船舶焊接生产基础 (2).pdf
Mastering the internal structure of metal and its influence on metal properties is very important for selecting and processing metal materials lattice unit cell lattice constant Metallic Crystal Structures body-centered cubic lattice face-centered cubic lattice hexagonal close packed lattice body-centered cubic lattice body-centered cubic lattice BCC(BodyBCC(Body-Centered Centered Cube)Cube)lattice constant lattice constant：a=b=c,a=b=c,=9090 Number of atoms per Number of atoms per cell ncell n：n=n=1 1/8 88 8+1 1=2 2(个个)Body centered cubic lattice metalBody centered cubic lattice metal：-FeFe、CrCr、MoMo、W W、V V、NbNb face-centered cubic lattice FCC(FaceFCC(Face-Centered Centered Cube)Cube)lattice constant lattice constant：a=b=c,a=b=c,=9090 Number of atoms per Number of atoms per cell ncell n：n=n=1 1/8 88 8+1 1/2 26 6=4 4(个个)FaceFace-centered cubic lattice metalcentered cubic lattice metal：-FeFe、AlAl、CuCu、NiNi、AuAu、AgAg face-centered cubic lattice hexagonal close packed lattice HCP HCP(Hexagonal Close(Hexagonal Close-Packed)Packed)lattice constant lattice constant：c/a=c/a=1 1.633633,=120 120=9090 Number of atoms per cell nNumber of atoms per cell n：n=n=1 1/6 61212+1 1/2 22 2+3 3=6 6(个个)Densely packed hexagonal lattice metalDensely packed hexagonal lattice metal：MgMg、GeGe、ZnZn、BeBe hexagonal close packed lattice face-centered cubic lattice hexagonal close packed lattice body-centered cubic lattice The form of a metal in a solid state Some metals have two or more lattice forms in a solid state Most metals in a solid state have only one crystal structure When changing from one lattice to another,it causes changes in volume and compactness,resulting in deformation and cracking of the workpiece.Marine iron carbon alloy,aluminum alloy,etc.,undergo heating and cooling during welding,the lattice form will change during this process.Isomerism:The change of a metal from one lattice to another in a solid state with temperature.Isomers Isomers at room at room temperature are temperature are usually denoted by usually denoted by the Greek letter,the Greek letter,while isomers at while isomers at higher temperatures higher temperatures are denoted by,are denoted by,and.and.The allotropic transformation of a metal The actual crystal structure of a metal Polycrystalline:An entire metallic material is composed of many small crystals with different orientation.Grain each of the small,irregularly shaped crystals of a polycrystal is called a grain.Grain boundary:The interface between grains is called grain boundary.Polycrystalline isotropy.Pseudo isotropy Schematic diagram of polycrystalline structure of metal Microstructure of pure iron The incompleteness of the irregular arrangement of atoms in a crystal is called crystal defect.planar defect line defect point defect vacancy Point defect gap atom substitutional atom line defect edge dislocation grain boundary subboundary p l a n e d e f e c t Although pure metal has excellent electrical and thermal conductivity,its mechanical properties are poor and expensive,so it is greatly limited in use.Our widely used metallic materials are alloys,especially iron and carbon alloys alloy alloy Phase structure of alloy:solid solution metal compound Phase:A homogeneous part of an alloy having the same chemical composition and structure Liquid substance is called liquid phase.Two or more liquid metals are single phase.example 25 twotwo-phasephase water single phasesingle phase Liquid nickel oil Liquid Cu pure iron BiphasicorBiphasicor polyphasicpolyphasic steel twotwo-phasephase single phasesingle phase single phasesingle phase 1）Tm theoretical crystallization temperature 2）Tn Actual crystallization temperature 3）degree of supercooling TTm-Tn 4）The greater the cooling rate during crystallization,the greater the degree of supercooling Reasons for horizontal line segment:The The latent heat of crystallization latent heat of crystallization compensates for the heat lost to the compensates for the heat lost to the outside worldoutside world Cooling Curve of Pure Metals Cooling curve and degree of undercooling The crystallization of metals is a process of continuous formation and growth of crystal nuclei,which is completed on a horizontal line in the cooling curve.crystal nucleus Crystallization Process of Pure Metals 01 Increase the degree of supercooling 02 modification 03 extraneous vibration Grain size and control 01 Increase the degree of supercooling In practical production,it is usually In practical production,it is usually used to reduce the casting temperature used to reduce the casting temperature or use metal casting with good or use metal casting with good conductivity to improve the cooling conductivity to improve the cooling rate and increase the degree of rate and increase the degree of supercoolingsupercooling.Grain size and control The relation between nucleation rate N and growth rate G and undercooling degree T In liquid metal crystallization In liquid metal crystallization before adding some small before adding some small modifier,such as aluminum or modifier,such as aluminum or aluminum alloy,can add trace aluminum alloy,can add trace titanium or titanium salt to titanium or titanium salt to increase the nucleation rate,increase the nucleation rate,growth rate is reduced.growth rate is reduced.Grain size and control 02 03 Mechanical vibration,ultrasonic Mechanical vibration,ultrasonic wave,electromagnetic wave wave,electromagnetic wave vibration can be used to break vibration can be used to break the crystal branches,indirectly the crystal branches,indirectly increase the nucleation core,and increase the nucleation core,and also can play a role in grain also can play a role in grain refinementrefinement Grain size and control ThanksThanks forfor watchingwatching
