TECNOLOGIA MECÂNICA - ULisboa · 3 General Information Class slides Available at the course website Textbooks Rodrigues J. e Martins P., Tecnologia M ecânica vol. 1 e 2, Escolar

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MASTER IN MATERIALS ENGINEERING

TECNOLOGIA MECÂNICA

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General Information

Teaching staff Paulo Martins (Professor) [email protected] ext.: 3006 Beatriz Silva (Assistant Professor) [email protected] ext.: 1560

Topics Introdução aos processos de fabrico Plasticity theory Metal forming processes (forging, shearing, sheet metal forming – stamping

and deep drawing)

Metal cutting processes (orthogonal metal cutting - introduction to turning)

Assessment Final examination (2 dates)

Weekly timetable to clarify doubts Available at the course website

Where to find me? Tecnologia Mecânica, Pavilhão de Física, Piso 0

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General Information

Class slides Available at the course website

Textbooks Rodrigues J. e Martins P., Tecnologia Mecânica vol. 1 e 2, Escolar Editora, 2010.

Gouveia B., Rodrigues J. e Martins P., Tecnologia Mecânica , vol. 3. Escolar Editora, 2011.

Rodrigues J. e Martins P., Enunciados de Exercícios Complementares, 2013.

(download na página da disciplina)

Martins P., Corte por arranque de apara (transparências e enunciados de problemas disponibilizados aos alunos na página da disciplina)

Hosford W. and Caddell R., Metal forming – mechanics and metallurgy, Prentice-Hall, 1983.

Kalpakjian S., Manufacturing processes for engineering materials, Addison-Wesley, 1984.

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Course Outline

Livro de Tecnologia Mecânica Vol. I – Fundamentos teóricosLivro de Tecnologia Mecânica Vol. II – Aplicações industriaisLivro de Tecnologia Mecânica Vol. III – Exercícios resolvidosEnunciados na pagina internet da disciplina

Week Dates Theory Exercises Self-study Laboratories

1 19 Sep. – 23 Sep.IntroductionChapter 1

(2 lectures)3.2/2.1 (parcial) 3.6/2.2 -

2 26 Sep. – 30 Sep. Chapter 5(2 lectures)

5.1

1.4/1.5 5.41.6/1.7/1.8/ 1.12

Compression and ring tests

3 03 Oct. – 07 Oct.Chapter 4

(2 lectures)2.3, 2.9

5.3/5.5 1.1/1.3/1.9/1.10/1.11/1.13/1.16

Compression and ring tests

4 10 Oct. – 14 Oct.

Chapter 4(1 lecture)2.12 e 4.4

Chapter 6(1 lecture)

4.1/4.6 2.4/2.5/2.7/2.8/2.13/2.14/2.15

Tension and torsiontests

5 17 Oct. – 21 Oct.

Chapter 6(1 lecture)3.5 e 3.6


2.10/3.1 3.3/3.4/3.7/3.8/3.9

Tension and torsiontests

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Course Outline


6 24 Oct. – 28 Oct.

Chapter 7(1 lecture)4.3, 4.4


3.2 7.14.2/4.5/4.6 Forging

7 31 Oct. – 04 Nov. Chapter 14(2 lectures) 4.1/7.1 7.3 01 Nov. (holiday)

8 07 Nov. – 11 Nov.

Chapter 14(1 lecture)14.1, 14.2


7.4Forging 1 7.2/7.6 Forging

9 14 Nov. – 18 Nov. Chapter 17(2 lectures)

17.1Shearing 1 10.1/10.2 Shearing and

blanking


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Course Outline


10 21 Nov. – 25 Nov.

Chapter 17(1 lecture)Shearing 3


Shearing 2 10.3/10.6 Shearing and blanking

11 28 Nov. – 02 Dec. Chapter 22(2 lectures)

22.1Sheet metal forming 1 22.2/22.3 Sheet metal forming

12 05 Dec. – 09 Dec.


11.1

Metal cutting(1 lecture)

11.6 11.3 Sheet metal forming

13 12 Dec. – 16 Dec.Metal cutting(2 lectures)

Metal cutting 1.2Metal cutting 1.1 Preparation for the

exam Metal cutting

14 19 Dec. – 21 Dec.

Metal cutting(1 lecture)

Metal cutting 2.1 Metal cutting 1.3 Preparation for theexam Metal cutting


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Introduction to manufacturing processesMaterials

The main resources of our planet

27.7

8.1

53.62.82.6

2.1

0.44

1.06

46.6

Silicon Aluminium IronCalcium Sodium PotassiumMagnesium Titanium Other ElementsOxygen

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Introduction to manufacturing processesMaterialsThe selection of materials must take into consideration the ever-increasing variety of materials, each having its own characteristics, applications, advantages and limitations.

• Properties of the materials – physical, chemical and mechanical properties• Cost and availability - raw and processed materials / desired shapes, dimensions and

quantities / reliability of the supply sources• Appearance, service life and recycling – characteristics that influence a purchasing decision

and cope with the increasing recycling demands

Examples: density, melting temperature, corrosion resistance, toxicity, hardness, ductility

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The selection of materials can be performed by using diagrams that relate different physical, chemical and mechanical properties. However a good use of these diagrams is only possible through basic knowledge of materials science and engineering that is important to review.

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11

Introduction to manufacturing processes

Metals

Polymers

Ceramics

Composites

Materials - classification

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Introduction to manufacturing processesMetalsAre inorganic substances that are typically arranged in one of the three most common crystalline structures: (FCC) face-centered cubic(BCC) body-centered cubic(HCP) hexagonal close-packed

A metallic alloy is a mixture of two or more metallic elements and may also have some non-metallic elements such as, for example, carbon, nitrogen or oxygen.

Of all the metallic alloys in use today, the alloys of iron, aluminium, titanium, coper and magnesium make up the largest proportion both by quantity and commercial value.

FCC BCC HCP

AluminiumLead

CopperGoldSilver

CromiumIron (FCC at 910ºC)

ZincMagnesium

Titanium (BCC at 883ºC)

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Introduction to manufacturing processesMetalsThe order of magnitude of the distance between the atoms in theseCrystal structures is 0.1 nm.

The way in which these atoms are arranged can have a significant influence on the properties of a particular metal and the reason that different structures form is a matter of minimizing the energy required to form these structures.

Of the three most common crystalline structures, the FCC and HCP have the most densely packed configurations.

There are two basic mechanisms by which plastic deformation may take place in crystal structures: (i) by slipping of one plane of atoms over an adjacent plan under a shearing force and (ii) by twinning, in which a portion of the crystal forms structurally a mirror image of itself across the plane of twinning.

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Introduction to manufacturing processesMetalsThe combination of a slip plane and its direction of slip is known as a slip system. In general, metals with 5 or more slip systems are ductile, where those with less than 5 are not ductile.

It must however be remembered that what appears to be a single slip plane is actually a slip band consisting of a number of slip planes.

FCC BCC HC12 slip systems

Required shear stress is low

Moderate strength and good ductility

48 slip systemsRequired shear stress is high

(high b/a ratio)

Good strength and moderateductility

3 slip systemsMore systems become active at

elevated tempartaures

Some metals are brittle at roomtemperature

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Introduction to manufacturing processesMetalsMetallic engineering materials usually are grouped in ferrous and non-ferrous.

The group of ferrous metals includes carbon steels, alloy steels, stainless steels, tool steels and cast irons. These materials are characterized by having a high percentage of iron.

The group of non-ferrous metals includes aluminium, magnesium, copper, titanium, nickel, and other metallic alloys that do not contain iron or contain only a small amount of iron.

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Steels have less than 2% carbon while cast irons have 2 to 4% carbon and 0.5 to 3% of silicon.

Carbon and alloy steels are among the most commonly used materials and have a wide variety of applications. Various elements are added to steels to impart properties of hardenability, strength, hardness, toughness, wear resistance, workability, weldability and machinability.

Examples: The higher the carbon content, the higher is the hardenability of the steel and the higher is its strength, hardness and wear resistance and the lower is ductility, weldability and toughness.A chromium content of 10-12 percentage gives rise to a thin, hard adherent film of chromium oxide that protects the metal from corrosion.

Carbon and alloy steels

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Introduction to manufacturing processesCarbon and alloy steels

Serie(AISI standard) Type of steel

1000 Carbon steels (1000-1200)Low alloy carbon-manganese steels (1300)

4000

Alloy steelsChromium-molybdenum (4100)Chromium-molybdenum-nickel (1.83% Ni) (4300, 4700)Molybdenum (4400)Molybdenum-nickel (4600, 4800)

5000 Alloys steelsChromium (5000-5200)

6000 Alloy steelsChromium-vanadium (6100)

8000 Alloy steelsChromium-molybdenum-nickel (0.55% Ni) (8600-8800)

Cost:Low carbon steels – 0.65 €/kgAlloy steels – 1.5 to 8.0 €/kgStainless steels – 4 to 6.5 €/kg

Typical density: 7.8 g/cm3

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Introduction to manufacturing processesEvolution of carbon steelsThe need to manufacture lightweight structures with high mechanical strengths was responsible for the performance of carbon steels to evolve in recent years.This need has been led by the automotive industry who seeks to reduce the weight of vehicles in order to improve its levels of consumption and reduce pollutant emissions.

IF – Interstitial Free Steels IS – Isotropic SteelsBH – Bake Hardening Steels CMn – Carbon Manganese SteelsHSLA – High Strength Low Alloy Steels CP – Complex Phase SteelsDP – Dual Phase Steels MART – Martensitic SteelsTRIP – Transformation Induced Plasticity Steels BH

IF

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Introduction to manufacturing processesComplex phase steelsSmall amounts of martensite, austenite and perlite dispersed in a ferrite/bainite matrix.

Door bar Suspension arm

Seat Flange

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Introduction to manufacturing processesDual phase steels (biphasic structure)Martensite or bainite dispersed in a soft ductile ferritic matrix

B-pillar

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Introduction to manufacturing processesOther carbon steelsTRIP – austenite embedded in a ferrite matrix with hard phases of martensite or bainite

TWIP – austenite matrix at room temperature due to a high percentage of manganese (17 to 24%) Plastic deformation mechanisms by slipping and twinning that allow reaching high deformation and high levels of strain hardening

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Introduction to manufacturing processesStainless steels

Serie(AISI Standard) Type of steel

2xx

AusteniticChromium– 15 a 30% ; Nickel– 6 a 20% ; Manganese; (FCC)Not heat treatable, non-magnetic, excelent corrosion resistance andformabilityEx: 201 a 205

3xx

AusteniticChromium– 15 a 30% ; Nickel– 6 a 20% ; (FCC)Not heat treatable, non-magnetic, excelent corrosion resistance andformabilityEx: 304, 316

4xx

FerriticChromium– 12 a 18% ; Carbon– 0.10% ; (BCC)Not heat treatable, magnetic, good corrosion resistance and formabilityEx:409, 430

MartensiticChromium– 12 a 18% ; Carboon– 0.15 a 1.0%Heat treatable, magnetic, moderate corrosion resistance, high strengthand good formabilityEx: 410, 420, 440

Stainless steels were developed in the early 1900’s and are produced bytechniques similar to those used in other types of steelmaking, usingelectric furnaces or the basic-oxygen process.

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Aluminium alloys

Cost: 2.4 €/kg


Serie(Aluminum

Association)Type of aluminium alloy

1000 Aluminium percentage above 99.0%

2000 Aluminium-copper alloys

3000 Aluminium-manganese alloys

4000 Aluminium-silicon alloys

5000 Aluminium-magnesium alloys

6000 Aluminium-magnesium-silicon alloys

7000 Aluminium-zinc alloys

8000 Aluminium-other elements (example: Aluminium-lithium)

9000 Free series


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Copper alloys

Cost: 4.5 ~ 5.0 €/kg


Serie(Copper Development

Association)Type of copper alloy

C10000Copper (Cu > 99.3%)Alloys with Cu > 96% (beryllium bronzes C17000)

C20000 Copper-zinc alloys (brasses)(red brass C23000, cartridge brass C26000)

C30000Copper-zinc-lead alloys (brasses with lead)(free cutting brass C36000, architectural bronze C38500)

C40000 Copper-zinc-tin alloys (brasses with tin)(naval brass C46400 to C46700)

C50000Copper-tin alloys (tin bronzes)(phosphor bronze C51000, free cutting phosphor bronze C54400)

C60000 Copper-aluminium alloys (aluminium bronzes)Copper-silicon alloys (silicon bronzes)

C70000 Copper-nickel alloysCopper-nickel-zinc alloys (nickel silver C74500)


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Introduction to manufacturing processesMetalsComparing metals with other engineering materials it may be concluded that their densities are usually much higher. With some exceptions, metals are very good thermal and electrical conductors and can withstand significant plastic deformations, as they have good ductility at room temperature.

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Introduction to manufacturing processesPolymers

Polymers are currently the most important group of non-metallic materials utilized in industrial applications. The earliest polymers were made of natural organic materials from animal and vegetal products (e.g. cellulose).

The first synthetic polymer was a phenol-formaldehyde namedBakelite (after LH Baekeland, 1906).

The development of modern plastics technology began in the 1920’s, when raw materials necessary for making polymers were extracted from coal and petroleum products.

Polymers are constituted by long chains or networks of organic molecules which form non-crystalline or semi-crystalline structures and are commonly classified into two different groups:

• Plastics (thermoplastics and thermosets) • Elastomers (rubbers)

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Thermoplastics are formed by simple organic molecules (monomers) that are linked together in a covalent bond forming long polymer chains. • (linear polymer chains) – acrylic, nylon, polyethylene and

polyvinyl chloride• (branched polymer chains) - polyethylene

The polymer chains are held together by secondary bonds (much weaker than primary bonds existing within each macromolecule) such as van der Waals bonds.

Thermosets are formed by network (highly three-dimensional cross-linked) arrangements of long polymer chains that are also held together in one giant molecule by strong covalent bonds. The network arrangement is formed during polymerization (curing) and is irreversible.Thermosets generally possess better mechanical (higher strength, higher dimensional stability but lower ductility), thermal and chemical properties than thermoplastics and they are not so much influenced by temperature and rate of deformation.

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Elastomers are amorphous polymers with the ability to undergo large elastic deformations without rupture and to fully recover their original (undeformed) shape after the load is removed.

Their structure is made of cross-linked arrangements of long polymer chains (with a degree of cross-linking smaller than that of thermosets) and justifies the reason why they can be disentangled and unwinder for accommodating elastic loading and unloading.

Vulcanization of rubber was discovered by C. Goodyear in 1839.

Cost:ABS (thermoplastic) – 1.6 €/kg PF – phenolic/bakelite (thermoset) – 1.3 €/kgElastomers – 1.3 to 2.7 €/kg

Typical densities: 0.9 to 2.2 g/cm3

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Thermoplastics Thermosets Elastomers

ABS

Nylon(polyamide)

Polyethylene

Plexiglas

PVC

Urea resins

Polyurethane

Phenolic resins(Bakelite)

Epoxy resins

Natural rubber(latex from rubber tree)

Silicone

Polyisoprene(synthetic rubber)

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Advantages:Density is low and close to the density of water. High corrosion resistance, low friction coefficient and low thermal and electrical conductivity.

Disadvantages:High coefficient of expansion and low mechanical strength. Dimensional stability greatly influenced by temperature (may not be used for temperatures above 100 °C).

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Introduction to manufacturing processesCeramics

Ceramics are inorganic substances consisting of metallic and non-metallic elements chemically bonded together by ionic and/or covalent bonds.Their structure may be totally crystalline or non-crystalline.

Ceramics are classified into two main groups: traditional ceramics andtechnical ceramics.

Traditional ceramics are obtained from clay Al2Si2O5(OH)4, silica SiO2, feldspar KAlSi3O8 e fluorite CaF2

Porcelane (density 2.5 g/cm3) Fluorite Silica

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Ionic bonds (CaF2) – significant differences in electronegativity (> 1.7 eV)Covalent bonds (SiC) – small differences in electronegativity

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Technical ceramics are artificially obtained from pure, or nearly pure, compounds made from oxides, carbides and nitrides.

Examples: Aluminium oxide Al2O3, silicon carbide SiC, silicon nitride Si3N4, zirconia ZrO2.

Silicon nitride(cost: 13.5 to 94 €/kg density: 3.4 g/cm3)

Zirconia(cost: 80 to 120 €/kg density: 5.7 g/cm3)

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Introduction to manufacturing processesCeramicsIn view of engineering applications, the importance of ceramics derives from their high abundance in nature and also from the fact that its chemical and physical properties are very different from those of metals and polymers.

35

Introduction to manufacturing processesCompositesComposite materials are combinations or mixtures of two or more materials that can be classified into three different groups: • polymer-ceramic• metal-polymer • metal-ceramic

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Introduction to manufacturing processesComposites

Composite materials are made from two or more constituent materials with significantly different physical, chemical or mechanical properties that, when combined, produce a material with characteristics different (and in some aspects better) from the individual constituent materials.

Composites are made to obtain materials that are stronger, lighter, or less expensive than traditional materials.

Woody plants, both true wood from trees and from plants such as palms and bamboo, yield natural composites that were used pre-historically by mankind and are still used widely in construction and scaffolding.

Plywood was developed by the Ancient Mesopotamians; gluing wood at different angles gives better properties than natural wood

Concrete is the most common artificial composite material of all and typically consists of loose stones (aggregate) held with a matrix of cement.

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Introduction to manufacturing processesCompositesThe structure of artificial composites is formed by a binder material (which serves as a matrix) and a reinforcing (or filling) material.

There are three principal kinds of reinforcement in composites: particles, continuous fibersshort (discontinuous) fibers and laminate or sandwich composite structures using a foam or honeycomb core.

The reinforcement usually has a higher strength than the matrix, which provides the ductility of the resulting material. In ceramic based composites, however, the matrix is brittle, and the fibers provide barriers to propagating cracks, increasing toughness of the material.

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Introduction to manufacturing processesComposites

CMP (polymer matrix) CMM (metallic matrix) CMC (ceramic matrix)

Epoxy reinforced with aramid(cost 37.5 €/kg, density 1.34 g/cm3)

Epoxy reinforced with carbon(cost 48 €/kg, density 1.53 g/cm3)

Polyester resin reinforced with fiber glass(cost 2 to 4 €/kg, density 1.8 to 2 g/cm3)

Aluminium matrix reinforced with carbon particles

Fibers/carbon particles(cost 67€/kg, density 1.75 g/cm3)

30% Titanium carbide (TiN) and 70% de aluminium oxide (Al2O3)

Al2O3 fibbers with ZrO2 matrix

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Introduction to manufacturing processesMaterials – resume of physical properties

METALDENSITY

(kg/m3)MELTING POINT

(¡C)SPECIFIC HEAT

(J/kgK)

THERMALCONDUCTIVITY

(W/m K)

COEFFICIENTOF THERMALEXPANSION

(µm/m¡C)AluminumAluminum alloysBeryliumColumbium (niobium)CopperCopper alloysGoldIronSteelsLeadLead alloysMagnesiumMagnesium alloysMolybdenum alloysNickelNickel alloysSiliconSilverTantalum alloysTitaniumTitanium alloysTungsten

27002630-2820

185485808970

7470-8940193007860

6920-913011350

8850-113501745

1770-1780102108910

7750-88502330

10500166004510

4430-470019290

660476-654

127824681082

885-126010631537

1371-1532327

182-326650

610-62126101453

1110-1454142396129961668

1549-16493410

900880-920

1884272385

337-435129460

448-502130

126-18810251046276440

381-544712235142519

502-544138

222121-239

14652393

29-23431774

15-5235

24-46154

75-13814292

12-631484295417

8-12166

23.623.0-23.6

8.57.116.5

16.5-2019.311.5

11.7-17.329.4

27.1-31.126.026.05.113.3

12.7-18.47.6319.36.58.35

8.1-9.54.5

NONMETALLICCeramicsGlassesGraphitePlasticsWood

2300-55002400-27001900-2200900-2000400-700

-580-1540

-110-330

-

750-950500-850

8401000-20002400-2800

10-170.6-1.75-10

0.1-0.40.1-0.4

5.5-13.54.6-707.86

72-2002-60

Ford Duratec V-6

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Introduction to manufacturing processesMaterials – resume of economical aspects

These graphics show a summary of the costs of major engineering materials per kg and per m3.The most widely used engineering materials (steel, aluminium, polymers, and glass) have an average cost ranging between 0.5 and 2 € / kg.The cost per m3 of polymers is lower than that of metals because its density is lower.

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Introduction to manufacturing processesMaterials and products

Automóveis e electrodomésticos

Bicicletade corrida

neveSkis de

10000Implantes

1000

100

10

1

Civil

Particular

Automóvelde corrida

TorradeiraSecador

Automóvel Familiar

de ténisRaquete

de lavarMáquina

Produtos

normalBicicleta Produtos de desporto

Aviação

Biomedicina

Próteses

Militar

1000

Cermets

Ligas de Ni

Polímeros

Cerâmicos

Ligas de Ti

Materiais

Ligas de Pb

Madeiras

Ligas de Al

100

10

1

-110

Cus

to (E

uro/

Kg)

Aço baixa liga

Aço carbono

Aço ferramenta

Aço inoxidável

Ligas de Zn

Ligas de W

Prata

Navio detransporte

Pontemetálica

Iate de luxo

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Introduction to manufacturing processesFabrication processes and modification of properties

The transformation of materials (raw materials) into products is carried out by means of primary fabrication processes and by secondary processes focused on the modification of physical, chemical and mechanical properties.

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Introduction to manufacturing processesCastingIn casting, a liquid (melted) metal is poured or forced into a mould where it solidifies by cooling. The filling of the mould is carried out by flow under its own weight (gravity casting) or under pressure (centrifugal casting and pressure die casting).

The moulds can be classified as permanent and non-permanent:Non-permanent sand moulds for one-off castings are cheap.Permanent moulds are made of metals, are suitable for large batches and can be expensive.Between these extremes lie other moulding methods: shell, investment and plaster moulds.

Sand casting is a simple and versatile technique to manufacture parts in a very wide range of geometric shapes. The moulds are not usable after breaking and removing the part but the sand from which they were made can be reused.

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Introduction to manufacturing processesCasting

Elaborate shapes can be moulded, but at the penalty of complexity in the mould shape and in the way it separates to allow removal. In fact, the parts must be designed with uniform section thicknesses for easy flow of liquid metal to all cavities of the mould and for progressive solidification without trap pockets of liquid giving shrinkage cavities.

Die casting allows high production rates of high-quality parts with complex shapes, good strength, good dimensional accuracy and surface details, thus requiring little or no subsequent machining or finishing operations.

The dimensional accuracy of die casting is better than that of sand casting.

Die casting forces the liquid metal into the moulds at pressures ranging from 0.7 to 700 MPa.

The moulds are permanent and expensive because they must withstand repeated application of pressure, temperature and wear involved in separating and removing the parts.

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Introduction to manufacturing processesPolymer processingPolymer processing is the transformation of the pellets or plastic powders into sheet, plate, rod, tubing, profiles or finished parts.

The technologies used for the processing of plastics are very dependent on the type of material. • Thermosets, are usually shaped by compression, transfer and injection

moulding. • Thermoplastics are usually shaped by thermo-forming, extrusion, injection

and blow moulding.

46

Introduction to manufacturing processesPolymer processing

In injection moulding material is molten and injected as a viscous liquid into the mould. The process allows producing complex shapes with good dimensional accuracy but because the molten parts shrink during cooling (typically 1.5 to 7 % in volume) allowance must be made for this when the mould is designed.

47

Introduction to manufacturing processesPolymer processing

The moulds for plastic processing must have good finishes with elimination of burrs, scratches and other marks resulting from cutting tools in the final operations of grinding and polishing. This is necessary to ensure that the flow of molten plastics takes place in a suitable manner and that the surface quality of the final parts is good. The moulds may be subjected to surface treatments (e.g. coatings) in order to increase its resistance to wear and abrasion and to improve its chemical resistance against certain types of materials, such as, for example, PVC.

Blow moulding

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Introduction to manufacturing processesPlastic deformation processes (forming processes)

Plastic deformation processes are operations that induce shape changes on the raw materials by plastic deformation under forces applied by various tools and dies. These processes are very efficient in the use of raw materials and in the reduction of waste, in contrast to other fabrication processes.

Plastic deformation processes can be grouped by type of operation, temperature, and size and shape of the workpiece.Regarding size and shape of the workpiece, plastic deformation processes are classified as either bulk or sheet forming.

Bulk forming

49

Introduction to manufacturing processesPlastic deformation processes (forming processes)

Sheet forming

50

Introduction to manufacturing processesCutting processes

Cutting processes remove material from the workpiece to produce the desired final shape.

Turning, drilling, milling and grinding remove material from the surface of the workpiece in the form of chips.

Turning, drilling and milling (machining processes) involve single-point or multi-point cutting tools with a clearly defined geometry. These processes were introduced in the 1700’s and the first computer-controlled machines started to become available during the 1940’s and 1950’s.

Grinding is an abrasive based cutting process that removes micro-ships from the surfaces of the workpiece.

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Electrical-discharge machining is based on the erosion of metals by spark discharges (leaving very small craters).

In water-jet cutting, water acts like a saw and cuts material under running pressures of 500 to 1500 MPa.

In thermal-cutting (oxyfuel-gas cutting, plasma arc cutting) the driving mechanism is oxidation and burning of the material or the use of ionized gas.

Shearing is a plastic deformation based cutting process that subjects material to be cut to shear stresses developed between a punch and a die.

52

Introduction to manufacturing processesCutting processesAlmost all engineering parts, whether made of metal, polymer or ceramic are subjected to some kind of machining or grinding in their production system consisting of the workpiece, the tool and the machine.

Machining operations are often finishing operations, and thus determine finish and tolerance.

Metals differ greatly in their machinability (i.e. in the ability to give a smooth surface and the ability to give economical tool life).Polymers machine easily but they deflect elastically limiting the tolerance and finish.Ceramics can be ground to high tolerance and finish.

53


In punching, the sheared slug is discarded whereas in blanking, the slug is the part and the rest is scrap.Raw materials subjected to punching or blanking can be supplied in the form of sheets, plates, rods and profiles.

54

Introduction to manufacturing processesPowder metallurgyPowder metallurgy creates the desired shapes by pressing and sintering fine particles (powders) of the material. The powders can be cold pressed and then sintered (heated at up to 0.8 Tm to give bonding), can be pressed in a heated die (die pressing) or contained in a thin preform that is heated under hydrostatic pressure (HIP – hot isostatic pressure).

Metals and ceramics which are too high-melting to cast and too strong to deform, can be made (by chemical methods) into powders and then shaped in this way.The process is most widely used for producing small metallic parts like bearings and gears for vehicles and appliances, cutting inserts and almost all engineering ceramics.

55

Introduction to manufacturing processesWelding and joining

Joining is made possible by a different number of techniques.

• Welding, brazing and soldering• Bolting and riveting• Adhesive bonding• Weldbonding (hybrid joining process)

56

Introdução aos processos de fabricoModification of metallurgical structures (mechanical treatment)

The term mechanical treatment describes processes applied to the surface of the components. They include polishing, abrasive cleaning and finishing, shot peening (abrasive projection) and are aimed to improve surface smoothness, remove corrosion and oxidation and improve surface strength.

57

Introduction to manufacturing processesModification of metallurgical structures (heat treatment)The term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally because heating and cooling often occur incidentally during manufacturing processes such as hot forming or welding.

Heat treating involves the use of heating or cooling, normally to extreme temperatures, in order to achieve a desired result such as hardening or softening of a material.

Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, normalizing and quenching.

58

Introduction to manufacturing processesModification of metallurgical structures (heat treatment)

SteelThe hardness and toughness of steels is controlled by quenching from the austenitising temperature (about 800ºC) and tempering.

Internal stresses caused by fabrication processes can be removed by stress relief annealing, which is another type of heat treatment process.

Aluminium, titanium and nickel alloysThe hardness and strength of these alloys derives from a precipitate produced by a controlled heat treatment: quenching from a high temperature followed by ageing at a lower temperature.

59

Introduction to manufacturing processesModification of chemical composition and coatings

This group describes other type of treatments applied to the surface of the components. They include plating, carburizing, anodizing and painting and are aimed to improve aesthetics, protect against corrosion, oxidation and wear.

60

Introduction to manufacturing processesProcess attributes and selection

Process attributes include the materials it can handle, the size range of the things it can make, their shape, their complexity, their accuracy, the quality of the finish and the speed at which it operates.

The objective is to match the attribute profiles of available processes to that specified by the design.

Documents

TECNOLOGIA MECÂNICA - ULisboa · 3 General Information Class slides Available at the course website Textbooks Rodrigues J. e Martins P., Tecnologia M ecânica vol. 1 e 2, Escolar