MANUFACTURING PROCESS - METAL POWDERS FOR POWDER METALLURGY

METAL POWDERS FOR PM

          

METAL POWDERS PLAY AN EXTREMELY IMPORTANT ROLE IN POWDER METALLURGY. THESE ARE HIGHLY ENGINEERED MATERIALS. THE PARTICLE SIZE, SHAPE AND SIZE DISTRIBUTION OF METAL POWDER AFFECT THE CHARACTERISTICS AND PROPERTIES OF THE COMPACTED PRODUCT. A LARGE NUMBER OF TYPES AND GRADES OF POWDERS AVAILABLE WHICH MAKES POSSIBLE THE PRODUCTION OF A WIDE VARIETY OF COMPONENTS FOR MEETING NUMEROUS PERFORMANCE REQUIREMENTS. ALL METALS CAN BE PRODUCED IN POWDER FORM BUT NOT ALL HAVE THE DESIRED PROPERTIES WHICH ARE NECESSARY FOR ECONOMICAL PRODUCTION. SOME WIDELY USED METAL POWDERS FOR MANUFACTURING PM PARTS ARE LISTED IN TABLE 11.1. THE CHARACTERISTIC OF POWDERS GIVEN IN THIS TABLE ARE SIGNIFICANT FROM THE VIEWPOINT OF APPLICATION FEASIBILITY FOR PM PARTS

TABLE 11.1 WIDELY USED METAL POWDERS

PURE METALS: ALUMINUM ANTIMONY BERYLIUM BISMUTH CADMIUM CHROMIUM COBALT COPPER IRON LEAD MANGANESE MOLYBDENUM NICKEL PRECIOUS METALS(GOLD, SILVER, PLATINUM) RHENIUM SILICON TANTALUM TIN TITANIUM TUNGSTEN VANADIUM ZINC

ALLOYS: ALUMINIUM-IRON BRASS COPPER-ZINC-NICKEL NICKEL-CHROMIUM NICKEL-CHROMIUM-IRON NICKEL-COPPER NICKEL-IRON SILICON-IRON SOLDER STAINLESS STEEL   COMPOUNDS: BORIDES(CHROMIUM, TUNGSTEN, ETC.) CARBIDES (MOLYBDENUM, TUNGSTEN, ETC.) MOLYBDENUM DISILICIDE NITRIDES (SILICONN TITANIUM, ETC.) ZIRCONIUM HYDRIDE

POWDER PRODUCTION

ALL METAL POWDERS, BECAUSE OF THEIR INDIVIDUAL PHYSICAL AND CHEMICAL CHARACTERISTICS, CANNOT BE PRODUCED IN THE SAME WAY.

THERE ARE SEVERAL METHODS FOR PRODUCING METAL POWDERS EACH GIVING DIFFERENT SIZE AND STRUCTURE OF THE PARTICLES. TABLE 11.2 GIVES IMPORTANT CHARACTERISTICS OF POWDERS PRODUCED BY SOME COMMERCIAL METHODS. ALSO GIVEN IN THIS TABLE ARE ADVANTAGES AND DISADVANTAGES OF THESE METHODS. A BRIEF DESCRIPTION OF SOME OF THESE METHODS FOLLOWS.

TABLE 11.2 METAL POWDER CHARACTERISTICS

APPARENT DENSITY THE APPARENT DENSITY OR SPECIFIC GRAVITY OF A POWDER IS EXPRESSED IN KG/M 3 . IT SHOULD BE KEPT CONSTANT. THIS MEANS THAT THE SAME AMOUNT OF POWDER SHOULD BE FED INTO THE DIE EACH TIME. CHEMICAL PROPERTIES THESE ARE THE PROPERTIES LIKE THE PURITY OF THE POWDER, AMOUNT OF OXIDES PERMITTED, AND THE PERCENTAGE OF OTHER ELEMENTS ALLOWED. COMPRESSIBILITY COMPRESSIBILITY IS THE RATIO OF THE VOLUME OF INTIAL POWDER TO THE VOLUME OF THE COMPRESSED PIECE. IT VARIES CONSIDERABLY AND IS AFFECTED BY THE PARTICLE-SIZE DISTRIBUTION AND SHAPE. COMPRESSIBILITY AFFECTS THE GREEN STRENGTH OF A COMPACT. FINENESS FINENESS REFERS TO THE PARTICLE SIZE AND IS DETERMINED BY PASSING THE POWDER THROUGH A STANDARD SIEVE OR BY MICROSCOPIC MEASUREMENT. FLOWABLITY FLOWABLITY IS THE CHARACTERISTIC OF A POWDER THAT PERMITS IT TO FLOW READILY AND CONFORM TO THE MOLD CAVITY. IT CAN BE DESCRIBED AS THE RATE OF FLOW THROUGH A FIXED ORIFICE. PARTICLE-SIZE DISTRIBUTION PARTICLE-SIZE DISTRIBUTION REFERS TO THE AMOUNT OF EACH STANDARD PARTICLE SIZE IN THE POWDER. IT INFLUENCES THE FLOWABLITY AND APPARENT DENSITY AS WELL AS POROSITY OF THE PRODUCT. SINTERING ABILITY SINTERING ABILITY IS THE SUITABILITY OF A POWDER FOR BONDING OF PARTICLES BY THE APPLICATION OF HEAT.

ATOMIZATION: IT IS AS EXCELLENT AND VERY WIDELY USED METHOD OF PRODUCING METAL POWDERS. IN CASE OF LOW MELTING POINT METALS, THE MOLTEN METAL IS KEPT IN A TANK. IT IS RAISED BY THE SUCTION PRODUCED BY HOT AIR, THROUGH A PIPE TO THE ATOMIZING NOZZLE. A FINE STREAM OF MOLTEN METAL IS BROKEN INTO SMALL DROPLETS, WHICH SOLIDIFY INTO METAL POWDER PARTICLES. THE SIZE OF PARTICLES CAN BE CONTROLLED BUT THE SHAPE OF PARTICLES REMAINS IRREGULAR. HOWEVER, THE TECHNIQUE USED FOR HIGH MELTING POINT METALS IS SLIGHTLY DIFFERENT. A STREAM OF MOLTEN METAL COMING FROM AN ORIFICE AT THE BOTTOM OF A RESERVOIR IS BROKEN UP BY A JET OF ATOMIZING FLUID (WHICH MAY BE INERT GAS, AIR , WATER OR STEAM) INTO METAL POWDER PARTICLES. IT IS POSSIBLE TO CONTROL THE POWDER CHARACTERISTICS (AVERAGE PARTICLE SIZE, PARTICLE SHAPES, PARTICLE SIZE DISTRIBUTION, PARTICLE CHEMISTRY, AND PARTICLE STRUCTURE) BY CHANGING THE PROCESS VARIABLES (SUCH AS TEMPERATURE, STREAM VELOCITY, ETC.) IN THE ATOMIZING PROCESS.

ELECTROLYSIS: ELECTROLYTIC DEPOSITION OR ELECTROLYSIS IS A WIDELY USED METHOD OF PRODUCING POWDERS OF IRON, COPPERS, SILVER, AND SEVERAL OTHER METALS. FOR PRODUCING IRON, FOR EXAMPLE, A TANK CONTAINING A SUITABLE ELECTROLYTE IS TAKEN. IN IT STEEL PLATES ARE PLACED AS ANODE AND STAINLESS STEEL PLATES ARE PLACED AS CATHODE. THE TWO ELECTRODES ARE CONNECTED TO A POWERFUL DE SOURCE. IN ABOUT 50 HOURS, A 2 MM THICK DEPOSIT OF IS OBTAINED ON THE CATHODE PLATES. THIS DEPOSIT OF ELECTROLYTIC IRON IS STRIPPED, WASHED, SCREENED, AND SIZED. THE IRON POWDER MAY BE ANNEALED IF ITS BRITTLENESS IS TO BE REDUCED.

REDUCTION: IN THIS PROCESS, METAL OXIDE IS REDUCED TO METAL POWDER THROUGH CONTACT WITH A REDUCING GAS AT TEMPERATURE BELOW THE MELTING POINT. FOR EXAMPLE, IN CASE OF IRON THE IRON OXIDE IS CRUSHED AND PASSED THROUGH A FURNACE. THE HYDROGEN ATMOSPHERE IN THE FURNACE REACTS WITH THE OXYGEN OF IRON OXIDE AT A TEMPERATURE OF NEARLY 1050 0 C AND PURE IRON WITH SPONGE – LIKE STRUCTURE IS OBTAINED. IN ADDITION TO IRON, OTHER COMMONLY PRODUCED COMMERCIALLY BY THIS METHOD INCLUDE NICKEL, COBALT, MOLYBDENUM, AND TUNGSTEN.

MACHINING AND GRINDING. MACHINING HAS BEEN USED TO PRODUCE COARSE MAGNESIUM POWDER. MILLING AND GRINDING PROCESSES UTILIZE VARIOUS TYPES OF ROTARY MILLS, STAMPING MILLS, CRUSHERS, AND GRINDERS, BREAK DOWN BRITTLE METALS INTO POWDERS OF ALMOST ANY FINENESS BUT OF IRREGULAR SHAPED PARTICLES.

THERE ARE SEVERAL OTHER METHODS INVOLVING PRECIPITATION, CONDENSATION AND OTHER CHEMICAL PROCESSES, THAT ARE EMPLOYED FOR PRODUCING METAL POWDERS.

POWDER MIXING

MIXING OF POWDERS PRECEDES COMPACTING.

THE PROCESS OF MIXING INCLUDES MIXING OF VARIOUS METAL POWDERS WITH LUBRICANTS AS A RESULT OF WHICH THE POWDERS ARE THOROUGHLY INTERMINGLED. THIS IS CARRIED OUT IN BATCH MIXERS. THE SURFACE FRICTION PROPERTIES OF THE POWDERS TO BE MIXED SIGNIFICANTLY AFFECT THE PROPERTIES OF THE MIXTURES. IF THE POWDERS DIFFER TO MUCH IN DENSITY, SEGREGATION OF THE HEAVIER POWDER MAY OCCUR BECAUSE GRAVITATIONAL FORCES MAY BE STRONGER THAN THE FRICTIONAL FORCES.

THE TEMPERATURE DURING MIXING AFFECTS THE FRICTION BETWEEN POWDER PARTICLES. WITH INCREASING TEMPERATURE, THE FRICTION COEFFICIENT BETWEEN MOST MATERIALS INCREASES AND THE FLOW OF POWDERS IS IMPAIRED. IT IS THEREFORE DESIRABLE TO MAINTAIN LOWER MIXING TEMPERATURE.

WHEN PARTS ARE PRESSED IN RIGID DIES, THE USE OF LUBRICANT BECOMES ESSENTIAL IN ORDER TO REDUCE FRICTION BETWEEN POWDER PARTICLES AND BETWEEN THE COMPACT BEING PRESSED AND THE DIE WALL AND CORE ROD. THE LUBRICANT ALSO REDUCES THE PRESSURE REQUIRED TO EJECT COMPACTS FROM THE DIE. THE LUBRICANT, WHICH IS GENERALLY ½ TO 1% BY WEIGHT, IS INTRODUCED AS A FINE POWDER MIXING TIME AND THE INTENSITY OF MIXING POWDER AND LUBRICANT AFFECT FLOW AND APPARENT DENSITY OF THE POWDER MIXTURE.

References :- www.nptel.iitm.ac.in