Comprehensive Study Guide on Sintered Materials and Powder Metallurgy Technology

Fundamentals of Sintering and Powder Metallurgy

Sintering is the core technology of powder materials (tehnologija prašnatih gradiv).
It is a production technology where products are made from basic raw materials in powder form.
The process involves pressing the powder into a specific metallic mold (forma) to create a compact.
The compact is subsequently heat-treated, a process known as sintering.
In some instances, materials undergo a two-stage thermal process: pre-sintering at a lower temperature followed by final sintering at a higher temperature.
Sintered parts are extensively used across various sectors, including:
- The automotive industry.
- The agricultural machinery industry.
- The electrical industry.

Applications and Justifications for Sintering

Sintering is utilized under specific industrial and material conditions:

Insoluble Components: To manufacture parts from components that do not mix (total or partial insolubility).
High Melting Points: To produce parts from different metals and alloys that cannot be synthesized via traditional smelting due to extremely high melting points or significant differences in melting temperatures ( $T_t$ ).
High Porosity: To create materials where high porosity is a functional requirement.
Mass Production: In the large-scale manufacturing of small, high-load products or those with complex geometries.
Cutting Tools: For the fabrication of tools from high-speed steel (HSS) and cemented carbides (karbidne trdine).

Specific material examples and applications include:

Graphite and Copper: Graphite is used for its sliding properties while copper provides electrical conductivity; this combination is used for sliding contacts.
Diamond Metals: Sintering of diamond grains or other hard substances within a metallic binder.
Slide Bearings: Porous structures that are impregnated with oil for self-lubrication.
Filters: Utilizing controlled porosity for filtration.
Gearbox Components: Synchronizers and gears.
Tough Binder alloys: Tungsten ( $W$ ), Molybdenum ( $Mo$ ), and Tantalum ( $Ta$ ) combined with a tough Cobalt ( $Co$ ) binder.
Composite Combinations: Iron ( $Fe$ ), Nickel ( $Ni$ ), and Cobalt ( $Co$ ) in combination with Zinc ( $Zn$ ), Cadmium ( $Cd$ ), or Lead ( $Pb$ ).
Electrical Contacts: Combinations of $W$ or $Mo$ with Copper ( $Cu$ ) or Silver ( $Ag$ ).

Limitations of the Sintering Process

Production Volume: Sintering is not suitable for single-piece or low-volume production because the tooling (the mold representing the product's shape) is very expensive to manufacture.
Size Constraints: It is generally not possible to manufacture very large products. The weight limit is typically around $1 kg$ due to the immense pressures required and the difficulty in achieving uniform pressing across large volumes.

Stages of Sintering Technology

The comprehensive technology of sintering involves several distinct phases:

Extraction of metal powders.
Powder preparation for pressing, including the addition of necessary additives.
Pressing of the powder.
Sintering of the compacts.
Post-sintering additional processing (this may include re-pressing and re-sintering).

Metal Powder Extraction Methods

The quality of the final product is heavily dependent on the method used to obtain the powder. Sintered products can also be heat-treated afterward, so it is crucial they contain a sufficient amount of carbon.

Mechanical Processes

Crushing and Milling: Suitable for brittle and porous metals.
- Input Materials: Shavings or short wires.
- Output: Fine grains/powder with sizes ranging from $0.01 - 0.4\,mm$ .
- Characteristics: The particles are flat-shaped, which allows for good compressibility.
- Atmosphere: A neutral atmosphere is maintained inside the crusher to prevent contamination or oxidation.
Molten Metal Atomization: Spraying a stream of molten metal.
- Process 1: Molten metal is passed through a nozzle.
- Process 2: An inert gas is used to atomize the stream.
- Process 3: Fully solidified and partially oxidized droplets fall into a collecting vessel.
Atomization on a Plate or in an Air Stream:
- Produces grains of various sizes.
- Grain size depends on the thickness of the metal stream, the rotation speed of the plate, and water pressure.
- Utilizing an air stream allows for the creation of very fine granularity.

Physical-Chemical Processes

Condensation Process: Used to obtain powders of very pure metals. High pressure and carbon monoxide are used to turn ore into liquid metal carbonyls (e.g., $Fe(CO)_5$ ). These are then vaporized in electric arc furnaces at higher temperatures into metal vapors. Condensation techniques then turn these vapors into fine spherical powders of pure metal.
Oxide Reduction (with Hydrogen): Used for obtaining $W$ and $Mo$ , and for the reduction of magnetite to obtain pure iron powder. This results in a "spongy" powder that is highly compressible.
Electrolysis: Metal is deposited on the cathode as a brittle coating, which is then mechanically crushed into fine powder. By adding specific additives to the electrolyte, metal powder can be made to collect directly on the cathode. This powder exhibits excellent compressibility.

Powder Preparation for Pressing

The goal is to compose a mixture (from one or multiple components) that yields the required mechanical, physical, and chemical properties (grain size, shape, quality, chemical composition, surface quality).

Classification (Sieving): Used to obtain uniform granularity within a specific range using various sieves.
Mixing: Achieved in mixers to ensure a uniform mixture. Additives such as water, gasoline, or alcohol are introduced to reduce friction between individual grains and accelerate mixing. Organic substances may be added to increase compressibility; these evaporate or burn off during sintering.
Heat Treatment of Powder: The powder is heated in a reducing atmosphere at temperatures between $400 - 1000^\circ\text{C}$ . This removes surface oxides, moisture, and gas inclusions. It also eliminates internal stresses caused during mechanical extraction.

Pressing (Compaction) Process

Pressing is critical as it determines the porosity and the subsequent mechanical and physical properties of the object.

Matrices (Molds): Powder is pressed into specially shaped containers called matrices, made from hard alloy steels.
Dosing: The amount of powder must be precisely measured by weight or volume.
Presses: Mechanical or hydraulic presses are used. The required pressure depends on product size, powder type, and desired properties.
Temperature: Pressing typically occurs at ambient (room) temperature.
Strength of the Compact: The strength achieved during pressing is very low and depends on the density and plastic deformation of the grains. This is influenced by the product size, shape, powder type, and the speed of pressing.
Pressing Speed: Slower pressing allows for greater plastic deformation of the grains.
Unilateral vs. Bilateral Pressing: Differences exist in density distribution depending on whether pressure is applied from one side or both.
Extrusion: Powder is mixed with liquid additives to form a kneadable paste. This paste is continuously extruded through a matrix of various shapes. This results in uniform density across the cross-section. It is used for making tubes and profiles, particularly from cemented carbides.
Density Distribution Example: In a single-sided pressing scenario, density varies along the height ( $H$ ) of the matrix. Values indicated: $7.3, 7.2, 7.1, 7.0, 6.9, 6.8$ .
Pressing Ratio: Defined as $H/h$ .

Sintering Process and Parameters

Sintering is a thermal treatment process similar to diffusion annealing.

Mechanism: Heating the compacts to the sintering temperature, which is below the metal's melting point (except in rare cases where melting occurs, though this causes shrinkage issues).
Results: Increases the strength, hardness, and density of the part.
Temperature Ranges: $600 - 3200^\circ\text{C}$ , depending on the metal.
Duration: Typically up to one hour.
Atmosphere: To prevent oxidation at high temperatures, sintering takes place in protective atmospheres (Nitrogen or Hydrogen) or vacuum furnaces.
Furnace Types: Batch furnaces or tunnel furnaces, usually heated by electric current.

Technological Specifications for Sintering

Grain Size: $1 - 500\,\mu\text{m}$ .
Pressure: $1000 - 10,000\,\text{bar}$ .
Temperature Examples (Diffusion Bonding):
- Copper alloys: $600 - 800^\circ\text{C}$ .
- Iron alloys: $1000 - 1300^\circ\text{C}$ .
- Cemented Carbides (KT): $1400 - 1600^\circ\text{C}$ .
- Refractory metals ( $Mo, W, Ta$ ): $2000 - 2900^\circ\text{C}$ .

Porosity Levels in Post-Sintered Products

Up to 60%: Used for filters.
Up to 30%: Used for slide bearings.
15 – 20%: Used for standard machine parts.
Up to 5%: Used for high-strength machine parts.

Specialized Processes and Post-Treatments

Hot Pressing

Combines pressing and sintering into a single step. The metal powder and matrix are heated by electric current and pressed simultaneously.
This process is highly limited due to heavy demands on tooling (high temperatures and pressures) and problems with powder oxidation.

Additional Post-Sintering Treatments

Heat Treatment: Hardening (kaljenje), carburizing (cementiranje).
Infiltration: To reduce porosity, another metal with a lower melting point than the sintering temperature is placed on the compact (e.g., Copper on Iron). During sintering, this metal melts and is absorbed into the pores. This maximizes strength and toughness while minimizing porosity.
Double Sintering: For higher requirements, the process can be repeated: Pressing + Sintering + Re-pressing at higher pressures + Final sintering at a higher temperature.
Calibration: Re-pressing at higher pressures to reduce dimensional variances and increase precision (tolerances).
Impregnation: Soaking porous slide bearings with oil, preferably in a vacuum, for lifelong lubrication.
Machining: Turning, milling, grinding, etc.
Coating: Applying surface layers to increase hardness, corrosion resistance, and wear resistance, and to decrease roughness.

Classification and Typical Uses of Sintered Materials

Low Friction Materials: Coefficient of friction $\mu = 0.004 - 0.008$ . Includes $Fe, Cu, C$ , with $15-20\%$ porosity and lubricant.
High Friction Materials: Coefficient of friction $\mu = 0.1 - 0.15$ . Includes $Fe, Cu, \text{quartz}, \text{asbestos}, \text{oxides}$ . Dry lubricants: $Pb, C$ , sulfides, barite.
Electrical Materials: Soft magnetic, hard magnetic, and electrical contacts.
Filters: Used for liquids, gases, flame arrestors, and sound mufflers. Materials: Bronzes, $Ni$ , Monel, stainless steels. Pore sizes: $2 - 100\,\mu\text{m}$ .
Sintered Magnets.
Hard Metals (Cemented Carbides): Used for cutting, drawing, cold forging, forming tools, and rock drilling.

Hard Metals and Cermets

Hard metals are a subgroup of Cermets.

Composition:
- Hard Phases: Carbides, nitrides, carbonitrides, or solid solutions of $Ti, Mo, W, Ta, Nb, V, Al$ .
- Metallic Binder Phases: $Ni, Co, Mo$ or their alloys.

Properties at High Temperatures (Example Table)

Cermet	Hardness HV at $1000^\circ\text{C}$	Flexural Strength (MPa) at $900^\circ\text{C}$	Oxidation Resistance ( $mg/cm^2 \times h$ )
$TiC - 16.5\,Ni - 9\,Mo$	$500$	$1050$	$11.8$
$TiC - 20\,TiN - 15\,WC - 10\,TaC - 5.5\,Ni - 11\,Co - 9\,Mo$	$650$	$1360$	$1.66$

Characteristics of Cemented Carbides (KT)

Compressive Strength: $3.4$ to $7\,GPa$ .
Toughness: Relatively low.
Modulus of Elasticity: Inversely proportional to $Co$ concentration. $E_{KT}$ is $2$ to $3$ times larger than that of steel.
ISO Classifications (ISO TC – 29/726 – 1963):
- P (Blue): $WC$ and others; for cutting steel.
- M (Yellow): Less $TiC$ ; for processing steel, $Ni$ superalloys, and nodular cast iron.
- K (Red): Only $WC$ ; for grey cast iron, non-ferrous metals, and non-metals.
- Additional Codes: $N, S,$ and $H$ .
- G: For forming tools.
- Numerical Suffixes: A higher number indicates greater toughness; a lower number indicates greater hardness.

Selection of Cutting Materials by ISO Group (1988)

Processed materials are classified into six groups: P (Steel), M (Stainless steel), K (Cast iron), N (Non-ferrous metals), S (Special materials/superalloys), and H (Hard materials).

Tool Material Compatibility Matrix

Standard HSS ( $S\,6-5-2$ ): Possible use in P, M, K, N, S.
Coated HSS ( $HSS-TiN$ ): Preferred across P, M, K, N, S.
Uncoated Carbides:
- HW-P01 to P40: Steel (P).
- HW-M10: Stainless (M).
- HW-K10 to K30: Cast Iron (K).
- HW-N10: Non-ferrous (N).
Coated Carbides ( $HC-P10, HC-K10$ ): Steel, Cast Iron.
TiC-based Cermets ( $HT-P01$ ): Steel.
Ceramics:
- Oxide Ceramic ( $CA-K10$ ): Cast Iron (K).
- Mixed Ceramic ( $CM-K05$ ): Cast Iron (K).
- Whisker-reinforced ( $CM-S20$ ): Special materials (S).
- Nitride Ceramic ( $CN-K20$ ): Cast Iron (K).
Super-hard Materials:
- Cubic Boron Nitride ( $BN-H05$ ): Hard materials (H).
- Polycrystalline Diamond ( $DP-N15$ ): Non-ferrous (N).