Materials and ManufacturingThe selection of the material is an important step in the design of a machine element. Fracture Behavior

Ductile material – Significant plastic deformation and energy absorption (toughness) before fracture.Characteristic feature of ductile material - neckingBrittle material – Little plastic deformation or energy absorption before fracture. Characteristic feature of brittle materials – fracture surface perpendicular to the stress.

SteelBefore and after fracture

The Concept of Stress

0AF

=σ

Uniaxial tensile stress: A force F is applied perpendicular to the area (A). Before the application of the force, the cross section area was AO

Engineering stress or nominal stress: Force divided by the original area.

00

0

ll

lll Δ=

−=εEngineering Strain or Nominal Strain: Change

of length divided by the original length

In general:•ductile materials are limited by their shear strengths•brittle materials (ductility< 5%) are limited by their tensile strengths

True stress and strain

Notice that past maximum stress point, σ decreases.Does this mean that the material is becoming weaker?

Necking leads to smaller cross sectional area!

Recall: Engineering Stress =oA

F=σ Original cross sectional area!

True Stress =i

T AF

=σ

True Strain =o

iT l

lln=ε

Ai = instantaneous areali = instantaneous length

If no net volume change (i.e. Ai li = Ao lo)

)1ln()1(

εεεσσ

+=+=

T

TOnly true at the onset of necking

Strain Hardening Parameter (n)

Strain hardening parameter 0.02<n<0.5 for ductile metals.Useful as a measure of the resistance to

necking

nTT Kεσ =

Compression TestsA ductile sample will not fracture under compression. Brittle materials will fracture when compressed. A material that has different tensile and compressive strength are called uneven materials.

Torsion Test• Ductile material twist• Brittle material fractures

GITL

P

=φL

GrMAX

φτ =

Stress-Strain Behavior of Ceramics

Flexural Strength: the stress at fracture under the bending tests. It’s also called Modulus of rupture, fracture strength, or the bend strength

3-point Bending tests

3

223

RLF

bdLF

ffs

ffs

πσ

σ

=

=

CharpyIzod

h’h

Energy ~ h - h’

Impact Test (testing fracture characteristics under high strain rates)Notched-bar impact tests are used to measure the impact energy (energy required to fracture a test piece under impact load), also called notch toughness. It determines the tendency of the material to behave in a brittle manner.Two classes of specimens have been standardized for notched-impact testing, Charpy (mainly in the US) and Izod (mainly in the UK).

Ductile-to-brittle transitionAs temperature decreases a ductile material can become brittle - ductile-to-brittle transition.FCC metals show high impact energy values that do not change appreciably with changes in temperature.

BCC metals, polymers and ceramic materials show a transition temperature, below which the material behaves in a brittle manner. The transition temperature varies over a wide range of temperatures. For metals and polymers is between -130 to 93oC. For ceramics is over 530oC.

High Carbon Steel

Charpy Test

Stainless Steel

16

• Pre-WWII: The Titanic • WWII: Liberty ships

• Problem: Used a type of steel with a DBTT ~ Room temp.

Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.1(a), p. 262, John Wiley and Sons, Inc., 1996. (Orig. source: Dr. Robert D. Ballard, The Discovery of the Titanic.)

Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.1(b), p. 262, John Wiley and Sons, Inc., 1996. (Orig. source:Earl R. Parker, "Behavior of Engineering Structures", Nat. Acad.Sci., Nat. Res. Council, John Wiley and Sons, Inc., NY, 1957.)

DESIGN STRATEGY: STAY ABOVE THE DBTT!

HardnessHardness: a measure of a material’s resistance to localized plastic

deformation (eg. Small dent or scratch).

Correlation between Hardness and Tensile Strength

TS (MPa) = 3.45xHB

TS (psi) = 500xHB

Note:No method of measuring hardness uniquelyindicates any other single mechanical property.Some hardness tests seem to be more closely associated with tensile strength, others with ductility, etc.

Under fluctuating / cyclic stresses, failure can occur at loads considerably lower than tensile or yield strengths of material under a static load: FatigueEstimated to cause 90% of all failures of metallic structures (bridges, aircraft, machine components, etc.). Fatigue failure is brittle-like (relatively little plastic deformation) - even in normally ductile materials. Thus sudden and catastrophic!

Fatigue failure proceeds in three distinct stages: crack initiation in the areas of stress concentration (near stress raisers), incremental crack propagation, final catastrophic failure.Cyclic stresses characterized by maximum, minimum and mean stress, the range of stress, the stress amplitude, and the stress ratio.

Fatigue :Failure under fluctuating/cyclic stress

Fatigue limit occurs for some materials (some Fe and Ti alloys). S—N curve becomes horizontal at large N. Stress amplitude below which the material never fails, no matter how large the number of cycles is. It has values between 0.4 to 0.25 the TS of the materialIn most alloys (ex. FCC), S decreases continuously with N. Fatigue strength: stress at which fracture occurs after specified number of cycles (e.g. 107). Fatigue life: Number of cycles to fail at specified stress level.

• Forming Operations– Forging– Rolling– Extrusion– Drawing

• Casting• Powder Metallurgy• Welding

Fabrication of Metals

Classification of Metal Fabrication Techniques

• Forming Operations ⎯ are those in which the shape of a metal piece is changed by plastic deformation

• Forming processes are commonly classified into hot-working and cold-working operations.

Hot Working

Temperature in Metalworking

Hot-working is defined as deformation under conditions of temperature and strain rate such that recrystallization takes place simultaneously with the deformation. Relatively high T– Recrystallization leads to very large deformation– Hot-working processes such as rolling, extrusion, or forging

typically are used in the first step of converting a cast ingot into a wrought product

– Deformation energy requirements are less than for cold work– Most metals experience some surface oxidation, which results

in material loss and a poor final surface finish.

The deformation is carried out at low temperatures, where recovery / recrystallization do not take place. Relatively low T

• Cold-working operations are usually carried out in several steps, with intermediate annealing operations introduced to soften the cold-worked metal and restore the ductility

• A higher quality surface finish and closer dimensional control of the finished piece

• Cold-working of a metal results in an increase in strength or hardness and a decrease in ductility.

Cold Working

Cold WorkingCold working: plastic deformation of a metal or alloy at a

temperature where dislocations are created faster than they are annihilated

100%0

0 ×⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

AAACW d

Intermediate Annealing during Cold Working

• When cold-working is excessive, the metal will fracture before reaching the desired size and shape. In order to avoid such difficulties, cold-working operations are usually carried out in several steps, with intermediate annealing operations introduced to soften the cold-worked metal and restore the ductility

• This sequence of repeated cold-working and annealing is frequently called the cold-work-anneal cycle

• Casting: a fabrication process whereby a totally molten metal is poured into a mold cavity having the desired shape; upon solidification, the metal assumes the shape of the mold but experiences some shrinkage.

• Casting techniques are used when1. The finished shape is so large or complicated that any other

method would be impractical2. A particular alloy is so low in ductility that forming by either

hot or cold working would be difficult3. In comparison to other fabrication processes, casting is the most

economical.

• Sand Casting• Die Casting • Investment Casting (lost-wax casting)• Continuous Casting

Classification of Casting

Casting

Sand, Investment, and Lost Foam Casting

• Use gravity to fill the mold• Mold is destroyed to remove casting• Metal flow is slow• Walls are much thicker than in die casting• Cycle time is longer than die casting

because of inability of mold material to remove heat

Aluminum Piston

Aluminum piston for an internal combustion engine. (a) As cast; (b) after machining.

Mold

Schematic illustration of the permanent mold used to produce aluminum pistons, showing the position of four cooling channels.

Investment Casting

• Create Wax Pattern• Assemble Wax Tree• Coat with Ceramic• Melt out wax• Pour in molten

metal• Break off ceramic

Die Casting• Liquid metal injected into

reusable steel mold, or die, very quickly with high pressures

• Reusable steel tooling and injection of liquid metal with high pressures differentiates die casting from other metal casting processes

Casting of Single Crystal Components

Jet engine turbine blades

Forging• Forging is the working of metal into a useful shape

by hammering/pressing. Forging is usually carried out hot.

• Forged articles have outstanding grain structures and the best combination of mechanical properties.

• Wrenches, and automotive crankshafts and piston connecting rods are typical articles formed by forging

Stages in the forging of a crankshaft

A macroetched section through a forging indicates that the grain flow follows the contour of the component, which often maximizes strength in the direction of greatest operating stress.(Metallurgy, by B. J. Moniz, American Technical Publishers, Inc., 1994)

Grain Flow

Open Die Forging Closed Die Forging

RollingRolling is the most extensively used metal forming process and its share is roughly 90%The material to be rolled is drawn by means of friction into the two revolving roll gapThe compressive forces applied by the rolls reduce the thickness of the material or changes its cross sectional areaThe geometry of the product depend on the contour of the roll gap.

ExtrusionA plastic deformation process in which metal is forced under pressure

to flow through a single, or series of dies until the desired shape is produced.

(a) Direct (b) indirect (c) hydrostatic (d) impact

Drawing

Deep Drawing• Blank is allowed to

draw into the die, and thickness is normally unchanged.

• Limiting Drawing Ratio (LDR)

LDR=d0 max / Dp

• Constraint of blank-holder gives improved process control and quality

Stamping Failure Diagnosis using Grid Marks

Hydroforming

Before PressureDie Open

Die Closing

1st Pressure Stage

Die Closing2nd Pressure

StageDie Closed

(a)(b)

(c)

(d)

Automotive Structural PartHydroformed Dodge Dakota

Radiator EnclosureStamped Dodge Dakota

Radiator Enclosure

Stamped Radiator Closure

17 components

36.4 Ibs/16.5 kg

Hydroformed Radiator Closure

8 components ( -9 )

25.4 lbs/11.5 kg (-11 Ibs, -30%)

Spin forming

• A fabrication technique that involves the compaction of powderedmetal, followed by a heat treatment to produce a more dense piece.

• Competitive with processes such as casting, forging, and machining. • Used when (a) melting point is too high (W, Mo); (b) there is a

reaction at melting (Zr); (c) material too hard to machine; (d) very large quantities are required. Near 70% of the P/M part production is for automotive applications.

• Good dimensional accuracy. Controllable porosity.• Size range from tiny balls for ball-point pens to parts weighing 100

lb. Most are around 5 lb.

Powder Metallurgy

pressure

heat

point contact at low T

densification by diffusion at higher T

area contact

densify

Upper trip lever for a commercial irrigation sprinkler, made by P/M. This part is made of unleaded brass alloy; it replaces a die-cast part, with a 60% savings.

Main-bearing powder metal caps for 3.8 and 3.1 liter General Motors automotive engines.

Examples of typical parts made by powder-metallurgy processes.

FINISHEDPRODUCTS

P/M ProcessOPTIONAL

OPERATIONSSINTERINGFORMINGMIXINGRAWMATERIALS

Elemental orAlloy MetalPowders

Additives(graphite, die,

lubricants)

Mixing

IsostaticExtrusion

Die CompactingSpraying

Pressureless-Sintering

Die CompactingIsostaticRolling

Injection MoldingSlip Casting

Cold Compaction

Hot Compaction

AtmosphereVacuum

High Temperature

SinteringRepressing

ConingSizing

RepressingForging

RerollingMetal Infiltration

OptionalManufacuring Steps

MachiningHeat Treating

Steam TreatingPlastic Impregnation

PlatingTumbling

Oil ImpregnationShot Peening

OptionalFinishing Steps

FinishedProducts