What is a Filler Metal?

In metal joining processes like brazing, soldering, and welding, the filler metal is a critical component. It’s the material that flows into the joint between two base metals, creating a strong bond and ensuring the structural integrity of the assembly. Selecting the right filler metal is essential, as it determines the joint’s strength, durability, and resistance to stress and corrosion.

In this article, we’ll explore what a filler metal is, how it works in brazing and other joining methods, the types available, and tips for choosing the ideal filler metal for your specific project. By understanding the role of filler metals, you’ll be better equipped to achieve optimal results in your metal joining applications.

What is a Filler Metal?

A filler metal is an alloy or metal that, when melted, flows between the surfaces of two base metals to bond them together. Unlike adhesives or mechanical fasteners, a filler metal creates a metallurgical bond, which distributes stress evenly across the joint and ensures high durability. This is especially important in industries such as HVAC, automotive, aerospace, and plumbing, where strong, reliable joints are essential.

Filler metals vary depending on the joining process used:

• Brazing Filler Metals: Typically operate at temperatures above 450°C, and are designed to create high-strength bonds between different types of metals.
• Soldering Filler Metals: Operate at lower temperatures, usually below 450°C, and are suited for electrical connections and applications requiring minimal heat.
• Welding Filler Metals: Melts along with the base metals, forming a fused joint with both base materials.

In brazing, the filler metal does not melt the base metals, instead flowing into the joint by capillary action. This allows for precise, clean joints and preserves the properties of the base metals.

How Filler Metals Work in Brazing

In brazing, the filler metal is introduced to the joint as the base metals are heated to a temperature just below their melting points. As the filler metal melts, it is drawn into the gap between the base metals through capillary action. Once the filler metal cools and solidifies, it forms a strong, durable bond between the metals.

Key Characteristics of Brazing Filler Metals

1. Low Melting Point: The filler metal has a lower melting point than the base metals, which allows it to flow without melting or distorting the joined parts.
2. Good Flow and Wetting: A good brazing filler metal has strong flow and wetting characteristics, ensuring it spreads evenly through the joint area.
3. Strength and Durability: The filler metal creates a metallurgical bond that can withstand mechanical stress, temperature changes, and, in some cases, corrosive environments.

Common Brazing Filler Metals

Some common brazing filler metals include silver-based alloys, copper-phosphorus alloys, and nickel-based alloys. Each of these fillers has unique properties, making them suitable for specific applications and metal combinations.

Types of Filler Metals for Brazing and Metal Joining

Filler metals vary in composition and application, and each type is designed for particular metals, environments, and joint requirements. Below are some of the most commonly used filler metals in brazing and other metal joining processes:

1. Silver-Based Filler Metals

Silver-based filler metals, commonly known as silver brazing alloys, are popular due to their low melting point and excellent flow characteristics. They work well with non-ferrous metals like copper, brass, and stainless steel and are widely used in HVAC, plumbing, and electrical applications.

• Applications: Joining copper to steel or brass to stainless steel.
• Advantages: High strength, good corrosion resistance, and clean appearance.

2. Copper-Phosphorus Filler Metals

Copper-phosphorus filler metals are specifically formulated for copper-to-copper and copper-to-brass joints. These filler metals contain phosphorus, which acts as a natural flux when joining copper, making them ideal for plumbing and refrigeration.

• Applications: Used in HVAC, plumbing, and refrigeration systems.
• Advantages: No need for flux when brazing copper-to-copper; strong and leak-proof joints.

3. Nickel-Based Filler Metals

Nickel-based filler metals are preferred for high-temperature and high-stress applications due to their resistance to oxidation and corrosion. These alloys are widely used in aerospace, automotive, and heavy machinery where components must endure harsh environments.

• Applications: Joining stainless steel, high-performance alloys, and other challenging metals.
• Advantages: High strength, excellent corrosion and temperature resistance.

4. Aluminium-Based Filler Metals

Aluminium-based filler metals are designed specifically for joining aluminium and are commonly used in automotive radiators and HVAC components due to their lightweight and good corrosion resistance.

• Applications: Joining aluminium-to-aluminium or aluminium-to-copper.
• Advantages: Lightweight, corrosion-resistant joints suitable for aluminium applications.

5. Tin-Based Soldering Filler Metals

While primarily used in soldering, tin-based filler metals are worth mentioning as they offer good electrical conductivity and are commonly used in electronics and plumbing. Tin-based solders are often alloyed with other metals like silver or copper to improve strength and performance.

• Applications: Circuit boards, electronic components, and small-scale plumbing joints.
• Advantages: Low melting point, safe for heat-sensitive components, and suitable for electrical conductivity.

Selecting the Right Filler Metal for Your Application

Choosing the correct filler metal is essential for creating a reliable, long-lasting joint. Here are some factors to consider:

1. Metal Compatibility: Ensure that the filler metal is compatible with the base metals you’re joining. For instance, silver-based alloys work well with copper and stainless steel, while copper-phosphorus alloys are ideal for copper-to-copper joints.
2. Joint Requirements: Consider the joint’s strength, corrosion resistance, and temperature resistance. For high-stress or high-temperature environments, nickel-based alloys are a good choice.
3. Application Environment: Think about whether the joint will be exposed to moisture, chemicals, or extreme temperatures. Choose a filler metal with the appropriate corrosion resistance for the environment.
4. Flow Characteristics: In brazing, filler metals with good flow characteristics are essential for even distribution and complete joint coverage. Silver-based alloys are known for excellent flow, making them ideal for intricate joints.

Benefits of Using Filler Metals in Brazing and Metal Joining

Filler metals provide several advantages in metal joining processes:

• Strong Metallurgical Bond: Filler metals form a metallurgical bond that is often as strong as, or even stronger than, the base metals. This makes them ideal for high-performance applications.
• Versatility Across Metal Combinations: Brazing filler metals can join various metals, including dissimilar ones, such as copper to stainless steel or aluminium to copper.
• Leak-Proof Joints: Brazed filler metals create sealed joints that are leak-proof, making them ideal for plumbing, HVAC, and refrigeration systems.
• Minimal Distortion: Since brazing filler metals melt at lower temperatures than the base metals, they produce minimal distortion, preserving the base metals’ properties.

Why Choose CuP Alloys for High-Quality Filler Metals?

At CuP Alloys, we specialise in providing a wide selection of brazing filler metals tailored to meet the demands of various industries. Whether you need silver-based alloys for precise joints or nickel-based alloys for high-temperature applications, our products are designed for quality and reliability.

With over 100 years of combined experience, we offer expert guidance to help you select the right filler metal for your specific needs. And with free UK delivery on all orders, CuP Alloys makes it easy to get the materials you need without added costs.

Explore our range of brazing alloys or contact us for tailored advice on your project. Ensure strong, durable joints with CuP Alloys—the trusted name in filler metals for brazing and beyond!