Manufacturing Process

Introduction

Optimization - 시험

• Define manufacturing and describe the technical and economic consideration : A kind of optimization
• Relationship among product design(functionality & costs) and engineering(processes) and factors(materials)
• Important trends in modern manufacturing and minimizing of production costs(optimization)

Manufacturing(production)

• The process of converting raw materials into products
  • Design of desires
  • Realization of goods
  • Through various production methods
• Raw material became a useful product : Maximizing Added value

Product Design & Concurrent Engineering

• Product design(제품 설계)
  • 70~80%의 개발 비용
  • 제품에 대한 기능 및 성능의 이해 필요
  • CAD, CAM, CIM을 통해 설계와 제작이 순차적으로 이루어짐
  • 하지만 현대는 즉각적으로 이뤄짐 : 동시 공학
• Concurrent engineering(동시 공학)
  • 설계 및 제조를 통합하는 즉각적이고 체계적인 접근
  • 제품의 수명주기와 관련된 모든 요소 최적화

Design for manufacture, assembly

시험

• Design For Manufacture(DFM)
  • 제품 설계 공정을 재료와 통합
  • Manufacturing methods
  • Process planning
  • Assembly, testing, and quality assurance
• Design For Assembly(DFA)
  • 전반적인 제조 운영
  • 특히, 주요 부품 간의 조립 공차 관리
• Design principle
  • Simple design
  • Appropriate material
  • Permissible accuracy
  • Optimized manufacturing processes considering cost down

Enviromental issues

• Design For Recycling(DFR) & Design For Enviroment(DFE)
  • 버려지는 재료 감소
  • 위험한 재료 사용의 감소
  • 모든 폐기물의 적절한 처분
  • 폐기물 처리 및 재활용의 개선
• Product Life Cycle(PLC) - 시험
  • 설계, 개발, 제작, 판매, 사용, 처분, 재활용 모두 포함 - 설계자가 고려
• Product Life Cycle Management(PLCM) - 시험
  • 설계, 개발, 제작, 판매, 사용, 처분, 재활용 모두 고려한 제작 전략 - 경영자가 고려

Manufacturing processes

• Casting(주조)
• Forging(단조)
• Extrusion(압축)
• Cutting(절삭)
• Welding(용접)

Traditional manufacturing processes

• Casting(주조)
• Forming(성형) : Forging and Extrusion
• Machining : Turning and Drilling and Milling

Advanced manufacturing processes

• Laser beam cutting
• MEMS(Micro Electro Mechanical Systems)

CIM(Computer Integrated Manufacturing)

• Computer Numerical Control
• Adaptive Control
• Computer-Aided Process Planning
• Group Technology
• Just-In-Time production : Inventory management

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Fundamentals of the Mechanical Behavior of Materials

Review of Eng. Materials

• Deformation($\delta$) : 변형
• Displacement : 변위
• Force($F$)
• Stress($\sigma$)
• Strain($\epsilon$)

Thermal deformation - 시험

$$
E=\frac{\sigma}{\epsilon}
$$

Tension test

아래 그림 시험

• 강도-변형의 특성 확인
• 실험적인(경험적인) Data
• load cell로 힘 측정
• Specimen(시편) 사용 - Dog bone
• Instron gauge

시험

• E는 탄성구간의 기울기
• E가 크면 동일한 힘에 대해서 변형율 작음
• E가 작으면 동일한 힘에 대해서 변형율 큼

$$
Poisson’s\ ratio\ :\ \nu=-\frac{\epsilon(lateral)}{\epsilon(longitudinal)}=-\frac{\epsilon_y}{\epsilon_z}
$$

• 분모는 힘이 가해지는 방향(길이 방향)
• $\nu<0.5$ : Elastic(탄성)
• $\nu=0.5$ : Plastic(소성)
• 소성을 주로 다룸

Ductility(연성)

How large a strain the material withstands before fracture

• Ductile material
  • Most metals
  • Elastic and plastic region
  • Same performance in tensile / compressive force
  • Deformation due to tension(normal) and torsion(shear)
• Brittle(취성) material
  • Glass, chalk, etc
  • Elastic region(without plastic region)
  • Good performance in compressive force comparing with tensile force

Effect of temperature in manufacturing process

Temperature : Major parameter in Manufacturing

• 온도가 오른다면
  • Elastic(Young’s) modulus - Decreasing
  • Tensile strength(UTS) - Decreasing
  • Yield strength - Decreasing
  • Elongation(연신율), Ductility(연성) - Increasing
  • 하지만 연신율이 약 60퍼 이상의 온도일 경우 보통 액체 상태로 존재

Bauschinger effect

• Tension and then compression or vice versa; Yield stress decrease
• Strain softening(변형연화)
• Work softening(가공연화)

Fatigue & Creep

• Fatigue(피로)
  • S-N curve(Stress - Number of cycles)
  • Cycle or Periodic stress
    • Fluctuating mechanical loads
    • Thermal stresses
  • Aluminum better regarding vibration/chattering
• Creep
  • Permanent elogation of material
  • Static load maintained for a period of time
  • High temperature application(Ex. Turbine blade)
  • A kind of inertia effect

Residual stresses

• 잔류응력
• Remaining stresses after material deformation
• Critical reason of an instability of dimensions and shapes
• Annealing process(온도를 천천히 올리고 내림, 풀림 공정) for stress-relief

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Structure and Manufacturing Properties of Metals

The crystal structure of Metals

• Structure greatly influences their properties and behavior
• Normal casting(a), Directionally solidified(b), Single crystal(c)
• Single crystal(단결정) : 고온에서 강도 유지
• 밀도가 높을수록 강도가 높지만 연성이 낮아짐
• 밀도가 낮을수록 강도가 낮지만 연성이 높아짐

Deformation and Strength of Single Crystals

Two basic mechanisms of plastic deformation(소성영역은 Slip과 Twinning이 발생하는 영역)

• (a) Slip : One plane of atoms slips over an adjacent plane under shear stress
  • 임계전단응력 : 소재에 영구변형을 발생시키는 전단응력
• (b) Twinning : Portion of the crystal forms a mirror image of itself across the plane of twinning

Mechanical fibering

시험

• 기계적 섬유화 : 특정한 방향으로 강도나 연성이 낮아져 발생하는 현상
• Anisotropy(이방성) (<-> 등방성 : 균일한 성질)
• Preferred orientation(선택적 방향성) : strength, hardness, etc.
• Example : Plywood - strong in planar direction but weak in thinkness direction

• Rolling
  • 소재의 강도를 높임(밀도 증가)
  • 얇은 두께의 고강도 철판 제작
  • 기계적 섬유화 발생

Plastic deformation due to shear stress

• Slip under critical shear stress
• Generation of Slipband(소성변형) in single crystal

Correlation between temperature and mechanical properties

시험

• In plastic deformation : Temp & Time(소성변형의 주요한 두 제어변수)
  • Recovery(< recrystallization temp.)
    • 기계적 성질은 큰 변화 없음
    • 연성 약간 증가
  • Recrystallization(= 0.3 ~ 0.5$T_m$)
    • $T_m$은 Melting temp.
    • 기계적 성질 변화
    • 새로운 결정 생성
    • 강도 감소
    • 경도 감소
    • 연성 증가
    • 소재내의 저장된 에너지가 많으면 recrystallization temp. 감소
  • Grain growth(> recrystallization temp.)
    • Orange-peel effect - Grain이 커지므로 표면이 거칠어지는 현상
    • Grain size가 커지면 등방성이 아닌 이방성에 가까워지며 일반적으로 기계적 성질에 좋지 않음
• High temperature and cooling slowly
  • Decrease : Residual stresses, Strength, Hardness
  • Increase : Ductility, Grain growth

Cold, Warm, and Hot working

• Homologous temperature(상사온도) : $T/T_m$
  • $T_m$ : 용융온도
• Plastic deformation : Hot working
  • Less dimensional accuracy due to thermal expansion
  • Rough surface due to oxide layer
  • 작은 힘으로 성형할 수 있음
• 온도변화가 잦으면 산화층에 의해 금속표면 부식

Process	$T/T_m$
Cold working(냉간가공)	< 0.3
Warm working(온간가공)	0.3 to 0.5
Hot working(열간가공)	> 0.5

Annealing process for Metals

시험

• 풀림 공정
• Totally structure and characteristic are changed
• Control porperties of metals
  • 탄성계수(크면 덜 변형, 작으면 잘 변형)
  • 포아송비
  • 강도
  • 경도
  • 강성
  • 연성
  • 인성

• 일반적으로 온도 증가 :
  • Increasing of ductility(연성 증가)
  • Decreasing of tensile strength(강성 감소)

Transition temperature

• 천이 온도
• Sharp change in ductility and toughness across a narrow temperature(소재의 특성이 아주 급격하게 변하는 온도)
• Transition temperature is increased
  • High rate(fast deformation)
  • Abrupt change in shape
  • Surface notch - 결함(표면 가공 필요)

Physical properties

• 경도(Hardness)
  • 물체의 단단한 정도
• 강도(Strength)
  • 끊어지지 않고 버티는 정도
• 인성(Toughness)
  • 소재나 재료가 지닌 점성의 강도
• 소성(Plastic)
  • 재료에 외력을 가하면 원형으로 복귀되지 않는 성질
• 탄성(Elastic)
  • 재료에 외력을 가하면 원형으로 복귀되는 성질
• 취성(Brittle)
  • 재료에 외력을 가하면 변형되지 않고 부서지는 성질
• 연성(Ductility)
  • 재료를 늘일 때 파괴되지 않고 계속 늘어나는 성질
• Example - 시험
  • 유리 - 경도는 높고 강도는 낮음
  • 나무 - 경도는 낮고 강도는 높음
  • 꿀 - 인성(점도)이 높음 : 충격에 가해졌을 때 파단이 아닌 휘어짐
    • 물의 점도는 1
• Density(밀도)
  • Density depends on weight, radius and packing of the atoms
  • Lower density is important for air craft or aerospace structure, automobiles and high speed equipment(minimizing inertia effect)
• Melting point(용융점)
  • Depends on the energy required to separate all atoms
  • Tool wear in machine tools(frictional heat)
  • Apparently important in casting process
• Specific heat(비열)
  • Specific heat is the energy required to raise the temperature of a unit mass(단위질량) of a material by 1’C
• Thermal conductivity(열전도도)
  • The rate at which heat flows within and through the material
• Thermal stress(열응력)
  • Thermal deformation(열변형)
  • Shrink fit(열박음, using thermal expansion phenomenon)
• Thermal fatigue(열피로)
  • Fatigue - 주기적인, 반복적인 무언가
  • Results from thermal cycling repetitive heating and cooling
  • Particularly important in Forging / Cutting process
• Thermal shock(열충격)
  • Development of cracks after a single thermal cycle
• Superconductivity(초전도성)
  • Zero resistivity below critical temperature
• Piezoelectric effect(압전효과)
  • Reversible interaction between an elastic strain and an electric field used in making transducers(mechanical strain <-> electric current)

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Surface, Tribology, Dimensional characteristics, Inspection, and Product quality assurance

Objectives of this chapter

• Important considerations
  • Surface structure(표면 구조), texture(표면 조직), and surface properties(표면 특성)
  • Friction(마찰), Wear(마멸), and lubrication(윤활) - tribology(윤활공학)
  • Surface treatment(표면 가공)
  • Inspection methods(destructive(파괴검사) / nondestructive(비파괴검사))
  • Statistical techniques for quality assurance of products

Introduction

• Surface directly influences several important properties:
  • Friction(마찰) and wear(마멸) properties
  • Effectiveness of lubrication(윤활)
  • Appearance and geometric features
  • Initiation of cracks due to surface defect
  • Thermal and electrical conductivity of contacting bodies
• Tribology : Surface phenomena of friction, wear and lubrication
• Surface treatment(표면 처리) : Method to improve surface properties (mechanical, thermal, electrical, chemical, etc.)

Surface structure of metals

• Depending on the composition and processing history
• Oxide layer(산화층) : Harder than the base metal(brittle & abrasive)
• Beilby(amorphous) layer : Melting, surface flow and rapid quenching(급속 냉각 담금질)
• Work-hardened layer(가공-경화층) : Processing and extent of frictional sliding(잔류응력 존재)

Surface texture

• Waviness(파상도) : Low frequency - recurrent deviation from a flat surface
  • Deflection of tools, dies, and of the workpiece
  • Warping from forces or temperature
  • Uneven lubrication
  • Periodic vibration(mechanical & thermal)
• Surface roughness(표면 거칠기) : High frequency irregular deviation
  • Arithmetic mean value : $R_a$
  • Root-mean-square : $R_{rms}$
  • Maximum roughness height : $R_t$(peak-valley)

Tribology : Friction, Wear, and Lubrication

Tribology : Science and technology of interacting surfaces

• Friction : Resistance to relative sliding between two contacting bodies under normal load(thermal deformation can be induced)
  • Coefficient of friction(friction force = uN)
• Wear : Progressive loss or undesired removal of material from a surface
  • Surface damage(loss)
  • Reducing surface roughness(benefit)
• Adhesive wear(응착 마멸) : Shearing of the junctions takes takes at the original interface between two bodies under tangential force
  • Scuffing defects
  • Smearing defects
  • Tearing defects
  • Galling defects
• Lubrication : Interface between tools, dies, molds, and workpieces is subjected to a wide range of variables
  • Contact pressure(elastic to plastic deformation)
  • Speed
  • Temperature
  • Friction and wear will be increased under high pressure, high speed, and high temperature : Minimization of friction, wear with lubrication
• Functions of metal working fluids(금속 가공용 윤활제)
  • Reduce friction : Reduction of force or energy requirement(production cost)
  • Reduce wear
  • Improve material flow
  • Act as thermal barrier : Prevention of cool-down of workpiece in hot working process

Surface treatment

• Improve resistance to wear, corrosion, oxidation, and indentation
• Control friction
• Reduce adhension(응착)
• Improve lubrication
• Improve fatigue resistance
• Rebuild surfaces on components
• Improve surface roughness

Dimensional tolerances(치수 공차)

• Tolerance is the acceptable variation in the dimensions of a part
  • height
  • width
  • depth
  • diameter
  • angles
  • etc.
• Tolerance is unavoidable and important when parts are to be assembled

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Metal-Casting Processes and Equipment; Heat Treatment

냉각 속도 중요 - 금속의 성질이 다르지 않게

Important Factor(주요 인자)

• Solidification(응고)
  • 치수의 변화 주의
• Flow of molten metal into the mold cavity(용탕의 주형으로의 유입)
  • 용탕이 얼마나 잘 내부 공간을 채우는지 - 사형 가열로 해결
• 주형(Mold)에서 금속의 응고 및 냉각 시 발생하는 열전달(Heat transfer) 정도
• Mold material

Casting examples

• Typical gray cast iron castings
  • Transmission valve body, hub rotor with disk-brake cylinder in automobile

Solidification of Metals

• Pure metals have defined melting points and solidification takes place at a constant temperature
• When temperature is reduced to the freezing point, latent heat(잠열) of fusion is given off
  • A -> B
• Alloys(합금) solidify over a range of temperatures
  • Solidification begins below liquidus and is completed at solidus
  • Two phases are co-exicst between solidus and liquidus

Cast Structure

• Solid solution(고용체) - 시험
  • Solute(용질, Minor) is added to the solvent(용매, Major) to form a solution
• Cast structure developed during solidification of metals and alloys depends on
  • Composition rate
  • Heat transfer rate
  • Flow of the liquid metal
• Pure metals
  • Chill zone(칠영역)
  • Columnar zone(주상정영역)
• Alloys
  • Chill zone(칠영역)
  • Columnar zone(주상정영역)
  • Equiaxed zone

Alloy

시험

• Solidification begins when temperature drops below the liquidus($T_L$), and is completed at solidus($T_S$)
• Freezing rage(응고범위) : $T_L-T_S$
  • Determines with of murshy zone
• Columnar dendrite(주상수지상정)
• Mushy zone
  • 고상과 액상이 공존하는 영역
  • 합금은 응고범위가 존재하기 때문에 존재
  • 응고범위가 작으면 Mushy zone 작음
  • 응고범위가 크면 Mushy zone 큼
• 냉각 속도 빠르면
  • 조직이 미세해짐
  • 가지사이의 간격이 좁아짐 - 잘 휘지 않음
  • 강도 증가
• 냉각 속도 느리면
  • 조직이 커짐(조대입자)
  • 가지사이의 간격이 넓어짐
  • 연성 증가

Fluid Flow and Heat Transfer

• Bernoulli’s theorem : Conservation of energy in a fluid system
  • $h_1+\frac{p_1}{\rho g}+\frac{v_1^2}{2g}=h_2+\frac{p_2}{\rho g}+\frac{v_2^2}{2g}+f$
• Law of mass continuity : The rate of flow is constant for an incompressible liquid
  • $Q=A_1v_1=A_2v_2$

Flow chracteristics

• Reynolds number($R_e$)
  • Prescence of turbulence is an important consideration in fluid flow
  • Reynolds number, $R_e$, is used to characterize this aspect
  • $R_e<2000$ : Laminar flow
  • $2000<R_e<20000$ : Mixture of laminar and turbulence
  • $R_e>20000$ : Severe turbulence
    • Erosion of mold
    • Air entrainment
    • Dross formation(Undesirable aspects in casting system)

$$
R_e=\frac{intertia}{friction}=\frac{vD\rho}{\eta}
$$

• $v=velocity\ of\ the\ liquid$
• $D=diameter\ of\ the\ channel$
• $\rho=density$
• $\eta=dynamic\ viscosity\ of\ the\ liquid(Ns/m^2)$

Fluidity of Molten Metal

• Fluidity : The ability of molten metal to fill the mold cavities
  • Characteristics of molten metal
  • Casting parameters
• Characteristics of molten metal
  • Viscosity(점도)
  • Surface tension(표면장력)
  • Inclusions(개재물)
  • Solidification pattern of the alloy : Inversely proportional to the freezing range

Casting parameters

• Mold design(주형설계)
• Mold material and its surface characteristics : The higher thermal conductivity of the mold and the rougher its surface, the lower the fluidity
• Superheat improves fluidity
• Rate of pouring : The lower it is, the lower fluidity because of high cooling rate
• Heat transfer
• Regarding metal shirink(Dimensional changes and cracking) during solidification and cooling

Casting Processes

• Ingot casting
  • Ingot : 초기의 후속 가공을 위해 만들어지는 금속 부품
• Continusous casting
• Sand casting(사형주조) : Expendable mold(소모성 주형)
  • Sand mold
    • Inexpensive and resistance to high temperatures
    • Silica($SiO_2$) sand is used
    • Sand with fine and round grain is closely packed and forms a smooth mold surface(High strength and low permeability)
    • A good permeability allows gases and steam evolved during casting to escape easily
  • Pattern
    • Shape of the casting
    • Material selection depends on size and shape, dimensional accuracy, accuracy, quantity of casting
    • Draft angle 고려
  • 미세한 주물사를 사용한 주형을 통해 만들어진 금속
    • 주형 강도 증가(결합 면적 증가) - 형태 잘 유지(좋은 표면)
    • 통기도 감소(가스 배출 어려움) - 금속 내부 공간(균일하지 않은 강도)
• Shell-mold casting : Expendable mold(소모성 주형)
  • Close dimensional tolerances and good surface finish at low cost
  • Light and thin(usually 5~10mm) : Thin shell allow gases to escape during solidification of the metal
  • Smooth wall of the mold wall : Low resistance to molten metal flow - shaper corner, thinner sections, and smaller projections
• Precision(정밀) casting : Expendable mold(소모성 주형)
  • Plaster-mold casting
    • Precision casting method : High dimensional accuracy, good surface finish
    • Low thermal conductivity(Slow cooling) : Uniform grain structure, Less warpage, Better mechanical properties
  • Ceramic-mold casting
    • Somewhat expensive
    • Good dimensional accuracy and surface finish
    • Wide range of size, intricate shapes
    • Suitable for high-temperature application(Stainless steel, tool steels, etc.)
• Investment casting : Expendable mold(소모성 주형)
  • Labor and materials are costly but no finishing is required
  • Suitable for casting high-melting-point alloys with good surface finish and close dimensional tolerances
    • Mecahnical components : Gears, Cams, Valves, ratchet, etc.
• Permanent mold casting processes
  • Molds are used repeatedly and are designed so that the casting can easily be removed
  • Metal molds
    • Better heat conductor - High rate of cooling - Microstructure and grain size
    • Maintain strength under high temperature
    • Expensive
  • Controlling of the rate of cooling speed with graphite(흑연) / refractory(내화액)
    • 코팅 두께로 조절
  • Good mechanical properties and surface finish
• Die casting : Permanent mold
  • Molten metal is forced into the die cavity at high pressures(~ 700MPa)
  • The machines are large comparing with the size of the product
  • Dies are cooled by circulating water or oil to improve die life and rapid cooling : Maximizing productivity
  • Mass production thorough automation
• Centrifugal / Squeeze casting : Permanent mold
  • Centrifugal casting(원심주조법)
    • Force the molten metal into the mold cavities by the inertial force(Rotation)
    • Good dimensional tolerance and surface
    • By centrifugal force : Inner surface of the casting remains cylindrical, Lighter elements(Dross, Impurities) on the inner surface
  • Squeeze casting
    • Combination of forging and casting
    • The higher cooling rate results in fine microstructure and good mechanical properties
    • Minimization of microporosity

표 5.8 - 시험

Post processes

• Annealing
  • Reduce hardness and strength
  • Modify its microstructure
  • Relieve resiual stress
  • Improve dimensional stability and machinability
  • Annealing sequence
    • Heating the workpiece to a specific range of temperature
    • Holding it at that temperature for a period of time
    • Cooling it slowly(Minimization of surface oxidation)
• Tempering
  • Increase ductility and toughness
  • Reduce residual stress and brittleness
  • Heating a specific temperature and cooled at a prescribed rate

Defects in castings

Seven basic categories

• Metallic projections(금속돌출)
  • Ex) Rough surface or massive projection such as swell
• Cavities(기공)
  • Ex) Blow hold, Pin hold
• Discontinuities
  • Ex) Crack, Tearing, Coldshut
• Defective surface
  • Ex) Surface fold, Laps, Scars
• Incomplete casting
  • Ex) Misruns(Premature solidification), Runout(Due to loss of metal after pouring)
• Incorrect dimensions of shape
  • Ex) Improper shrinkage allowance, Uneven contraction
• Inclusion(개재물)
  • Stress raisers and reduce the strength of the casting, break tools during machining operation

Porosity(기공)

• Detrimental to the ductility and surface finish
• Develop when the liquid metal solidifies and shrinks between dendrites
• Microporosity is from gases expelling
• Macroporosity(Shrinkage cavities) is from shirinkage
• Chills(냉각쇠) are used in castings to eliminate macroporosity caused by shrinkage
• Porosity due to gases(Microporosity)
  • Spherical and smooth walls(Normally)
• Porosity due to shrinkage(Macroporosity)
  • Rough and angular(Normally)

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Bulk Deformation Processes

• Metal forming process
  • Bulk deformation(팽창 변형) : Involves the plastic deformation of materials under various force / power conditions
    • Forging(단조)
    • Rolling(압연)
    • Extrusion(압출)
    • Drawing(인발)
    • Swaging
  • Sheet-metal forming

Forging(단조)

강성 증가

• Forging is the manufactruing process where plastic deformation of material takes place by compressive forces
• Cold or hot working : can be carried out at room or at elevated temperature
• Open-die forging(자유 단조)
  • Involves placing a solid cylindrical workpiece between two flat dies and reducing its height by compressive force(it’s called “upsetting”)
  • Die : may be flat or have cavities of various shape
  • Barreling(배부름) - 시험
    • Caused by frictional forces at the die-workpiece interfaces and upsetting of hot workpieces between cool dies
    • Non-uniform deformation
    • Minimized by lubricant or ultrasonic vibration of the platens
• Impression-die(Closed-die forging)
  • 형 단조
  • Workpiece acquires the shape of the die cavity while deformed between the closing dies
  • Flash : high frction encourages the filling of the die cavities - 시험
  • Qualities depending on operational performance and control
• Precision forging(정밀 단조) : Near net shape with Aluminum, Magnesium
• Closed-die forging(폐쇄 단조) : No flash, completely filled
• Isothermal forging(등온 단조) : Hot die forging, same Temp. workpiece-die, expensive
• Incremental forging(점진 단조) : Several small steps(Incremental), low force-low noise
• Coining : Closed-die forging coin / medal, no lubricant, improving qualities
• Heading : Upsetting operation
• Cogging : Drawing out, thickness is reduced by sucessive steps
• Roll forging(압연 단조) : Passing workpiece through set of grooved rolls

Forging defects

시험

• Due to improper the material flow patterns in the die cavity
• Buckling defect(겹침 결함) : The excess material in the web
• Internal crack(내부 크랙) : Due to oversized billet
• Effect of radius
  • Die radius(다이의 모서리 형상) significantly affect formation of forging defects
  • Material flows better around large radius
  • Cold shut : Material can fold over itself with smaller radius
  • These defects can lead to fatigue failure during the service life of the forged component
  • 설계변수 : 다이의 모서리 형상

Forgeability

Capability to be shaped without cracking and requiring low forces

• Upsetting test
  • Upset of cylindrical specimen
    • Measure reduction in height prior to cracking
  • The higher reduction, the greater the forgeability of the metal
  • As the friction increases, the specimen cracks at a lower reduction in height
• Hot-twist test
  • A torsion test of a long and round specimen
    • Twisted continuously until it fails
  • Performed at various temperatures and the number of turns in observed
  • Useful for determining the forgeability of steel

단조성이 높다 : 균열 없이 크게 변형, 동일한 변형량을 유지할 때 소요되는 하중 작음

Die design parameters

General considerations for die design

• Parting line : Two dies meet
• Flash : 3% of the maximum thickness of the forging
• Draft angles : Removal of the part from the die(Internal > External)
• Die radius for conrners and fillets
  • Smooth flow of the metal in the die cavity
  • Improvement of die life
  • Small radius : Stress concentration
  • Small radius in fillets : Fatigue cracking
• Land의 길이는 Falsh 두께의 5배

Rolling(압연)

• Process of reducing the thickness of long workpiece by compressive forces applied through a set of rolls
• Good strength and ductility : Reduce grain size and refine the microstructure - 시험
• 90% of all metals produced by metalworking processes

Mechanics of Flat Rolling

• For constant volume rate of metal, velocity of strip must increase as it moves through roll gap
• No slip point : The two velocities (Strip / Roll) are the same

• Roll forces in hot rolling
  • Two difficulties in calculation of forces and torque
    • Proper estimation of the coefficient of friction at elevated temperature(Generally 0.2 ~ 0.7)
    • Strain-rate sensitivity of materials at elevated temperature
• Roll deflections and roll flateening
  • Roll forces will bend the rolls and results in a strip that is thicker at its center than at its edges(Crown)
  • Flattening of roll with camber
    • Camber : Curvature in diameter variation, typically less than 0.5mm on the roll diameter - 시험
• Spreading
  • Rolling plates and sheets with high strip width-to-thickness ratios is essentially a process of plane strain
  • Width increases during rolling, known as spreading
  • Spreading decrease with
    • Increasing width-to-thickness ratios of the strip
    • Increasing of friction
    • Increasing ratios of roll radius to strip thickness

Defects in Rolling

• Structural defects(구조적 결함)
  • Wavy edges(파도형 결함)
    • Roll의 손상
  • Zipper cracks in the center of strip(중앙부 터짐)
    • Ingot 내부의 기공
    • 연성 부족
    • 과한 Camber
  • Edge cracks(측면 터짐)
    • 연성 부족
    • 적은 Camber
  • Alligatoring
    • 넓은 기공
• Residual stresses
  • Due to inhomogeneous plastic deformation
    • Small rolls / Small reduction in thickness
      • 소재의 표면만 소성변형
      • 표면 : 압축잔류응력
      • 중앙부 : 인장잔류응력
    • Large rolls / Large reduction in thickness
      • 소재의 내부 위주의 소성변형
      • 표면 : 인장잔류응력
      • 중앙부 : 압축잔류응력

Roll arrangement

Miscellaneous rolling operations

• Shape rolling
• Ring rolling
• Thread rolling
  • Cold-froming process
  • Good strength
  • Requested sufficient ductility
  • Requested good lubrication

Extrusion(압출)

• Cold and hot working process
• Four basic types of extrusion
  • Direct extrusion(직접, 전방 압출)
  • Indirect extrusion(간접, 후방 압출)
  • Hydrostatic extrusion(정수압 압출)
  • Impact extrusion

Metal flow in extrusion

• Main factors in metal flow
  • Friction at billet-container and billet-die interface
  • Thermal gradients within the billet
    • The most homogeneous flow pattern under no friction at the interfaces
    • Dead-metal zone develops under high friction
    • High-shear zone extends farther back(“Pipe defect” - 내부에 공간 존재)

Cold extrusion

• Combination of processes particularly extrusion combined with forging
• Cold extrusion advantages
  • Improved mechanical properties(Minimize thermal defects)
  • Good control of dimensional tolerances
  • Improved surface finish
  • High production rates and low cost

Defects in extrusion

• Surface cracking(표면균열)
  • High extrusion temperature under high speed and friction
  • Cracks are inter-granular
  • Cracks are the results of hot shortness(적열취성)
  • Minimization of surface cracking by using lower temperature and low speed
• Extrusion defect(= Pipe defect = Tailpipe = Fishtailing)
  • Type of metal flow will draw surface oxides and impurities toward the center of billet
  • Minimization of pipe defect by modifiying the flow pattern, machining the billet surface
• Internal cracking(= Chevron cracking)
  • Develop at the center of an extruded product
  • Major factors are die angle, extrusion ratio, friction

Drawing(인발)

• A bar or tube is reduced or changed in shape by pulling through a converging die under tension(Extrusion is carried out with compressive force)
• Rod and wire drawings are finishing processes and are further processed into other shapes

Defects in drawing

• Internal defects increases with increasing
  • Die angle
  • Friction
  • The presence of inclusion in the material
• Seam defect(솔기결함)
  • A type of surface defect in only drawing
  • Longitudinal scratches or folds in the material
  • Major reason of alligatoring defect

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Sheet-Metal Forming Process

• Sheet metal forming involves the workpiece with high ratio of surface area to thickness
  • Plate(후판) : Thicker than 6mm
  • Sheet(박판) : Less than 6mm
• Sheet metal is produced by rolling process
  • If thin : Coiled after rolling(Wound roll form)
  • If thick : Available as sheet or plate
• Forming of sheet metals is carried out by tensile forces in the plane of the sheet
  • Compressive force -> Buckling, Folding and Wrinkling of the sheet
  • Thickness change due only to Poisson’s effect unlike bulk deformation

Shearing(판재의 전단작업)

• Cutting sheet metal, plates, bars, and tubing into pieces using punch and die
• Workpiece is subjected to shear stress
• Important variables are punch force, speed, edge condition of the sheet, materials, corner radii of punch and die, punch-die clearance(간극) and lubrication
• As clearance increases, edges become rougher and deformation zone is larger : Generally 2-8% of sheet thickness, and 1% of it for fine blanking
• The ratio of the burnished-to-rough area on the sheared edge increases with increasing ductility of the sheet metal and decreases with increasing sheet thickness and clearance
• As clearance increases, the material tends to be pulled into the die, rather than being sheared
• Burr height increases with increasing clearance and increasing ductility of the metal

Die Cutting

• Punching : The sheared slug is discarded
• Blanking : The slug is the part itself, and the rest is scrap
• Parts produced have various uses
  • Perforating : Punching holes
  • Parting : Shear into two or more pieces
  • Notching : Remove the material at edge
  • Slitting
  • Lancing : Leave a tap without removing material

Fine Blanking

• Very smooth and square edges
• V-shaped impingement(Stinger) grabs the sheet tightly in place
• Clearances on the order of 1% of the sheet thickness with sheet thickness of 0.5 ~ 13mm(8% in ordinary shearing operation)
• Suitable sheet hardness : 50 ~ 90HRB

Slitting

• Carried out with a pair of circular blades
• Two types of slitting equipment
  • Driven type : The blades are powered
  • Pull-through type : The strip is pulled through idling blades

Shearing Die

• Qualities of a sheared part can be directly influenced by clearance
• The smaller the clearance, the better is the quality of the sheared edge
• Shaving : Extra material from a rough sheared edge is trimmed by cutting
  • Part requiring multiple operations such as punching, bending, and blanking are made at high production rate using progressive dies

Bending of Sheet and Plate(판재의 굽힘작업)

• Bending : One of the most common metalworking operations
• Bending force is a function of the material’s strength, legnth and thickness of the part, and the width of the die opening
• Bend allowance($L_b$, 굽힘허용부) : The length of the neutral axis in the bend area and is used to determine the blank length for a bent part
• Bend radius($R$, 굽힘반경) is measured to the inner surface of the band
• Length of bend($L$, 굽힘길이) is the width of the sheet
• Minimum band radius : Crack appears on the outer surface of the band

Bendability

Factors affecting bendability

• Bendability increases by increasing its tensile reduction of area
• As length($L$) increases, minimum bend radius increases(under 10t)
• Bendability decreases as edge roughness increases
• Cold rolling direction : Anisotropy because of the alignment of impurities, inclusions, and voids(It is called “mechanical fibering”)

Springback

• In bending, elastic recovery is called springback
  • Plastic deformation is always followed by elastic recovery upon removal of the load
• A quantity characterizing springback is the springback factor $K_s$(ex : $K_s=1$ is no springback and $K_s=0$ is full springback)

Compensation for Springback

• Springback is usually compensated by using
  • Overbending
  • Coining : High localized compressive forces between the tip of the punch and the die surface
  • Strength bending : Applying tension while being bent
  • Bending in high temperature : Springback decreases as yield stress decreases(Elastic region decreases)

Common bending operations

• Bending : The edge of the sheet is bent into the cavity of a die
• Flanging : Bending the edges of sheet metal, typically to 90 degree
• Dimpling : Hole is first punched and then expanded into a flange
• Hemming : Edge of the sheet is folded over
• Roll forming : Bending continuous lengths of sheet metal
• Tube bending
  • Method is to pack the inside with loose particles(Typically sand)
  • Prevent the tube from buckling inward

Miscellaneous Forming Processes

• Stretch forming
  • Sheet metal is clamped around it edges and stretched over a die or form block
  • Aircraft - wing skin panels, automobile door panels
• Bulging
  • Expanding it with a rubber or polyurethane
  • Embossing

Spinning

Forming of axisymmetric parts over a rotating mandrel, using ride tools or rollers

• Conbentional spinning
  • Circular blank of flat or preformed sheet metal is held against the rotating mandrel
  • Suitable for conical or curvilinear shape
• Shear spinning
  • Basically the same as conventional spinning except for that the diameter keeps constant
• Tube spinning
  • Tubes or pipes are reduced in thickness by spinning them on a cylindrical mandrel using rollers
  • External or internal spinning

Peen forming

• Produce curvatures on thin sheet metals by shot peening
• Induced compressive surface residual stresses, thus improving the fatigue strength of the sheet

Deep Drawing

Flat sheet-metal blank is formed into a cylindrical or box-shaped part by means of a punch that presses the blank into the die cavity

• Variables in deep drawing
  • Properties of the sheet metal
  • Ratio of the blank diameter to the punch diameter
  • Sheet thickness
  • Clearance between the punch and the die
  • Corner radius of the punch and die
  • Blankholder force
  • Speed of the punch
  • Friction at the punch, die and workpiece interfaces

Ironing

• Thickness has to be reduced by a defromation when the thickness of the sheet as it enters the die cavity is more than the clearance between the punch and the die : It is called Ironing
  • Producing a cup with constant wall thickness
  • Correcting earing defect

Deep Drawing Practice

• Clearances and radius
  • Clearances are 7 - 14% greater than the original thickness of the sheet
  • Ironing increases as clearance decreases
  • Radius is too small -> Fracture
  • Radius is too large -> Wrinkle
• Draw beads
  • Draw bead diameters may range from 13 to 20mm
  • Draw bead help in reducing the blank holder forces
• Blankholder pressure
  • 0.7 to 1.0% of the sum of the yield and the UTS of the sheet metal
  • Too high pressure -> Tearing of the cup wall
  • Too low pressure -> Wrinkle in the flange
• Equipment
  • Punch speed : 0.1 ~ 0.3m/s(Punch speed in not important in drawability)

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Material-Removal Process : Cutting

Machining(Cutting) is a general term to describe material removal on a workpiece and modification of its surfaces

Machining processes in manufacturing

• Advantage of machining
  • Closer dimensional accuracy
  • External and internal geometric features
  • Special surface characteristics or textures
  • Economical when the number of final product is low
• Disadvantage of machining
  • Waste material
  • Need more time to make something

Mechanics of Chip Formation(칩 형성역학)

• Material removal -> Chip production
• A chip is produced ahead of tool by shearing along the shear plane

Input(Independent) variables(Controllable parameters)	Output(Dependent) variables
Type of cutting tool	Types of chip produced
Tool shape	Force and energy required
Workpiece material	Temperature rise
Cutting condition(Speed, Feed, Depth of cut)	Wear, chipping and failure of tool
Type of cutting fluid	Surface finish of workpiece

• Orthogonal cutting(Two dimensional model)
  • Two-dimensional orthonogonal cutting : The edge of tool is perpendicular(Orthogonal) to cutting direction
  • Tool has a rake angle, relief of clearance angle

Chip Morphology

• Type of chips produced influences surface finish, integrity and machining operation
• Actual chips are significantly different from the ideal model shown on previous slides
• The tool side of the chip surface is shiny(Burnished), which was caused by rubbing of the chip
• The other side of the chip surface has a jagged and steplike appearance
• Continuous chip
  • Typically formed at high cutting speeds and high rake angles
  • Generally produce good surface
  • Chip usually becomes harder and stronger and less ductile than the original workpiece
  • But continuous chips tend to get tangled around the tool
  • Chip breaker can be used to solve this problem
• Built-up-edge(BUE) chip, 구성인선 - 시험
  • Forms at the tip of the tool during cutting
  • Built-up(구성) : material from the workpiece that are gradually deposited on the tool
  • BUE has 3 times higher hardness than bulk workpiece
  • BUE affects surface finish and integrity in machining
  • BUE generally undesirable, but a thin stable BUE is regarded as desirable(It protects the tool surface)
• Major factors(BUE)
  • Adhension of the workpiece material to the rake face of the tool
  • Ceramic cutting tool have much lower affinity to form BUE
  • Growth of the successive layers of adhenred metal on the tool
  • Tendency of the workpiece material for strain hardening
• Build-up Edge decreases as
  • the cutting speed($V$) increases
  • the depth of cut($t_0$) decreases
  • the rake angle($\alpha$) increases
  • tip radius of the tool decreases
  • an effective cutting fluid is applied
• Chip curl
  • Possible factors contributing to chip curl
    • Stresses distribution in shear zone
    • Thermal gradients
    • Work-hardening characteristics of the workpiece
    • Geometry of rake face of the tool
  • The radius of curvature decreases(The chip becomes curlier) with decreasing depth of cut, increasing rake angle, and decreasing frcition at the tool-chip interface -> Chip curl increases
• Chip breaker
  • Long chips needs to be broken since they tend to be entangled and interfere with machining operation
  • It is troublesome in high speed automated machinery

Temperature

• As temperature increases, it will
  • adversely affect the properties(Strength, Hardness, Wear resistance of the cutting tool) of the cutting tool
  • adversely affect dimensional accuracy
  • induce thermal damage to the machined surface, its properties and service life
  • adversely affect dimensional control(distortion of the machine itself due to temperature gradients)
• Heat generation
  • Shearing on the tool - chip interface
  • Overcoming friction on the rake face of the tool - chip interface
  • Tool tip rubbing against the machined surface when the tool is worn

• Max temperature is away from the tool tip
• temperature increases with cutting speed
• The chip plays a role of good heat sink in that it absorbs and carries away most of heat generated
• Large proportion of the heat generated is carried away by the chip(In the form of heat), as cutting speed

Tool Wear and Failure

• Cutting tools are subjected to high force, elevated temerature and sliding; all these conditions induce wear of the cutting tools
• Tool wear is one of the most important aspects of machining operations; because of its effect on the quality of the machined surface and the economics of machining
• The war behavior of cutting tools are flank wear, crater wear, nose wear, and chipping of the cutting edge; wear is generally a gradual process, chipping of the tool is a kind of catastrophic failure

Flank Wear

• Flank wear is due to
  • sliding of the tool along the machined surface(depanding on materials involved)
  • temperature rise(adverse effects on the tool material properties)
• Tool-life curves
  • A tool life decreases as cutting speed increases
  • The workpiece material influences the tool life
  • The microstructures of the workpiece

Crater Wear

• Most significant factors are temperature and the degree of chemical affinity between the tool and the workpiece(The factor affecting flank wear also influence crater wear)
• The location of maximum depth of crater wear coincides with the location of maximum temperature at the tool - Chip interface
• Abrupt increase in crater wear rate above certain temperature is observed

Chipping

• Chipping is a phenomenon that results in sudden loss of tool material
• The breaking away of a piece from the cutting edge of the tool
  • Microchipping : Chipped pieces are very small
  • Gross chipping : Chipped pieces are large fragment

Surface Finish and Surface Integrity

• Surface influences a dimensional accuracy of machined parts as well as properties of the parts such as fatigue strength
• Surface finish describes the geometric features of surface
• Surface integrity pertains to properties such as fatigue life and corrosion resistance
  • Major factors that influence surface integrity
    • Temperatures generated during processing
    • Residual stresses
    • Metallurgical transformations
    • Plastic deformation, tearing, and cracking of the surface

Dull Tool in Orthogonal Cutting and Feed Marks

• Dull tool : Lacks sharpness has a large radius along its edges
  • Large radius along its edges
  • Rake angle becomes negative
  • Dull tool rub over the machine surface
    • Friction heat
    • Surface residual stresses
    • Surface damages : Cracking, Tearing, etc.
• Feed mark : The tool leaves a spiral profile
  • The higher feed rate and the smaller the radius(R), the more significant feed marks

Machinability

• One of material properties but difficult to express quantitatively
  • Surface finish and integrity of the machined part
  • Tool life
  • Force and power requirements
  • Chip contorl
• Good machinability
  • Good surface finish and integrity
  • Long tool life
  • Low force and power
  • Good elimination of the chip

Cutting-Tool Material

• Tool material is one of most important consideration in machining process
• A cutting tool must have the following characteristics
  • Hot hardness
  • Toughness
  • Wear resistance
  • Chemical stability or inertness

Carbon steels

• The oldest of tool materials since 1880s
• Inexpensive and easily shaped and sharpened
• Insufficient hot hardness for machining at high cutting speed
• Insufficient wear resistance for machining at high cutting speed

High-speed steels(HSS)

• Developed to machine at speeds higher than previously possible
• The largest number of tool materials