41FabricationandProcessingofmetalMaterialsMaterials.pptx

Lecturer:*第第10次课次课Fabrication and Processing of metal Materials MaterialsCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA4.1 Introduction4.2 Forming Operations4.3 Casting4.4 Other techniquesContentCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAforming techniquesCasting welding4.1 IntroductionCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAThe methods chosen depend on several factors:1 The ductility properties of the metalplastic deformationforming techniquesCastingso low in ductility4.1 IntroductionCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA2 the size and shape of the finished piece weldingtwo or more metal partsCastingso large or complicated4.1 IntroductionCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA3 costCastingmost economical.4.1 IntroductionCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA4.2 Forming OperationswConcept:Concept:Those in which the shape of a metal piece is changed by plastic deformation(塑性变形)；for example,forging(锻造),rolling(扎压),extrusion(挤压),and drawing(拉拔)are common forming techniques.wDeformation ConditionsDeformation Conditions:The deformation must be induced by an external force or stress,the magnitude of which must exceed the yield strength(屈服强度)of the material.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAHot Working&Cold WorkingwConcept:When deformation is achieved at a temperature above that at which recrystallization(再结晶)occurs,the process is termed hot working;otherwise,it is cold working.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAwHot-working operationAdvantages:Advantages:(1)Large deformation and repeat(2)requiring less deformation energy.Disadvantages:Disadvantages:(1)Surface oxidationresulting in material loss and a poor final surface finish.wCold-working operationAdvantages:Advantages:(1)An increase in strength(2)Higher quality surface finish(3)Better mechanical properties(4)Closer dimensional control of the finished pieceDisadvantages:Disadvantages:(1)The decrease in ductility.(2)The total deformation is accomplished in a series of steps in which the piece is successively cold worked a small amount and then process annealedan expensive and inconvenient procedure.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAForging(锻造)w wConceptConcept:is mechanically working or deforming a single piece of a normally hot metal;this may be accomplished by the application of successive blows or by continuous squeezing.w wProductsProducts:having outstanding grain structures and the best combination of mechanical properties.Such as wrenches(扳手),automotive crankshafts(汽车机轴)and piston connecting rods(活塞连杆)College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA(1)Closed die (闭式锻模):A force is brought to bear on two or more die halves having the finished shape such that the metal is deformed in the cavity between them.(2)Open dieOpen die(开口锻模):Two dies having simple geometric shapes(e.g.,parallel flat,semicircular)are employed,normally on large work pieces.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINARolling wConsists of passing a piece of metal between two rolls;a reduction in thickness results from compressive stresses exerted by the rolls.w wCold rollingCold rolling(冷轧)sheet(薄板),strip(带),and foil(金属薄片)with high quality surface finish.w wGrooved rollsGrooved rolls(带槽轧辊)circular shapes(环形轧材)as well as I-beams(I型梁)and railroad railsCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAExtrusion(挤压)w wConceptConcept:a bar of metal is forced through a die orifice by a compressive force that is applied to a ram;the extruded piece that emerges has the desired shape and a reduced cross-sectional area.Products Products:rods and tubing that have rather complicated cross-sectional geometriesCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINADrawing(拉拔)w wConceptConcept:is the pulling of a metal piece through a die having a tapered bore by means of a tensile force that is applied on the exit side.A reduction in cross section results,with a corresponding increase in length.wProductsProducts:rod,wire,and tubing College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA4.3 Castingw wConceptConcept:a fabrication process whereby a totally molten is poured into a mold cavity having the desired shape;upon solidification,the metal assumes the shape of the mold but experiences some shrinkage.w wEmployed conditions:Employed conditions:(1)The finished shape is large or complicated(2)A particular alloy is low in ductility.(3)Economical.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAwSand casting(砂型铸造)wDie casting(模铸)wInvestment Casting(熔模铸造)wContinuous Casting(连铸)4.3 CastingCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINASand casting(砂型铸造)w wConcept:Concept:Sand is used as the mold material.A two-piece mold is formed by packing sand around a pattern that has the shape of the intended casting.w wProducts:Products:automotive cylinder blocks(汽车气缸座),fire hydrants(消火栓),and large pipe fitting(大型管道配件).College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINADie casting(模铸)w wConcept:Concept:The liquid metal is forced into a mold under pressure and at a relatively high velocity,and allowed to solidify with the pressure maintained.A two-pieces permanent steel mold or die is employed;when clamped together,the two pieces form the desired shape.w wCharacter:Character:1.Rapid casting rates 2.A single set of dies may be used for thousands of casting;3.Lend itself only to relatively small pieces and to alloys of zinc(Zn),aluminum(Al),and magnesium(Mg),which have low melting temperature.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAInvestment Casting(熔模铸造)wFor investment(sometimes called lost-wax)casting,the pattern is made from a wax or plastic that has a low melting temperature.Around the pattern is poured a fluid slurry,which sets up to form a solid mold;plaster of paris(熟石膏)is usually used.The mold is then heated,such that the pattern melts and is burned out,leaving behind a mold cavity having the desired shape.Application Application:high dimensional accuracy,reproduction of fine detail,and an excellent finishCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAContinuous Casting(连铸)wThe refined and molten metal is cast directly into a continuous strand which may have either a rectangular or circular cross section;solidification occurs in a water-cooled die having the desired cross-sectional geometry.w wCharacterCharacter:the chemical composition and mechanical properties are more uniform throughout the cross sections;highly automated and more efficient.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA4.4 Other techniques(其它技术)wPowder metallurgy(粉末冶金)wWelding(焊接法)College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAConcept:Concept:fabrication technique involves the compaction of powdered metal,followed by a heat treatment to produce a more dense piece.Powder metallurgy(粉末冶金)College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAPowder Metallurgy is suitable forwmetals having low ductilities.wMetals having high melting temperatures are difficult to cast,and fabrication is expedited using power metallurgy.wParts that require very close dimensional tolerances(尺寸公差)e.g.,bushing(套管)and gears(齿轮)College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAWeldingWhy may welding be considered to be a fabrication techique:wIn welding,two or more metal parts are joined to form a single piece when one-part fabrication is expensive or inconvenient.w Both similar and dissimilar metals may be welded.wThe joining bond is metallurgical rather than just mechanical,as with riveting(铆接)and bolting(拧紧).College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAwarc and gas welding(电弧焊和气焊)wLaser beam welding(激光焊)College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA the work pieces(工件)to be joined and the filler material(填充材料,i.e.,welding rod)are heated to a sufficiently high temperature to cause both to melt;upon solidification(凝固),the filler material forms a fusion joint between the work pieces.Arc and gas weldingCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAFigure:Schematic cross-sectional representation showing the zones in the vicinity of a typical fusion weld.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAPossible alterations:1.If the work piece material was previously cold worked,this HAZHAZ may have experience recrystallization and grain growth,and thus a diminishment(减小)of strength,hardness,and toughness(韧性).2.Upon cooling,residual stresses(残余应力)may form in this region that weaken the joint.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA3.For steels,the material in this zone may have been heated to temperatures sufficiently high so as to form austenite(奥氏体).upon,cooling to room temperature,the microstructural products that form depend on cooling rate and alloy composition.For plain carbon steels(普碳钢)that have low harden-abilities,normally pearlite(珠光体)and a proeutectoid phase(先共析相)will be present.However,for alloy steels(合金钢),one microstructural product may be martensite(马氏体),which is ordinarily undesirable because it is so brittle(脆).4.Some stanless stress may be“sensitized(激活)”during welding,which renders them susceptible to intergranular corrosion(粒间腐蚀)College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINALaser beam welding(激光焊)Feature:Feature:wIt is a relatively modern joining technique.1.A highly focused and intense laser beam is used as the heat source2.The laser beam melts the parent metal,and upon solidification,a fusion joint is produced;often a fillter material need not be used.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAThe advantages of this techniques1.It is a noncontact process,which eliminates mechanical distortion of the work pieces;2.It can be rapid and highly automated;3.Energy input to the work piece is low,and therefore,the HAZHAZ size is minimal;4.Welds may be small in size and very precise;5.A large variety of metals and alloys may be joined using this technique;College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA3 Calculate the radius of an iridium atom giventhat Ir has an FCC crystal structure,a densityof 22.4 g/cm3,and an atomic weight of 192.2g/mol.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA4 For a bronze alloy,the stress at which plastic deformation begins is 275 MPa,and the modulus of elasticity is 115 GPa.(a)What is the maximum load that may be applied to a specimen with a cross-sectional area of 325mm2 without plastic deformation?(b)If the original specimen length is 115 mm,what is the maximum length to which it may be stretched without causing plastic deformation?College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA5 A cylindrical specimen of some alloy 8 mm in diameter is stressed elastically in tension.A force of 15,700 N produces a reduction in specimen diameter of 5 103 mm(2 104 in.).Compute Poissons ratio for this material if its modulus of elasticity is 140 GPa.