MM1504: Powder Metallurgy and Ceramics

Dr. Sanjay Kumar Vajpai [email protected]

Assistant Professor, Department of Metallurgical and Materials Engineering, National Institute of Technology, Jamshedpur Syllabus and Lecture Schedule: MM1504: Powder Metallurgy and Ceramics (3-1-0)

UNIT-I: (8 Lectures)

Powder Production (Chemical Methods, Electrolytic Methods, Atomization, Mechanical Methods)

UNIT-II: (5 Lectures)

Powder Characterization (Chemical Composition and Structure, Particle Size and Surface Topography, Pyrophorocity and Toxicity)

UNIT-III: (12 Lectures)

Powder Compaction, Phenomenological Aspects of Sintering, Solid State Sintering, Analytical Approach to Sintering, Non Isothermal Sintering, Microstructural Evolution

UNIT-IV: (12 Lectures)

Liquid Phase Sintering, Stages of Liquid Phase Sintering, Super solidus Sintering, Activated Sintering, Pressure Assisted Sintering, Microwave Sintering, Select Case Studies.

UNIT-V: (13 Lectures)

General overview of Ceramics: Structure and properties of ceramics, Types according to various applications; Various consolidation methods, casting of ceramics, conventional and advanced sintering processes for ceramics References Books 1. Powder Metallugy Science, 2nd ed R.M. German.

2. Powder Metallurgy: Science, Technology and Materials by A. Upadhyaya, G.S. Upadhyaya,

3. ASM Handbook, Volume 7: Powder Metal Technologies & Applications (1998)

4. Introduction to Ceramics by Kingery W.D, Bowmen H. K., Uhlmann D.R Powder Metallurgy ➢ Raw Material: Powder form of Metals, Alloys, Ceramics Cost ➢ Extended ability to fabricate Complicated shapes Effectiveness ➢ Controlled Microstructure + ➢ Reduced Machining Cost: Near-Net-Shape Processing Uniqueness ➢ Easy to handle difficult-to-machine shapes ➢ Minimum wastage of material and Energy Savings ➢ Composites Conventional Powder Metallurgy Steps: Powder manufacture → Blending → Consolidation/Compaction → Sintering

Additive Manufacturing: A Recent Development of Powder Metallurgy Processing PM Automotive Parts Courtesy: Western Sintering Camshaft Cap in Engine The Roller Finger Follower is used in the valve Courtesy: GKN train of passenger cars as part of a cam follower system that is capable of shutting off a cylinder while the engine is running. As a very complex shape design with several High strength and wear resistant materials integrated functions, the PM Metal Injection High dimensional accuracy Moulding (MIM) process technology is the Optimized for minimum friction and valve perfect choice clearance compensation • Systems that optimize camshaft or valve timing are of increasing importance for fuel consumption and CO emissions. • PM has proven to be an ideal solution for variable valve timing (VVT) components. • For VVT Stators the PM process can facilitate freedom of design and deliver highly precise, complex products.

ADVANTAGES •Compact lightweight designs WT stator-sprocket •Reduced machining with multi part design •Low friction with custom surface geometry

Products 1. Porous products such as bearings and filters. 2. Tungsten carbide, gauges, wire drawing dies, wire-guides, stamping and blanking tools, stones, hammers, rock drilling bits, etc. 3. Various machine parts are produced from tungsten powder. Highly heat and wear resistant cutting tools from tungsten carbide powders with titanium carbide, powders are used for and die manufacturing. 4. Refractory parts such as components made out of tungsten, tantalum and molybdenum are used in electric bulbs, radio valves, oscillator valves, X-ray tubes in the form of filament, cathode, anode, control grids, electric contact points etc. 5. Products of complex shapes that require considerable machining when made by other processes namely toothed components such as gears. 6. Components used in automotive part assembly such as electrical contacts, crankshaft drive or camshaft sprocket, piston rings and rocker shaft brackets, door, mechanisms, connecting rods and brake linings, clutch facings, welding rods, etc. 7. Products where the combined properties of two metals or metals and non-metals are desired such as non-porous bearings, electric motor brushes, etc. 8. Porous metal bearings made which are later impregnated with lubricants. Copper and graphite powders are used for manufacturing automobile parts and brushes. 9. The combinations of metals and ceramics, which are bonded by similar process as metal powders, are called cermets. They combine in them useful properties of high refractoriness of ceramics and toughness of metals. They are produced in two forms namely oxides based and carbide based. Powder Rolling General Flow diagram of the PM processing Powder Production Methods

The methods for powder production affect the powder characteristics, hence, the method is selected wrt type of application and desired properties of the final product.

1.Chemical Methods:

1. For Metal Powders: Chemical Reduction and Chemical Decomposition of compounds (Oxides, Hydrides, halides, or any other salt) are the major techniques. Generally used for producing pure elemental powders.

2. Ceramic Powders: Carbonates, hydroxides, nitrates, sulphates, acetates, oxalates, alkoxides or any other metal salts are used as raw material.

2. Electrolytic Method:

It involves primarily electrodeposition process, i.e. dissolution of the impure metal from anode and subsequent deposition on the cathode. Generally used for pure elemental powders.

3. Atomisation Method: Widely used commercial process. Used for producing a wide variety of elemental powders as well as alloys.

4. Mechanical Methods: It can be used for metals, alloys, and ceramics. It is not a primary method. Chemical Methods

For Metal Powders

1. Solid State: Reduction of Iron or Tungsten Oxide (WO3) with a reducing gas.

2. Gaseous State: Reduction of Titanium Chloride (TiCl4) vapors with molten magnesium.

3. Aqueous Solution: Precipitation of cement Copper from Copper sulphate solution.

4. Direct Decomposition of Metal Hydrides: Ti, Zr, Hf V, Th, or U (MH)

5. Direct Decomposition of Metal Carbonyls (MCo5): e.g. Fe, Ni, etc.

For Ceramics Powders

There can be three methods, similar to metal powders, solid-state reactions, liquid solutions, and vapor phase reactions. Chemical Methods: Solid State Reduction The gas flow is required, and the gas/solid contact is achieved by mechanical stirring of the particulate bed by raking, by tumbling in the reactor -→ fluidized bed reactor

• Reduction With CO

3 Fe2O3 (s) + CO → 2 Fe3O4+ CO2 (g) Fe3O4 (s) + CO(g) → 2 FeO(s)+ CO2 (g) FeO(s) + CO(g) → Fe(s)+ CO2 (g)

C + O2 → CO2 CO2 + C → 2CO

• Reduction of higher oxide to lower oxide.

• For each oxide and reaction temperature, a critical Co/Co2 ratio in the gas mixture need to maintained. o • Fe2O3 (s) → Fe3O4 occurs between 200-500 C and a minimum -4 Co/CO2 ratio of approximately 10 . o • Fe3O4 (s) → FeO occurs between 500-900 C. • FeO (s) → Fe occurs between 900-1300 oC.

• Fe2O3 → Fe3O4 → FeO → Fe • Hematite → Magnetite → Wustite → Iron Chemical Methods: Solid State Reduction

• Reduction With H2 4WO (s) + H → W O + H O 3 Fe2O3 (s) + H2 → 2 Fe3O4+ H2O 3 2 4 11 2 W O (s) + H → WO (s)+ H O Fe3O4 (s) + H2 → 3 FeO(s)+ H2O 4 11 2 2 2 WO (s) + H → W(s)+ H O FeO(s) + H2 → Fe(s)+ H2O 2 2 2 Fe3O4 (s) + 4H2 → 3 Fe + 4H2O Fe2O3 (s) + 3H2 → 2 Fe + 3H2O

• Hydrogen is the best reduction gas dealing with high temperatures. • Other metals such as Ni, Co, etc. can also be produced using hydrogen reduction process. • The metals which form hydrides are generally not preferred through this process, especially at low temperatures.

• The reduction of halides by hydrogen can also be carried out, similar to oxides and sulphides, e.g., VCl3, ZrCl4, or TiCl4. • Hydrogen gas is costly, but it is preferred due to cleanliness of the process.

• CO-H2 mixture can also be used for reduction in few cases. Chemical Methods: Solid State Reduction

• Below Any Ellingham Line, the metal is stable relative to the Oxide.

• If the element A can reduce the Oxide BxOy in the diagram, the Ellingham line for AxOy lies below that for BxOy. • Certain Metal oxides can also be reduced using relatively more stable metals. Chemical Methods: Hydro-metallurgical Reduction

• Metals can directly be produced from aqueous solutions through, (i) Reduction with another metal (ii) Gaseous Reduction.

➢ 1. Reduction of metal ions from a solution of another metal is known as cementation process. for any reaction, Mn+ + ne → M, The reduction potential is given by E = Eo – (RT/nF) ln(aM/aM+) “The more negative the electrode potential, the more stable the ions are in the solution.” A comparison of the electrode potentials of different metals, the more stable metal ion can be determined.

➢ 2. In gaseous reduction, commercial gasses used are: H2S, SO2, Co, and H2. e.g. Reduction of metal species in solution by Hydrogen can only take place if the hydrogen is at a lower potential than the metal ions at the appropriate metal concentration.

2+ + Ni (aq) + H2 (g) = Ni(s) + 2H (aq) Chemical Methods: Direct Synthesis

• This method is carried out at high temperature for pure compound ceramics and intermetallics. • Also known as self-propagating high-temperature synthesis. • Porous compact is heated at high temperatures. • Exothermic seif-sustaining reaction. Production of Carbonyl Iron, Copper, and Sponge Titanium Powder Powder Production Methods

The methods for powder production affect the powder characteristics, hence, the method is selected wrt type of application and desired properties of the final product.

1.Chemical Methods:

2. Ceramic Powders: Carbonates, hydroxides, nitrates, sulphates, acetates, oxalates, alkoxides or any other metal salts are used as raw material.

2. Electrolytic Method:

It involves primarily electrodeposition process, i.e. dissolution of the impure metal from anode and subsequent deposition on the cathode. Generally used for pure elemental powders.

3. Atomisation Method: Widely used commercial process. Used for producing a wide variety of elemental powders as well as alloys.

4. Mechanical Methods: It can be used for metals, alloys, and ceramics. It is not a primary method. Electrolytic Method

• Electrodeposition of metal: dissolution of impure metal from Anode to deposition on Cathode.

• Mimpure → Mn+ +ne (Anode); Mn+ +ne → Mpure • Overall: Mimpure → Mpure

❖ Adjustment of the chemical and physical conditions during electrodeposition to deposit metal loosely on the cathode, as cake or flakes.

Factors promoting powdery deposits: ➢ High Current density ➢ Weak metal concentration Addition of colloids and acides ➢ Low temperature ➢ High viscosity ➢ Avoidance of agitation ➢ Suppression of convection Electrolytic Method

• General shape of the powder is dendritic. • Some specific powders produced by electrolysis: Copper, Iron, Titanium • Low Apparent and Tap density • Low Flowability

Electrolytic Copper Powders Particles Atomization Method

• Method can be used for any material that can be melted. • Disintegration of melt in to fine powders. • Independent of physical and mechanical properties. • Widely used due to ease of making highly pure metals and pre-alloyed powders.

Melt → passing through orifice → impingement of gas/liquid on the melt stream → fragmentation of liquid stream → formation of droplets → solidification of droplets in-flight.

In Atomization process: Gas (air, nitrogen, Ar, He etc.) and liquid (water) can be used as atomization media. Atomization Method Conditions for fine particles • Low metal viscosity • Low metal surface tension • Superheated metal • Small nozzle diameter, i.e. low metal feed rate • High atomizing pressure • High atomizing agent volume • High atomizing agent velocity • Short metal stream • Short jet length • Optimum apex angle

For Sphericity of metal powder: • High metal surface tension • Narrow melting range • High pouring temperature • Gas atomization, inert gas • Low jet velocity • Long apex angle • Long flight path Electrolysis Atomization Method

WA Fe-Powder

Atomization WA Bronze Powder Water Atomization Mechanism

• Liquid metal stream breaks up under impact from the water droplets. • No Shear mechanism involved. • The velocity component of the water normal to the liquid metal stream, rather than velocity component parallel to the stream, controls the mean

particle size, dm. Mean Particle Size 퐵 푑푚 = sin 훼 푉푤 −푛 푑푚 = 퐾푃

Vw = velocity of water jet a= angle between the axis of the metal stream and water jet B=Constant (approx. value 2750) P= Atomizing water pressure n= ranges from 0.6-0.8 for water pressures from 0.1 to 20 Mpa K= Constant (value depends on viscosity and surface tension of molten metal) 27 Water Atomization

• This method is significant for low- and high-alloy steels, including stainless steels. • Due to oxide formation, it is not preferred for highly reactive metals such as Titanium, Aluminum, and Superalloys. • Generally, water atomized powders are irregular in shape with a thick oxide layer on the surface. • Water pressure used in the range of 3.5-21 MPa, with water velocities from 40 ms-1 to 15 ms-1. • The particle cooling rate is approx. 103 Ks-1 to 104 Ks-1.

Water Atomized Cu-Al-Ni Powder particles

28 Gas Atomization: Mechanism of Melt Droplet Formation • In atomization, a liquid layer which can have the form of a cylinder, a column or a sheet, is acted upon by gas and is broken up into droplets. • The surface of the melt is at first disturbed by a perturbance resembling a sinusoidal oscillation and is then broken up into unstable liquid bodies, the ligaments, which in turn break down into droplets. • Initiation of a sinusoidal wave which rapidly increases in amplitude. • Detachment of the wave from the bulk of the liquid to produce a ligament. The dimension of the ligament depends on the wavelength at disintegration. • Breakup of the ligament in to spherical droplets. • The droplets may undergo further disintegration and are cooled mainly through convection as they travel downstream following the motion of the gas.

푑푚 = 푓(diameter of liquid metal stream, kinematic viscosity of the liquid metal and gas, flow rate of liquid and Stable sheet gas, surface tension of the melt, density of melt, etc)

Growth of waves sheet Ligament formation Ligament breakdown

29 Gas Atomization

Pure Ti powder Cu-Al-Ni Powder Ni-Cu Alloy (Monel)

• Gas Atomized powders are typically spherical, with smooth surface. • Higher pressure/smaller jet distances produce finer powders. • Typical Atomization pressures: 1.5 MPa to 4.0 MPa. • Typical gas velocities: 50 ms-1 to 15o ms-1. Typical particle quench rate = 100 K/s. • Generally used for preparing powders of superalloys, titanium, high-speed steels, and other reactive metals. • Low overall energy efficiency. • Expensive if inert gas is used.

30 Centrifugal Atomization (Rotating Electrode Process)

• Ejection of molten metal from a rapidly spiining container, plate or disc. • In Rotating Electrode Process (REP), a rod electrode is rotated rapidly, and melted at one end by an electric arc. • Molten metal spins off the bar and solidifies before hitting the walls of the inert gas filled container. • The range of particle size is controlled by controlling the rotation of the anode. • Generally spherical powders are produced by REP. • Very clean process, but expensive. • Mostly useful for reactive metals. • Powders produced through this method are used for specialized applications where consolidation is achieved by HIP, SPS, or any other high-temperature method.

31 Rotating Electrode Process

Co-Cr-Mo Alloy Powder Pure Titanium

Ti-6Al-4V Ti-Al Powder

32 Evaporation Methods

• Useful in preparing nano-scale particles. • The material is vaporized in low pressure Argon, at approx. 10% of atmospheric pressure. • The particle are nucleated from the vapor, homogeneously. • The particles are collected at a cold substrate. • Heating Sources: electron beam, lasers, Plasma flames, induction coils, etc. • The shape of the powders is agglomerated. • The powders produced are expensive. • Handling of powders is difficult due to high reactivity. Glove-box is required. • The small sized powders have health hazards. • It is well suited to produce ceramic powders.

33 Mechanical Methods

• Mechanical methods are, generally, not used as primary methods. • The mechanical comminution is possible by impact, attrition, shear etc. • These methods have been used as primary process for cases such as: ✓ Easy-to-fracture materials such as pure antimony and bismuth, and relatively hard and brittle alloys and ceramics. ✓ Reactive materials, e.g. beryllium and metal hydrides. ✓ Common metals when flaky powders are required.

Basic Aspects: ❖ Impact/Compression: falling or vibrating media. ❖ Shear: by seizer between two moving surfaces. ❖ Attrition: Frictional stresses.

A combination of the these factors work in practice.

34 Mechanical Methods Jar Mill Fixed Moving Jaw Jaw

• Wet Process (Process Control Agent, PCA is used) • Dry Process

35 Mechanical Methods Ball Milling- High Energy Planetary Ball Mill

Metal A

Metal B

Intermetallic

36 Mechanical Methods Initial Powders Ball Milling- High Energy Planetary Ball Mill

Cu Increasing Milling Time

Mechanical TEM of Severely Milling Milled Powder, Al Nanocrystalline Structure