Surfaces, Dimensional Characteristics, Inspection, and Quality Assurance

• Because of the various mechanical, physical, thermal, and chemical effects induced by its processing history, the surface of a manufactured part generally has properties and behavior that are considerably different from those of its bulk. Although the bulk material generally determines the component’s overall mechanical properties, the component’s surface directly influences several important properties and characteristics of the manufactured part.

• Surface properties– Friction and wear properties of the part during subsequent

processing when it comes directly into contact with tools and dies and when it is placed in service.

– Effectiveness of lubricants during the manufacturing processes, as well as throughout the part’s service life

– Appearance and geometric features of the part and their role in subsequent operations such as painting, coating, welding, soldering, and adhesive bonding, as well as the resistance of the part to corrosion

– Initiation of cracks because of surface defects, such as roughness, scratches, seams, and heat-affected zones, which could lead to weakening and premature failure of the part by fatigue or other fracture mechanisms

– Thermal and electrical conductivity of contacting bodies. For example, rough surfaces have higher thermal and electrical resistance than smooth surfaces.

• Friction, wear, and lubrication, now called tribology, are surface phenomena.

• Friction influences:– forces

– power requirements

– surface quality of parts

• Wear alters the surface geometry of tools and dies.

• Lubrication is an integral aspect of all manufacturing operations.

• Treatment of surfaces to modify their properties and characteristics is an important aspect in surface technology. Several methods are:– mechanical

– thermal

– electrical

– chemical

• Measurement of the relevant dimensions and features of parts is an integral aspect of interchangeable manufacture - the basic concept of standardization and mass production.

• Inspection and testing of manufactured products with destructive and nondestructive methods.

Surface Structure and Properties

• The depth and properties of the work hardened layer - the surface structure- depend on factors such as the processing method used and frictional sliding to which the surface was subjected.

• Unless the metal is processes and kept in an inert (oxygen free) environment - or it is a noble metal, such as gold or platinum - an oxide layer usually lies on top of the work hardened layer. Some examples are:– Iron has an oxide structure with FeO adjacent to the bulk metal,

followed by Fe3O4 and then a layer of Fe2O3, which is exposed to the environment.

– Aluminum has a dense amorphous layer of Al2O3 with a thick, porous hydrated aluminum-oxide layer over it.

– Copper has a bright shiny surface when freshly scratched or machined. Soon after, however, it develops a Cu2O layer, which is then covered with a layer of CuO. This gives copper it’s somewhat dull color.

– Stainless steels are “stainless” because they develop a protective layer of chromium oxide, CrO (passivation).

• Under normal environmental conditions, surface oxide layers are generally covered with adsorbed layers of gas and moisture. Finally, the outermost surface of the metal may be covered with contaminants such as dirt, dust, grease, lubricant residues, cleaning compound residues, and pollutants from the environment.– the oxide layer is much harder than the base metal, but brittle and

abrasive

– surface factors also apply to plastics and ceramics

– surface condition affects friction, wear, and lubrication

Surface Integrity• Describes the topological, mechanical, and metallurgical

properties and characteristics.

• Influences properties such as fatigue strength, resistance to corrosion, and service life.

• Lack of surface integrity may be due to a number of defects either present before machining, created by the process, or due to lack of proper parameter control resulting in excessive stresses and temperatures– cracks - external or internal separations with sharp outlines

– craters or pits - shallow depressions

– folds, seams, or laps - overlapping of the material

– heat-affected zones (HAZ) - zone subjected to thermal cycling without melting

– inclusions - nonmetallic impurities

– intergranular attack - weakening of grain boundaries by liquid metal embrittlement and corrosion

– metallurgical transformation - microstructural changes caused by temperature and pressure cycling. Included are phase transformations, recrystallization, alloy depletion, decarburization, molten and recast, resolidified, or redeposited material, as in electrical discharge machining

– plastic deformation - severe surface deformation caused by high stresses due to friction, tool and die geometry, worn tools, and the method of processing

– residual stresses - surface stresses (tension or compression) caused by nonuniform deformation and temperature distribution

– splatter - small resolidified molten metal particles deposited on a surface, as from welding

Surface Texture• All surfaces have unique characteristics called texture,

roughness, and finish– lay, or directionality, is the direction of the predominant surface

pattern, and is usually visible to the naked eye.

– roughness is closely spaced, irregular deviations on a scale smaller than that of waviness.

– Waviness is a periodical deviation from a flat surface.

Surface Roughness

• Described by two methods: arithmetic mean value (Ra) and root mean square average (Rq)

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Measuring Surface Roughness– surface profilometer

Surface Roughness in Practice

• Surface roughness design requirements for typical engineering applications can vary by as much as two orders of magnitude for different parts. The reasons and considerations for this wide range include:– Precision required on mating surfaces, such as seals, gaskets, tools,

and dies. For example, ball bearings and gages require very smooth surfaces, whereas gaskets and brake drums can be rough.

– Frictional considerations, that is, the effect of roughness on wear, friction, and lubrication.

– Fatigue and notch sensitivity, because the rougher the surface is, the shorter the fatigue life will be.

– Electrical and thermal contact resistance, because the rougher the surface is, the greater the resistance is.

– Corrosion resistance, because the higher the roughness, the greater the possibility of trapping corrosive media.

– Subsequent processing, such as painting and coating, in which a certain roughness can result in better bonding.

– Appearance

– Cost considerations, because the finer the finish is, the higher the cost will be.

• These factors should be carefully considered before a decision is made as to the recommendation about surface roughness for a certain product. As in all manufacturing processes, the cost involved in the selection should also be a major consideration.

Tribology

• Tribology is the science and technology of interacting surfaces, thus involving friction, wear, and lubrication.

• Friction:– The resistance to relative sliding between two bodies in contact

under a normal load. Dissipates energy in the form of heat.

– Adhesion theory of friction:

• In metalworking processes, the load at the interface is generally high and the normal stress nears the yield stress, forming microwelds. The cleaner the surface, then stronger the bonds. To pull the two surfaces apart requires a certain force much like pulling apart a welded joint.

applied force eCompressiv

force Tensile adhesion oft Coefficien

• Experimental observations indicate that this coefficient can be quite high with softer metals, and that, with few exceptions, it requires a certain finite force to pull the surfaces apart. The more ductile the materials, the greater the plastic deformation and the stronger the adhesion of the junctions.

– Coefficient of friction

• Sliding between two bodies under a normal load is possible only by application of a tangential force F. According to the adhesion theory, this is the force required to shear the junctions. The coefficient of friction µ is defined as:

• where is the shear strength of the junction and is the normal stress, which, for a plastically deformed junction, is the yield strength. Thus, the numerator is a surface property, whereas the denominator is a bulk property.

σ

τ

σA

τA

N

Fμ

r

r

• Because an asperity is surrounded by a large mass of material, the normal stress on an asperity is equivalent to the hardness of the material. Thus, the coefficient of friction can now be defined as:

• If the upper body is harder than the lower one, or if its surface contains protruding hard particles, then as it slides over the softer body it can scratch and produce grooves on the lower surface. This is known as plowing and is an important aspect in frictional behavior; in fact, it can be a dominant mechanism for situations where adhesion is not strong.

• The nature and strength of the interface is the most significant factor in friction. A strong interface requires a high friction force for relative sliding and one with little strength requires a low friction force. The coefficient of friction can be reduced, not only by decreasing the shear strength, but by increasing the hardness. This is what is accomplished by a lubricant.

hardness

– Coefficients of friction in sliding contact, as measured experimentally, vary from as low as 0.02 to 100, or higher. This range is not surprising in view of the many variables involved in the friction process. In metalworking operations that use various lubricants, the range for is much narrower, as shown.

• Temperature rise due to friction:– Almost all energy dissipated in overcoming friction is converted to

heat. The magnitude of the interface temperature rise depends on:

• friction force

• speed

• surface roughness

• physical properties of materials

• Reducing friction:– By selecting material with low adhesion, such as carbides and

ceramics

– by using surface films and coatings

– by using ultrasonic vibrations, at 20kHz

• Measuring friction:– The coefficient of friction is determined experimentally by

measuring forces or dimensional changes in the specimen.

– Ring compression test:

• the most common geometry has outer diameter to inner diameter to height proportions of the specimen of 6:3:2

• Friction in plastics and ceramics:– plastics possess low frictional characteristics (self lubricating)

– they are attractive for bearings, gears, seals, prosthetic joints, and general friction-reducing applications

– the plowing component of friction is a significant factor

– the effect of temperature rise at the sliding interfaces caused by friction

– adhesion and plowing at interfaces contribute to the friction force in ceramics

• Wear:– It is defined as the progressive loss of material from a surface.

– It affects the process, size, and quality of the parts produced.

– It alters the surface topography and may result in severe surface damage.

– It also has the beneficial effect of reducing surface roughness.

• Wear is classified as:– adhesive

– corrosive

– fatigue

– fretting

– impact

• Adhesive wear:– Because of factors such as strain hardening at the asperity contact,

diffusion, and mutual solid solubility, the adhesive bonds are often stronger than the base metals.

• mild wear - oxide layers on surfaces act as protective film

• severe wear - contacting surface layers are free from contaminants

• film of absorbed layer of gas and other contaminants affect the interfacial bond strength of contacting asperities.

• Abrasive wear:– Abrasive wear is caused by a hard and rough surface sliding across

the surface.

• Produces grooves or scratches on the softer surface

• Abrasive wear can be reduced by increasing the hardness of materials (by heat treatment and microstructural changes) or by reducing the normal load.

• Three body wear (a lubricant between the die and workpiece) may carry wear particles

• Corrosive wear:– Also known as oxidation or chemical wear, it is caused by

chemical or electrochemical reactions between the surface and the environment.

– Corrosive products are the wear particles

– Among corrosive media are water, seawater, oxygen, acids and chemicals, and atmospheric hydrogen sulfide and sulfur dioxide.

– Reduction of corrosive wear by:

• selecting materials resistant to environmental attack

• controlling the environment

• reducing operating temperatures to lower the rate of reaction

• Other types of wear:– Erosion - caused by loose abrasive particles on the surface

– Fretting corrosion - occurs at interfaces with very small movements

– Impact wear - removal of small amounts of material due to impact

• Fatigue wear:– Also called surface fatigue or surface-fracture wear.

– Caused when the surface of a material is subjected to cyclic loading.

– The wear particles are usually formed by spalling or pitting.

– Thermal fatigue - cracks on the surface are generated by thermal stresses from thermal cycling.

– Could be reduced by:

• lowering contact stresses

• reducing thermal cycling

• improving the quality of materials by removing impurities, inclusions, and various others flaws that may act as local points for crack initiation

• Measuring wear:– Less accurate:

• visual and tactile inspection

– More accurate

• measuring dimensional changes

• gaging the warn components

• profilometry

• weighing

• noise level

• radiography

• spectroscopy (the lubricant is analyzed for wear particles)

Lubrication• In manufacturing processes, the interface between tools,

dies, and workpieces is usually subjected to a wide range of variables. Among the major ones are:– Contact pressures, ranging from low values of elastic stresses to

multiples of the yield stress of the workpiece material

– Relative speeds, ranging from very low (such as in some superplastic metal-forming operations) to very high speeds (such as in explosive forming, thin wire drawing, grinding, and high-speed metal cutting)

– Temperatures, ranging from ambient to almost melting, such as in hot extrusion and squeeze casting

– If two surfaces slide against each other under these conditions, with no protective layers at the interface, friction and wear will be high. To reduce friction and wear, surfaces should be held as far apart as possible. This generally is done by metalworking lubricants, which may be solid, semisolid, or liquid in nature. Metalworking lubricants are not only fluids or solids with certain desirable physical properties, such as viscosity, but also are chemicals that can react with the surfaces of the tools, dies, and workpieces and alter their physical properties.

• Lubrication regimes– Basically, four regimes of lubrication are relevant to metalworking

processes.

• Thick film: the surfaces are completely separated by a fluid film having a thickness of about one order of magnitude greater than the surface roughness; thus, there is no metal-to-metal contact. The normal load is light and is supported by the hydrodynamic fluid film by the wedge effect caused by the relative velocity of the two bodies and the viscosity of the fluid. The coefficient of friction is very low, usually ranging between 0.001 and 0.002.

• Thin film: as the normal load increases, or as the speed and viscosity of the fluid decrease, the film thickness is reduced to about three to five times the surface roughness. There may be some metal-to-metal contact at the higher asperities; this contact increases friction and leads to slightly higher wear.

– Mixed lubrication: In this situation, a significant portion of the load is carried by the metal-to-metal contact of the asperities, and the rest of the load is carried by the pressurized fluid film present in hydrodynamic pockets, such as in the valleys of asperities. The film thickness is less than three times the surface roughness. With proper selection of lubricants, a strongly adhering boundary film a few molecules thick can be formed on the surfaces. This film prevents direct metal-to-metal contact and thus reduces wear. Depending on the strength of the boundary film and other parameters, the friction coefficient in mixed lubrication may range up to about 0.4.

– Boundary lubrication: The load here is supported by the contacting surfaces, which are covered with a boundary layer. Depending on the boundary film thickness and its strength, the friction coefficient ranges from about 0.1 to 0.4. Wear can be relatively high if the boundary layer is destroyed. Typical boundary lubricants are natural oils, fats, fatty acids, and soaps. Boundary films form rapidly on metal surfaces. As the film thickness decreases and metal-to-metal contact takes place, the chemical aspects of surfaces and their roughness become significant. Chemical aspects are not important in thick-film lubrication, except for the fact that they may corrode or stain.

• A boundary film can break down or it can be removed by being disturbed or rubbed off during sliding, or because of desorption due to high temperatures at the interface. The metal surfaces may thus be deprived of this protective layer. The clean metal surfaces then contact each other and, as a consequence, severe wear and scoring can occur. Thus the adherence of boundary films is an important aspect in lubrication.

• Geometric effects:– The overall geometry of the interacting bodies is an important

consideration in lubrication.

– Where is the viscosity, t is film thickness. Thus the pressure increases with increasing viscosity, speed, and workpiece diameter, and decreases with increasing film thickness.

• Metalworking fluids– Metalworking fluid functions can be summarized as follows:

• reduce friction, reducing force and energy requirements and temperature rise

• Reduce wear, seizure, and galling

• Improve material flow in tools, dies, and molds

• act as thermal barrier between the workpiece and tool and die surfaces, thus preventing workpiece cooling in hot-working processes

• act as a release or parting agent to help in the removal or ejection of parts from dies and molds

– Many types of metalworking fluids are now available to fulfill these requirements. Because of their diverse chemistries, properties, and characteristics, the behavior and performance of lubricants can be complex. In this section only the general properties of only the most commonly used lubricants are discussed.

• oils

– have high film strength on the surface of a metal

– low thermal conductivity and specific heat

– sources are: mineral (petroleum), animal, vegetable, and fish

– can be compounded with a variety of additives including sulfur, chlorine, and phosphorus (extreme pressure additives)

• an emulsion is usually a mixture of oil and water in various proportions, along with additives.

– direct and indirect emulsions:

» direct: mineral oil is dispersed in water as very small droplets

» indirect: water droplets are dispersed in oil (high cooling capacity)

• synthetic solutions are chemical fluids containing inorganic and other chemicals dissolved in water

• soaps are generally reaction products of sodium or potassium salts with fatty acids. Alkali soaps are soluble in water, but other metal soaps are generally insoluble.

• greases are solid or semisolid lubricants and generally consist of soaps, mineral oil, and various additives.

• Waxes may be of animal or plant (paraffin) origin and have complex structure.

• Solid lubricants:– Graphite is weak in shear along its layers and has a low coefficient

of friction in that direction (only in the presence of air or moisture). In a vacuum or in an inert gas atmosphere, friction is very high.

– Thin layers of soft metals and polymer coatings are used as solid lubricants. Suitable metals are lead, indium, cadmium, tin, silver, and polymers such as PTFE, polyethylene, and methacrylates.

– Glass becomes viscous at high temperatures, and hence can serve as a lubricant. Poor thermal conductivity makes glass attractive as a thermal barrier between a hot workpiece and a relatively cool die.

• Conversion coatings:– Acids are applied to the workpiece surface. The chemical reaction

will a generate rough and spongy surface that acts as a carrier for the lubricant. Borax or lime is used to remove any excess acid from the surface. A liquid lubricant, such as a soap, is then applied to the coated surface.

– Zinc phosphate conversion coatings are often used on carbon and low-alloy steels.

– Oxalate coatings are used for stainless steels and high temperature alloys.