with international standards such as ASTM F543 (US) or DIN 7504 (German) to ensure their suitability for global markets.

4. Compare Supplier Capabilities and After-Sales Service

Choose a bi-metal screw supplier with independent production capabilities to avoid markups imposed by middlemen. Investigate their production equipment (such as bi-metal composite production lines and thread rolling machines) and testing equipment (such as tensile testing machines and salt spray test chambers) to ensure consistent production capacity and quality. Also, focus on after-sales service. High-quality suppliers should provide technical selection guidance, sample testing services, and a return and exchange guarantee for product quality issues. Especially for bulk purchases, request a quality agreement that clearly defines the warranty period for key indicators such as rust protection life and strength.

V. Installation and Maintenance Tips for Bi-Metal Self-Tapping Screws

Proper installation and maintenance are crucial to maximize the performance of bi-metal screws. Improper handling can cause screw breakage, rust, or joint failure. The following are key tips:

1. Pre-Installation Preparation

Before installation, adjust tool parameters based on the workpiece material. For soft materials (wood, plastic), use an electric screwdriver with a speed of 1500-2000 rpm and a torque setting of 70% of the screw's nominal torque. For hard materials (metal, cast iron), reduce the speed to 800-1200 rpm and set the torque to 85%-90% of the nominal torque. Also, clean the workpiece surface from oil, rust, and debris to prevent impurities from becoming lodged in the threads during installation and affecting the tightness of the connection. For workpieces thicker than 15mm, it is recommended to pre-drill a pilot hole (with a diameter of 60%-70% of the screw's nominal diameter) to reduce screw insertion resistance and protect thread integrity.

2. Installation Procedures

When holding the power tool, keep it perpendicular to the workpiece to avoid tilting the screw, which may cause thread deflection or breakage. If significant resistance is encountered during screwing, stop immediately and check for foreign objects inside the workpiece or inappropriate screw specifications. Never force the screw. Once installed, the screw head should be firmly against the workpiece surface, with a clearance of no more than 0.1mm. There should be no head deformation or thread stripping. For critical connections (such as building curtain walls and vehicle chassis), use a torque wrench to control the installation torque to ensure uniform tightening force for each bi-metal screw.

3. Post-Installation Maintenance and Care

Regularly check the bi-metal screw connection during routine maintenance, especially in locations with frequent vibration or harsh environments. It is recommended to inspect the screw every 3-6 months. Tighten any loose screws promptly. If surface rust (mostly surface stains, not rust on the base material) occurs, wipe with a neutral detergent. Avoid using acidic or alkaline detergents, which can damage the stainless steel exterior. For bi-metal screws exposed outdoors for extended periods, apply a layer of rust-proof grease every one to two years to further enhance corrosion resistance. If a screw shows signs of failure, such as breakage or delamination, it should be replaced immediately and the cause of the failure investigated (e.g., incorrect specifications, excessive installation torque) to prevent recurrence.

VI. Bi-Metal Screw Industry Development Outlook

As the manufacturing industry transitions toward high-end, green, and intelligent manufacturing, bi-metal screws, as key fasteners, are experiencing three key development trends:

1. Upgrading Materials and Processes

In the future, bi-metal screws will develop toward higher strength and improved corrosion resistance. They may utilize the more corrosion-resistant 2205 duplex stainless steel for the outer layer and an ultra-high-strength titanium alloy substrate for the inner layer, meeting the demands of high-end applications such as aerospace. Laser composite technology will be widely adopted to achieve precise control of bimetallic bonding, increasing bond strength to over 300 MPa. Phosphorus-free passivation treatment will also be used to further enhance environmental friendliness.

2. Cross-Border Expansion of Applications

In emerging sectors such as new energy and semiconductors, the application of bi-metal screws will continue to expand. In photovoltaic power plants, weather-resistant bi-metal self-tapping screws can be used to connect photovoltaic modules to mounting brackets. In semiconductor equipment, cleanroom-grade bi-metal screws meet the stringent requirements of cleanrooms. Furthermore, with the development of smart homes, intelligent bi-metal screws equipped with sensors may become available, enabling real-time monitoring and early warning of connection status.

3. Intelligent Transformation of Production and Selection

On the production side, bi-metal screw companies will introduce automated production lines and digital twin technology to achieve full-process quality traceability from raw materials to finished products. On the selection side, AI-based selection systems will be developed. Inputting information such as workpiece material and environmental parameters will quickly match the optimal bi-metal screw specifications, improving selection efficiency and accuracy. Furthermore, the improvement of cross-border e-commerce channels will facilitate the global market access of high-quality bi-metal screw products, promoting industry standardization and internationalization.

Conclusion

Bi-metal screws (bi-metal screws and self-tapping screws), with their combined advantages of "corrosion resistance and high strength," are reshaping the fastener industry landscape. From architectural curtain walls to new energy vehicles, from home appliance manufacturing to high-end machinery, bi-metal screws are expanding their application, becoming a key factor in improving product reliability and reducing production costs. It is crucial for industry practitioners to understand its core characteristics, accurate purchasing methods, and standardized usage techniques. With the continuous upgrading of technology, bi-metal screws will surely demonstrate their value in more fields and become an important support for the high-quality development of the manufacturing industry.