Powder Metallurgy & Welding Processes: Advantages, Disadvantages & Techniques

Posted on Sep 3, 2024 in Chemistry

ITEM 5 – POWDER METALLURGY

Advantages

  • Minimizes raw material loss, using only the necessary amount of powder.
  • Provides precise control over composition limits.
  • Can eliminate or minimize machining operations.
  • All operations are suitable for automation.
  • Achieves good surface finishes without molding signals.
  • Allows for controlled porosity and highly distributed internal oxidation suitable for hardening.
  • Avoids segregation.
  • Enables the production of numerous parts not possible with conventional methods.

Disadvantages

  • Limited design possibilities due to the need for easy extraction from the matrix.
  • Limited part size due to press strength limitations.
  • Parts may not have the same mechanical properties as those obtained by conventional methods.
  • High cost due to the expensive matrix materials (alloy steel or tungsten carbide).

Compaction

This process shapes the metal powder into the desired size and provides the necessary strength for handling before sintering. Cohesion is achieved through cold soldering of the powder particles under pressure within a matrix (typically made of steel or tungsten carbide). Pressure can range from 800 to 20,000 kg/cm2, with most commonly using 40,000 kg/cm2.

Sintering

This crucial process provides cohesion and strength to the compacted part. The powder is heated to a temperature below its melting point (typically 2/3 to 4/5 of the melting temperature) in a protective atmosphere. This allows particles to bond, resulting in a component with sufficient mechanical strength.

TEMA 7 – WELDING

Welding is a permanent bonding process between metals, achieved through heat input at an appropriate temperature, with or without pressure and the addition of molten metal or alloy.

Types of Welds

Heterogeneous Welds

Joining dissimilar metals, with or without filler metal. Examples include soldering and brazing.

Homogeneous Welds

Joining similar metals with a filler metal of the same nature. Examples include forge welding, aluminothermic welding, gas fusion, ultrasonic, friction, arc welding, and resistance welding.

Soldering

Joining workpieces using a low-melting-point metal alloy (below 500°C). Easy to perform but has disadvantages:

  • Lower strength than the base metals.
  • Susceptibility to corrosion due to galvanic potential between dissimilar metals.
  • Low mechanical strength (10-15 kg/mm2).

Brazing

Uses a filler metal with a melting point above 500°C but below that of the base metals. Two types:

  • Yellow Brazing: Uses brass as a filler metal, operating between 650-950°C. Suitable for iron, copper, and their alloys.
  • Silver Brazing: Uses a silver-based alloy with copper, zinc, and cadmium. Requires tight clearances for capillary action. Commonly used for welding carbide inserts.

Forge Welding

Traditional method involving heating the metals to near melting point and then forging them together. Requires clean surfaces and perfect contact. Typically used for low-carbon steel.

Aluminothermic Welding

Uses the exothermic reaction between aluminum and iron oxide to generate molten iron as a filler metal. Suitable for welding thick sections, such as railroad rails and large shafts.

Ultrasonic Spot Welding

Applies ultrasonic vibrations to two plates under pressure, causing friction and ultimately welding them together.

Gas Welding

Uses a flame produced by the combustion of a gas (e.g., acetylene) with oxygen to melt the edges of the workpieces and filler metal. Suitable for thin pieces.

Oxyfuel Cutting

Uses a high-pressure oxygen stream to cut metal after preheating it with a flame. Commonly used for scrapping and underwater work.

Arc Welding

Utilizes the heat generated by an electric arc to melt the base metal and filler metal. Different types include:

  • TIG (Tungsten Inert Gas): Uses a non-consumable tungsten electrode and an inert gas shield.
  • MIG (Metal Inert Gas): Uses a consumable wire electrode and an inert gas shield.
  • MAG (Metal Active Gas): Similar to MIG but uses an active gas (CO2) as a shield.

Resistance Welding

Utilizes the heat generated by electrical resistance to join metal pieces. Common types include:

  • Spot Welding: Creates localized welds by passing a high current through overlapping metal sheets.
  • Seam Welding: Creates a continuous weld seam by passing a current through rotating electrodes.

Welding Metallurgy

Welding involves complex metallurgical processes that can affect the microstructure and properties of the welded parts. Potential defects include gas absorption, reactions with atmospheric gases, and internal stresses.

Weldability of Metals

Refers to the ability of a metal to be welded under specific conditions. Understanding weldability helps in selecting appropriate welding techniques and parameters to achieve a sound weld with desired properties.