CH-3: Powder Metallurgy (PM) 1. Powder Metallurgy:

Lecture by: M. K. PODDAR, Asst. Prof. SIT SitamarhiPersonal Web: www.ajourneywithtime.weebly.comCH-3: Powder Metallurgy (PM)1. Powder Metallurgy: Science of producing metal powder and making finished/semifinished objects or components by using initial material as metal powder is known as powdermetallurgy.1.1 Basic Steps in the PM process: Powder Production Blending or Mixing Compaction Sintering FinishingFirst very fine particles of metal powder are produced from raw material by crushing orgrinding. Mixing this metal powder with various additives to impart certain characteristicscalled blending of metal powder then this blended metal powder are pressed into the shapesin a dies called compaction, then sintering the compact shape by heating it without melting(upto 70 to 80 % of melting temperature) in controlled atmosphere to impart strength thensecondary finishing or sizing the powder metallurgy product.1. Metal Powder2. Mixing and Blending3. Compaction4. SinteringFig.1: Basic Steps in the PM process1.2 Production of Metal Powder:A Powder can be defined as a finely divided particulate solid. It is characterized by size andshape, density, flow, compressibility etc. The basic principle method by which metallicpowders are produced:i. Atomizationii. Chemical methodiii. Physical Method (e.g. Electrolytic Deposition)iv. Mechanical Method

Lecture by: M. K. PODDAR, Asst. Prof. SIT SitamarhiPersonal Web: tion is a process where liquid metal is fragmented into small droplets that cool andsolidify into fine particles by uses high pressure fluid jet. High quality powder of ferrous andnon-ferrous powder like Al, Cu, iron, brass, stainless steel, super alloys etc. are producedcommercially. Types of atomization are water atomization, gas atomization, vacuumatomization, centrifugal atomization.In conventional (gas or water) atomization , aliquid metal is produced by pouring moltenmetal through a tundish with a nozzle at itsbase. The stream of liquid is then broken intodroplets by the impingement of high pressuregas or water.Fig.2: Water or gas atomizationFig. 3: Centrifugal atomizationRotating electrodeMechanical Method:It is cheapest of the powder production method, these methods involve using mechanicalforces such as compressive, shear or impact to reduce powder size or particle size reductionof bulk metal. E.g. milling/ Grinding/crushing/impactingFig.4: (a) Roll crusher, (b) Ball mill

Lecture by: M. K. PODDAR, Asst. Prof. SIT SitamarhiPersonal Web: www.ajourneywithtime.weebly.com1.3 Mixing and Blending: Powders of different metals and other materials may be mixed andblended with various additives (lubricants or binders) in order to impart special physical andmechanical properties. Lubricant facilitates easy ejection of compact and minimizes wear of tools.Binders such as wax or thermoplastic polymers are added to improve green strength. Some commonequipment geometries used for blending powdersFig.4: (a) Cylindrical, (b) rotating cube, (c) double cone, (d) twin shell1.4 CompactionPress powder into the desired shape and size in dies using a hydraulic or mechanical press.Conventional compaction method is pressing, in which opposing punches squeeze thepowders contained in a die. The workpart after pressing is called a green compact, the wordgreen meaning not fully processed. It may be hot compaction or cold compaction depends onmetal powder.Fig.5 Stages of metal powder compaction Fig.6: 450 kN (50-ton) hydraulic pressforcompaction of PM parts (photo courtesy of DorstAmerica, Inc.).Increased compaction pressure– Provides better packing of particles and leads to porosity– localized deformation allowing new contacts to be formed between particles

Lecture by: M. K. PODDAR, Asst. Prof. SIT SitamarhiPersonal Web: www.ajourneywithtime.weebly.com1.5 SinteringIn the green condition of the compact shape possess low strength therefore by heating thecompact below the melting point (upto 70 to 80 % of melting temperature), strength can beachieved. This method of achieving strength in compact by heating is called as sinteringprocess. It is performed in the presence of inert atmosphere like argon gases or in thepresence of vacuum to avoid contamination of air (O2) with compact. The compact issubjected to heating process in oven or furnace which will form chemical bond among thepowder particles. Hence strength of the compact increases tremendously.Fig.7: sintering in furnaceSintering Sequence on a Microscopic ScaleFig.8: (1) Particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) pores betweenparticles are reduced in size; (4) grain boundaries develop between particles in place of necked regions1.6 Sizing & Finishing or secondary operationsOperations include repressing, grinding, plating can be done; They are used to ensure closedimensional tolerances, good surface finish, increase density, corrosion resistance etc.1.7 Advantages and Disadvantages of powder metallurgyRefers Class lecture notes, dated on 28.04.20181.8 Impregnation and InfiltrationPorosity is a unique and inherent characteristic of PM technology. It can be exploited tocreate special products by filling the available pore space with oils, polymers, or metals. Twocategories: (a) Impregnation; (b) Infiltration

Lecture by: M. K. PODDAR, Asst. Prof. SIT SitamarhiPersonal Web: www.ajourneywithtime.weebly.com(a) Impregnation: The term used when oil or other fluid is permeated into the pores of asintered PM part. Common products are oil-impregnated bearings, gears, and similarcomponents. Alternative application is when parts are impregnated with polymer resins thatseep into the pore spaces in liquid form and then solidify to create a pressure tight part(b) Infiltration: Operation in which the pores of the PM part are filled with a molten metal.The melting point of the filler metal must be below that of the PM part. Heating the fillermetal in contact with the sintered part so capillary action draws the filler into the pores;resulting structure is nonporous, and the infiltrated part has a more uniform density, as wellas improved toughness and strengthQ. What are the materials and products that seem most suited to powder metallurgy?Ans. A pure metal in particulate form can be produced. Common elemental powders it canproduce are Iron, Aluminum, and Copper etc. Elemental powders can be mixed with othermetal powders to produce alloys that are difficult to formulate by conventional methodsExample: tool Steels. It can also produce metal alloy powder like stainless steel, superalloys,Ti alloy powders etc.PM Products are Gears, bearings, sprockets, fasteners, electrical contacts, cutting tools, andvarious machinery parts etc. Advantage of PM: parts can be made to near net shape or netshape. When produced in large quantities, gears and bearings are ideal for PM because thereis a need for porosity in the part to serve as a reservoir for lubricant.Q. What is hot Iso-static pressing (HIP) in Powder metallurgy?Ans: In this method of compaction of powder metal apply hydrostatic pressure in order toachieve uniform compaction. In HIP, the container is usually made of a high melting pointsheet metal in which pressing is done and pressing medium is inert gas. The commoncondition for HIP are 100 MPa at 1100 (although at high pressure and temperature).The main advantages of HIP are its ability to [produce compact having almost 100% density;good metallurgical bonding of the particles and good mechanical properties.

Lecture by: M. K. PODDAR, Asst. Prof. SIT SitamarhiPersonal Web: www.ajourneywithtime.weebly.comQ. Explain briefly particle size, distribution and shape in powder metallurgy technique.Ans: Geometry features of engineering metal powder are particle size & distribution, particleshape and internal structure and surface area. Particle size distribution is specified in terms ofsieve analysis i.e amount of powder paving through different mess size. Particle sizedistribution influence the packing of powder and its behavior during compaction andsintering.Particle shapes in PM are spherical, rounded, cylindrical, spongey, acicular etc.References:1. Text book: Production Technology by R K Jain2. nptel.ac.in/courses/112107144/103. https://en.wikipedia.org/wiki/Powder metallurgy

secondary finishing or sizing the powder metallurgy product. 1. Metal Powder 2. Mixing and Blending 3. Compaction 4. Sintering Fig.1: Basic Steps in the PM process 1.2 Production of Metal Powder: A Powder can be defined as a finely divided particulate solid. It is characterized by size and shape, density, flow, compressibility etc.