Metal Processing - Sintering

Sintering to Bond Metal Materials

December 15, 2022

Metal sintering is a popular manufacturing method for metal products and components due to its efficiency and cost-effectiveness. This post will define metal sintering and explain the process, its applications, and its advantages over other metal fabrication techniques.

What is Metal Sintering

Metal sintering is a metal fabrication process that uses metal powders along with heat and pressure to facilitate atomic diffusion and bonding between different materials. This process allows you to alter the molecular structure of your materials and permanently bind them together.

Atomic diffusion occurs when atoms within a solid heat up and begin randomly vibrating, which causes them to travel through the material until they can cool down. For example, helium balloons slowly deflate because the helium atoms diffuse through the balloon wall and escape, leaving behind denser air molecules that sink. The rate of diffusion depends on the atom’s ability to diffuse and the severity of the concentration gradient.

Essentially, when you heat a metal to just below its melting point, molecules move slightly and form bonds with adjacent grains. This forms joints, or “necks,” between the grains, which creates a more rigid microstructure. Sintering can be used with most metals, but pure metals with minimal surface contamination yield the best results.

What Is the Purpose of Sintering?

Sintering enables you to bind materials together into one unified mass without melting them, making it a cost-effective and efficient process . It is also a highly versatile method with various applications across many different industries. Common sintered metal products include:

  • Gears
  • Couplings
  • Bearings and bearing retainers
  • Pulleys
  • Rotors
  • Sprockets
  • Electrical contracts
  • Cams
  • Armatures

The Sintering Process

Every substance has different properties, so you need to choose a metal powder that will yield the results you want. Metal powders typically contain metallic particles and various minerals that will affect your final product. They also contain a lubricant to improve the powder’s flow during the sintering process — this ingredient will evaporate in the furnace.

The basic sintering process involves the following steps:

  1. Preparation: First, you need to prepare your powder compact. You can join your components together using cold welds, 3D-printed lasers, pressing tools, or atmospheric pressure. Note that a highly controlled atmosphere containing protective gases such as hydrogen, carbon monoxide, or nitrogen is crucial for safely pressurizing your materials.
  2. Heating and consolidation: In this stage, the metal is heated to just below its melting point, which activates the formation of martensitic crystalline microstructures.
  3. Particle merging: In this intermediate phase, the particles become denser and eventually merge together. You can speed this step up using the liquid phase sintering (LPS) method.
  4. Solidification: The material cools and solidifies, creating a single unified mass.

Depending on the specific technique you use, you may need to add other steps or materials to the process.

Types of Sintering Processes

There are several different sintering techniques you can use to achieve different results:

  • Conventional sintering: The simplest sintering technique, conventional sintering, involves heating your prepared powder compact to the correct temperature without applying external pressure. You can use this technique with box furnaces or tube furnaces, but a controlled atmosphere is necessary to ensure safety and proper results.
  • High-temperature sintering: High-temperature sintering is like conventional sintering, but at elevated temperatures. It reduces surface oxidation that can inhibit metal-to-metal bonding, which improves the component’s mechanical properties, reduces porosity, and strengthens the metal. However, parts that have undergone high-temperature sintering tend to shrink more than expected.
  • Direct metal laser sintering (DMLS): Is what advanced 3D printers do when they produce metal components. It allows you to directly print metal parts using finely powdered metals. This method also enables you to combine plastic and metal materials.
  • Liquid phase sintering: In LPS, a liquid is present for some or all of the sintering process. This technique accelerates molecule densification and inter-particle bonding, which shortens the process time significantly. Traditionally, LPS was used exclusively for ceramic materials, but it has some applications in metal fabrication.

The right technique for your application depends on your component’s intent application and your desired results.

Sintering vs. Melting

Melting and sintering are both methods for binding metals together. However, while they are similar in nature, they are two very different processes.

Melting involves heating the metal to an extremely high temperature to induce a complete phase change from solid to liquid. Because you need such high temperatures to melt metals, processes that include melting require a high energy expenditure. Additionally, working with molten metal can be difficult due the fluidity of liquids. A component made through melting is more likely to contain imperfections than a sintered one.

With sufficient pressure, sintering can take place at a temperature significantly lower than the melting point, allowing you to work with metals and other materials that have high melting points without needing to use as much energy. As a result, you get more control over the process, leading to more consistent results.

What Are the Benefits of Metal Sintering?

Some of the advantages of metal sintering over other methods include:

  • Material conservation: The sintering process produces minimal waste, making it a highly sustainable metalworking process.
  • Flexibility: Sintering allows you to work with many materials, including those with different melting temperatures and properties.
  • Energy efficiency: Because it can be accomplished faster and at lower temperatures, sintering requires less energy than melting. Additionally, sintering decreases idle furnace time, which further conserves energy.
  • Cost-effectiveness: Sintering is less expensive than other metal fabrication methods because it produces less waste and requires less energy.
  • Increased control: Sintering allows you to bind together materials with different melting points, some of which can be very high. Because the metal remains solid, you have more control to fine-tune and adjust the process to achieve better, more consistent results.
  • Reproducible size and hardness: The high degree of control you have over the sintering process makes it easier to consistently reproduce excellent results.
  • Simplified processes: The high control allowed by the sintering process allows you to create complex shapes and parts, reducing the need for further machining. It also results in fewer process interruptions and faster startups, greatly increasing your potential for productivity.
  • Enhanced metal properties: Sintering reduces surface porosity, which enhances properties such as conductivity, corrosion resistance, and tensile strength.
  • Improved part appearance: Sintering produces less soot and oxidation than other metalworking techniques, resulting in brighter, cleaner, higher-performing parts.

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