Section B Question Paper Feb 2013(TQM)

POWDER METALLURGY - Preliminary

POWDER METALLURGY

         

POWDER METALLURGY (PM) IS A METAL WORKING PROCESS FOR FORMING PRECISION METAL COMPONENTS FROM METAL POWDERS. THE METAL POWDER IS FIRST PRESSED INTO PRODUCT SHAPE AT ROOM TEMPERATURE. THIS IS FOLLOWED BY HEATING (SINTERING) THAT CAUSES THE POWDER PARTICLES TO FUSE TOGETHER WITHOUT MELTING.

THE PARTS PRODUCED BY PM HAVE ADEQUATE PHYSICAL AND MECHANICAL PROPERTIES WHILE COMPLETELY MEETING THE FUNCTIONAL PERFORMANCE CHARACTERISTICS. THE COST OF PRODUCING A COMPONENT OF GIVEN SHAPE AND THE REQUIRED DIMENSIONAL TOLERANCES BY PM IS GENERALLY LOWER THAN THE COST OF CASTING OR MAKING IT AS A WROUGHT PRODUCT, BECAUSE OF EXTREMELY LOW SCRAP AND THE FEWER PROCESSING STEPS. THE COST ADVANTAGE IS THE MAIN REASON FOR SELECTING PM AS A PROCESS OF PRODUCTION FOR HIGH – VOLUME COMPONENT WHICH NEEDS TO BE PRODUCED EXACTLY TO, OR CLOSE TO, FINAL DIMENSIONS. PARTS CAN BE PRODUCED WHICH ARE IMPREGNATED WITH OIL OR PLASTIC, OR INFILTRATED WITH LOWER MELTING POINT METAL. THEY CAN BE ELECTROPLATED, HEAT TREATED, AND MACHINED IF NECESSARY.

THE RATE OF PRODUCTION OF PARTS IS QUITE HIGH, A FEW HUNDREDS TO SEVERAL THOUSANDS PER HOUR.

INDUSTRIAL APPLICATIONS OF PM PARTS ARE SEVERAL. THESE INCLUDE SELF – LUBRICATING BEARINGS, POROUS METAL FILTERS AND A WIDE RANGE OF ENGINEERED SHAPES, SUCH AS GEARS, CAMS, BRACKETS, SPROCKETS, ETC.

PROCESS DETAILS:

IN THE PM PROCESS THE FOLLOWING THREE STEPS ARE FOLLOWED IN SEQUENCE: MIXING (OR BLENDING), COMPACTING, AND SINTERING.

MIXING: A HOMOGENEOUS MIXTURE OF ELEMENTAL METAL POWDERS OR ALLOY POWDERS IS PREPARED. DEPENDING UPON THE NEED, POWDERS OF OTHER ALLOYS OR LUBRICANTS MAY BE ADDED.

COMPACTING: A CONTROLLED AMOUNT OF THE MIXED POWDER IS INTRODUCED INTO A PRECISION DIE AND THEN IT IS PRESSED OR COMPACTED AT A PRESSURE IN THE RANGE 100 MPA TO 1000 MPA. THE COMPACTING PRESSURE REQUIRED DEPENDS ON THE CHARACTERISTICS AND SHAPE OF THE PARTICLES, THE METHOD OF MIXING, AND ON THE LUBRICANT USED. THIS IS GENERALLY DONE AT ROOM TEMPERATURE. IN DOING SO, THE LOOSE POWDER IS CONSOLIDATED AND DENSIFIED INTO A SHAPED MODEL. THE MODEL IS GENERALLY CALLED “GREEN COMPACT.” AS IS COMES OUT OF THE DIE, THE COMPACT HAS THE SIZE AND SHAPE OF THE FINISHED PRODUCT. THE STRENGTH OF THE COMPACT IS JUST SUFFICIENT FOR IN – PROCESS HANDLING AND TRANSPORTATION TO THE SINTERING FURNACE. 

FIG 10.1 TYPICAL SET OF POWDER METALLURGY TOOLS.

TO ILLUSTRATE THE PROCESS, LET US TAKE A STRAIGHT CYLINDRICAL PART SUCH AS A SLEEVE BEARING. FIG 10.1 SHOWS A TYPICAL SET OF TOOLS USED FOR PRODUCING THIS PART. THE COMPACTING CYCLE FOR THIS PART (FIG 10.2) FOLLOWS THE FOLLOWING STEPS.

 FIG 10.2 POWDER METALLURGY COMPACTING CYCLE.

1. WITH THE UPPER PUNCH IN THE WITHDRAWN POSITION, THE EMPTY DIE CAVITY IS FILLED WITH MIXED POWDER.
2. THE METAL POWDER IN THE DIE IS PRESSED BY SIMULTANEOUS MOVEMENT OF UPPER AND LOWER PUNCHES.
3. THE UPPER PUNCH IS WITHDRAWN, AND THE GREEN COMPACT IS EJECTED FROM THE DIE BY THE LOWER PUNCH.
4. THE GREEN COMPACT IS PUSHED OUT OF THE PRESSING AREA SO THAT THE NEXT OPERATING CYCLE CAN START.

         THIS COMPACTING CYCLE IS ALMOST THE SAME FOR ALL PARTS. 

SINTERING: DURING THIS STEP, THE GREEN COMPACT IS HEATED IN A PROTECTIVE ATMOSPHERE FURNACE TO A SUITABLE TEMPERATURE, WHICH IS BELOW THE MELTING POINT OF THE METAL. TYPICAL SINTERING ATMOSPHERES ARE ENDOTHERMIC GAS, EXOTHERMIC GAS, DISSOCIATED AMMONIA, HYDROGEN, AND NITROGEN. SINTERING TEMPERATURE VARIES FROM METAL TO METAL; TYPICALLY THESE ARE WITHIN 70 TO 90% OF THE MELTING POINT OF THE METAL OR ALLOY. TABLE 10.1 GIVES THE SINTERING TEMPERATURES USED FOR VARIOUS METALS. SINTERING TIME VARIES WITH SIZE AND METAL OF PART. TABLE 10.1 ALSO GIVES TYPICAL RANGE OF SINTERING TIME NEEDED FOR VARIOUS METALS.

TABLE 10.1 SINTERING TEMPERATURE AND TIME FOR VARIOUS METAL POWDERS

MATERIAL	TEMPERATURE ( 0C)	TIME
COPPER, BRASS, BRONZE NICKEL STAINLESS STEELS FERRITES TUNGSTEN CARBIDE MOLYBDENUM TUNGSTEN TANTALUM	760-900 1000-1150 1100-1290 1200-1500 1430-1500 2050 2350 2400	10-40 30-40 30-60 10-600 20-30 120 480 480

          

SINTERING IS A SOLID STATE PROCESS WHICH IS RESPONSIBLE FOR PRODUCING PHYSICAL AND MECHANICAL PROPERTIES IN THE PM PART BY DEVELOPING METALLURGICAL BOND AMONG THE POWDER PARTICLES. IT ALSO SERVES TO REMOVE THE LUBRICANT FROM THE POWDER, PREVENTS OXIDATION, AND CONTROLS CARBON CONTENT IN THE PART. THE STRUCTURE AND POROSITY OBTAINED IN A SINTERED COMPACT DEPEND ON THE TEMPERATURE, TIME, AND PROCESSING DETAILS. IT IS NOT POSSIBLE TO COMPLETELY ELIMINATE THE POROSITY BECAUSE VOIDS CANNOT BE COMPLETELY CLOSED BY COMPACTION AND BECAUSE GASES EVOLVE DURING SINTERING. POROSITY IS AN IMPORTANT CHARACTERISTIC FOR MAKING PM BEARINGS AND FILTERS.

SECONDARY AND FINISHING OPERATIONS

          

SOMETIMES ADDITIONAL OPERATIONS ARE CARRIED OUT ON SINTERED PM PARTS IN ORDER TO FURTHER IMPROVE THEIR PROPERTIES OR TO IMPART SPECIAL CHARACTERISTICS. SOME IMPORTANT OPERATIONS ARE AS UNDER.

1. COINING AND SIZING. THESE ARE HIGH PRESSURE COMPACTING OPERATIONS. THEIR MAIN FUNCTION IS TO IMPART (A) GREATER DIMENSIONAL ACCURACY TO THE SINTERED PART, AND (B) GREATER STRENGTH AND BETTER SURFACE FINISH BY FURTHER DENSIFICATION.
2. FORGING. THE SINTERED PM PARTS MAY BE HOT OR COLD FORGED TO OBTAIN EXACT SHAPE, GOOD SURFACE FINISH, GOOD DIMENSIONAL TOLERANCES, AND A UNIFORM AND FINE GRAIN SIZE. FORGED PM PARTS ARE BEING INCREASINGLY USED FOR SUCH APPLICATIONS AS HIGHLY STRESSED AUTOMOTIVE, JET – ENGINE AND TURBINE COMPONENTS.
3. IMPREGNATION. THE INHERENT POROSITY OF PM PARTS IS UTILIZED BY IMPREGNATING THEM WITH A FLUID LIKE OIL OR GREASE. A TYPICAL APPLICATION OF THIS OPERATION IS FOR SINTERED BEARINGS AND BUSHINGS THAT ARE INTERNALLY LUBRICATED WITH UPTO 30% OIL BY VOLUME BY SIMPLY IMMERSING THEM IN HEATED OIL. SUCH COMPONENTS HAVE A CONTINUOUS SUPPLY OF LUBRICANT BY CAPILLARY ACTION, DURING THEIR USE. UNIVERSAL JOINT IS A TYPICAL GREASE – IMPREGNATED PM PART.
4. INFILTRATION. THE PORES OF SINTERED PART ARE FILLED WITH SOME LOW MELTING POINT METAL WITH THE RESULT THAT PART'S HARDNESS AND TENSILE STRENGTH ARE IMPROVED. A SLUG OF METAL TO BE IMPREGNATED IS KEPT IN CLOSE CONTACT WITH THE SINTERED COMPONENT AND TOGETHER THEY ARE HEATED TO THE MELTING POINT OF THE SLUG. THE MOLTEN METAL INFILTRATES THE PORES BY CAPILLARY ACTION. WHEN THE PROCESS IS COMPLETE, THE COMPONENT HAS GREATER DENSITY, HARDNESS, AND STRENGTH. COPPER IS OFTEN USED FOR THE INFILTRATION OF IRON – BASE PM COMPONENTS. LEAD HAS ALSO BEEN USED FOR INFILTRATION OF COMPONENTS LIKE BUSHES FOR WHICH LOWER FRICTIONAL CHARACTERISTICS ARE NEEDED.
5. HEAT TREATMENT. SINTERED PM COMPONENTS MAY BE HEAT TREATED FOR OBTAINING GREATER HARDNESS OR STRENGTH IN THEM.
6. MACHINING. THE SINTERED COMPONENT MAY BE MACHINED BY TURNING, MILLING, DRILLING, THREADING, GRINDING, ETC. TO OBTAIN VARIOUS GEOMETRIC FEATURES.
7. FINISHING. ALMOST ALL THE COMMONLY USED FINISHING METHOD ARE APPLICABLE TO PM PARTS. SOME OF SUCH METHODS ARE PLATING, BURNISHING, COATING, AND COLOURING.

PLATING. FOR IMPROVED APPEARANCE AND RESISTANCE TO WEAR AND CORROSION, THE SINTERED COMPACTS MAY BE PLATED BY ELECTROPLATING OR OTHER PLATING PROCESSES. TO AVOID PENETRATION AND ENTRAPMENT OF PLATING SOLUTION IN THE PORES OF THE PART, AN IMPREGNATION OR INFILTRATION TREATMENT IS OFTEN NECESSARY BEFORE PLATING. COPPER, ZINC, NICKEL, CHROMIUM, AND CADMIUM PLATING CAN BE APPLIED.

BURNISHING. TO WORK HARDEN THE SURFACE OR TO IMPROVE THE SURFACE FINISH AND DIMENSIONAL ACCURACY, BURNISHING MAY BE DONE ON PM PARTS. IT IS RELATIVELY EASY TO DISPLACE METAL ON PM PARTS THAN ON WROUGHT PARTS BECAUSE OF SURFACE POROSITY IN PM PARTS.

COATING. PM SINTERED PARTS ARE MORE SUSCEPTIBLE TO ENVIRONMENTAL DEGRADATION THAN CAST AND MACHINED PARTS. THIS IS BECAUSE OF INTER – CONNECTED POROSITY IN PM PARTS. COATINGS FILL IN THE PORES AND SEAL THE ENTIRE REACTIVE SURFACE.

COLOURING. FERROUS PM PARTS CAN BE APPLIED COLOUR FOR PROTECTION AGAINST CORROSION. SEVERAL METHODS ARE IN USE FOR COLOURING. ONE COMMON METHOD TO BLACKEN FERROUS PM PARTS IS TO DO IT CHEMICALLY, USING A SALT BATH.

8. JOINING. PM PARTS CAN BE WELDED BY SEVERAL CONVENTIONAL METHODS. ELECTRIC RESISTANCE WELDING IS BETTER SUITED THAN OXY- ACETYLENE WELDING AND ARC WELDING BECAUSE OF OXIDATION OF THE INTERIOR POROSITY. ARGON ARC WELDING IS SUITABLE FOR STAINLESS STEEL PM PARTS.

Macro and Micro Economics - Preliminary

What's the difference between macroeconomics and microeconomics?

Microeconomics is generally the study of individuals and business decisions, macroeconomics looks at higher up country and government decisions.Macroeconomics and microeconomics, and their wide array of underlying concepts, have been the subject of a great deal of writings. The field of study is vast; here is a brief summary of what each covers:

Microeconomics is the study of decisions that people and businesses make regarding the allocation of resources and prices of goods and services. This means also taking into account taxes and regulations created by governments. Microeconomics focuses on supply and demand and other forces that determine the price levels seen in the economy. For example, microeconomics would look at how a specific company could maximize it's production and capacity so it could lower prices and better compete in its industry. (Find out more about microeconomics in Understanding Microeconomics.) 

Macroeconomics, on the other hand, is the field of economics that studies the behaviour of the economy as a whole and not just on specific companies, but entire industries and economies. This looks at economy-wide phenomena, such as Gross National Product (GDP) and how it is affected by changes in unemployment, national income, rate of growth, and price levels. For example, macroeconomics would look at how an increase/decrease in net exports would affect a nation's capital account or how GDP would be affected by unemployment rate. (To keep reading on this subject, see Macroeconomic Analysis.)

While these two studies of economics appear to be different, they are actually interdependent and complement one another since there are many overlapping issues between the two fields. For example, increased inflation (macro effect) would cause the price of raw materials to increase for companies and in turn affect the end product's price charged to the public. 

The bottom line is that microeconomics takes a bottoms-up approach to analysing the economy while macroeconomics takes a top-down approach. Regardless, both micro- and macroeconomics provide fundamental tools for any finance professional and should be studied together in order to fully understand how companies operate and earnrevenues and thus, how an entire economy is managed and sustained. 

Cost of Quality (COQ) - Section B

Definition: 

Cost of Quality (COQ) is the sum of the costs incurred by a company in preventing poor

quality, the costs incurred to ensure and evaluate that the quality requirements are being

met, and any other costs incurred as a result of poor quality being produced. Poor quality

is defined as non-value added activities, waste, errors or failure to meet customer needs

and requirements. These COQ costs can be broken down into the three categories of

prevention, appraisal and failure costs. The COQ model is often referred to as the PAF

model after these three categories.

Prevention Cost:

The Planned costs incurred by an organization to ensure that errors are not made at any of the various stages during the delivery process of that product or service to customer. The delivery process many include design, development, production and shipping.

Examples: Education and training, continuous improvement efforts, quality administration staff, process control, market research, field testing and preventive maintenance.

Appraisal cost:

The costs of verifying, checking or evaluating a product or service at the various stages during the delivery process of that product or service to the customer.

Examples: Incoming inspection cost, internal product audit, inspection activities, inventory counts, Quality administration salaries, supplier evaluation and audit reports.

Failure cost:

The costs incurred by a company because the product or service did not meet the requirements and the product had to be fixed or replaced or the service had to be repeated. These failure costs can be further subdivided into two groups.

                Internal failure cost:

                Internal failure include all the costs resulting from the failures that are found before the product              or service reaches the customer.

                Examples: Scrap, rework, extra inventory, repair stations, re-design, salvage, corrective action

                reports and overtime due to nonconforming product or service.

                External Failure:

                External failures are all the costs incurred by the company resulting when the customer finds the             failure. These external failure costs do not include any of the customer’s personal costs.

                Examples: warranty, customer complaint administration, replacement product, recalls, shipping                               costs, analysis of warranty data, customer follow-up and field service departments.