ME215 - Comprehensive

Mechanical properties

• Strength

• Rigidity

• Resistance to fracture (toughness)

• Vibration and impact resistance

Physical properties

• Weight

• Electrical and thermal conductivity

• Luster and general appearance

• Corrosion and temperature resistance

• Magnetic properties

Unit strain

Amount of elongation per unit length

e = (change in length) / (length)

Strain

Force transmitted across an area. Ratio of a length to a length.

Stress

The force transmitted through an area.

S = F / A

Shear

Types of stresses or strains where forces acting on a body are offset with respect to one another.

Young’s modulus

The ratio of stress to strain.

Stiffness

Ability of a material to resist deflection or stretching when loaded.

Resilience

Amount of energy a material can absorb while in the elastic range.

Plastic deformation

Elongation beyond the elastic limit, where the material does not return to its original shape. Does NOT deteriorate properties.

Yield point

Where additional strain occurs without an increase in stress.

Tensile strength (ultimate strength)

The maximum load that a material can withstand before deforming or fracturing.

Breaking strength (fracture strength)

Stress at which failure occurs.

Ductility

The property describing the amount of plastic deformation that can occur before failure.

Brittle Fracture

Material fractures with no plastic deformation.

Toughness

The work per unit volume required to fracture a material. In other words, a resistance to fracture / failure.

Damping capacity

The ability of a material to absorb mechanical vibrations.

Hardness

The resistance to plastic deformation, penetration, and indentation. Also describes resistance to scratching, energy absorption under impact loading, and wear resistance.

Dynamic loading

• Sudden impacts or rapidly changing loads

• Repeated loading / unloading cycles

• Frequent changes in type of load (i.e. tension to compression)

Fatigue

When materials fail at less than their ultimate tensile strength (and usually less then their yield strength) due to repeated loads.

Fatigue strength

Max load that can be sustained for a specified number of loading cycles.

Endurance

Stress below which a material will not fail under repeated load cycles.

Creep

Long exposure to elevated temperatures that leads to failure.

Heat capacity (specific heat)

Amount of energy that must be added / removed to a material to produce a 1 degree change in temperature.

Anisotropy

Variation of properties with direction.

Heat treatment

The controlled heating and cooling of materials for the purpose of altering their structures and properties.

Annealing

General term for heat treatment operations.

Full annealing

Where steels are heated past recrystallization temperatures and slowly cooled at a controlled rate, typically inside of a furnace. Product is soft and ductile, and the structure and properties are uniform throughout.

Process annealing

When the workpiece is annealed repeatedly as it is worked to continuously restore ductility lost from other processes.

Hypoeutectoid steel

Steel with less than 0.77% carbon content.

Hypereutectoid steel

Steel with more than 0.77% carbon content.

Eutectoid steel

Steel with 0.77% carbon content.

Normalizing

When steel is heated past its recrystallization temperature, held at that temperature, then allowed to air cool. Generally increases strength, toughness, and ductility, but structure and properties vary throughout the workpiece.

Recrystallization

New crystals are formed to reduce internal energy. This can be achieved through heating the material, but the heating must be controlled to yield desired results.

Spheroidization

Type of annealing where small spheres of cementite form in a ferrite matrix. Increased overall machinability.

Solid-solution strengthening

Base metal dissolves other atoms either as substitutional or interstitial solutions.

Strain hardening

Produces an increase in hardness by means of plastic deformation under cold-working conditions.

Grain size refinement

Improves strength without a loss of ductility and toughness by achieving smaller grain size.

Precipitation hardening (age hardening)

Hardening produced through a 3-step heat treatment process.

Dispersion hardening

Hardness obtained by dispersing second-phase particles through a base material.

Phase transformation hardening

Hardening obtained by heating a material until it is all one phase then rapidly cooling it.

Coherency

Crystalline continuity. Solute atoms have the tendency to distort the host structure, and, if strain is great enough, those atoms break free to form their own crystal structure.

Tempering

Performed after hardening heat treatments, this operation reheats the steel to slightly lower temperatures, and is allowed to air cool afterwards. Leads to less hardness, but increased strength and ductility.

Maraging steels

Low carbon steels with superior strength and toughness while still being malleable.

Stress relief anneal

Metals heated to below recrystallization temperatures, then air cooled, to relieve internal stresses within the material.

Carburizing

Diffusion of carbon into FCC austenite steel at high temperatures.

Low carbon steel

0.2% carbon or less. Good formability.

Medium carbon steel

0.2% - 0.5% carbon. Best overall balance of properties.

High carbon steel

0.5% carbon or more. High hardness, low toughness, and poor formability.

Alloy steels

Steel containing more than 1.65% Mg, 0.6% Si, and 0.6% Cu.

Low alloy steel

Steels with less than 8% of total alloying element addition.

High alloy steel

Steels with more than 8% of total alloying element addition.

AISI - SAE identification system

• 1st digit: Identifies the major alloying elements

• 2nd digit: Designates a subgrouping within the major alloy system

• 3rd and 4th digits: Indicates the approximate amount of carbon in “points,” where one point equals 0.01% C.

Microalloyed steel

Steel with minimum carbon but high strength.

Stainless steel

Steel with chromium as an alloying element, which provides corrosion resistance.

Cast iron

Iron-carbon alloy with 2.11% or more carbon content. Produced specifically for casting.

Copper-zinc alloys

Forms brass, which has good corrosion resistance.

Copper-tin alloys

Forms bronze, which has good strength, toughness, wear resistance, and corrosion resistance.

Copper-nickel alloys

High thermal conductivity, and good corrosion resistance.

Aluminum and aluminum alloys

Nonferrous material that is lightweight, workable, corrosion resistant, and is thermally and electrically conductive.

Wrought aluminum alloy classification

• 1st digit: Indicates major alloying elements

• 2nd digit: Typically 0, but indicates changes to original alloy

• 3rd and 4th digits: Specifies the particular alloy within the overall family

Casting aluminum alloy classification

• 1st digit: Indicates the alloy group

• 2nd and 3rd digits: Identifies particular alloy or aluminum purity

• 4th digit: Separated by decimal point; indicates product form

Magnesium and magnesium alloys

Light, high strength-to-weight ratio, good damping capacity.

Titanium and titanium alloys

Strong, lightweight, resistant to corrosion.

Nickel-based alloys

Great strength and corrosion resistance at high temperatures.

Elastomers

Formed by linear polymers that are cross linked. Incredible amounts of deformation while retaining elasticity. Examples include rubber.

Plastics

Composed of hydrocarbons connected via covalent bonds; can be natural or synthetic.

Ceramics

Hard and brittle materials formed through primary bonds which lead to high strength. Boast extremely high temperature resistance.

Independent variables in forming processes

Aspects of a forming process where the engineer or operator has direct control, and are usually specified or selected during setup.

Includes:

• Starting material

• Starting temperature

• Workpiece / tool geometry

• Lubrication

Dependent variables in forming processes

Aspects of a forming process that are a direct consequences of the independent variables, or engineer / operator choice.

Includes:

• Force / power requirements

• Properties of the product

• Final (exit) temperature

• Surface finish and precision

Hot working

Plastic deformation that occurs above recrystallization temperature. Must closely control temperature to achieve desired properties.

Cold working

Plastic deformation that occurs below recrystallization temperature. Yields good properties and surface finish, but requires a lot of force. Produces anisotropy.

Bulk deformation processes

Deformation operations where the thickness / cross sectional area is reduced, or the shape of the material is significantly changed.

Sheet-forming processes

Deformation operations where the thickness and surface area of the material remain relatively constant.

Rolling (a bulk deformation process)

Reduce the thickness / cross-sectional area using the force between two rollers. Produces slabs, blooms, and billets.

Forging

Plastic deformation using localized forces. Uses dies to contact the workpiece and shape it.

Direct extrusion

A solid ram drives the material through a stationary die; lots of force required due to large friction force.

Indirect extrusion

A hollow ram pushes the die back through the material, which is stationary.

Drawing

Plastic flow of material over a curved axis, then formed into a part. Material is essentially pulled through the die.

Blanking

The die-and-punch operation where the piece punched out is the product and the rest is scrap.

Piercing

The die-and-punch operation where the piece punched out is the scrap and the rest is the product.

Progressive die sets

Sets of 2 or more dies and punches that are mounted and applied in tandem.

Transfer dies

Move the workpiece from one operation to another on a single machine.

Compound dies

Combine multiple processes under one stroke of the ram.

Bending

Plastic deformation along a linear axis that causes little to no change in surface area.

Casting

Metal heated above melting point, and molten metal is poured into a mold where it solidifies into the final part.

Parting line

Separates the cope and the drag.

Cope

Top half of the mold.

Drag

Bottom half of the mold.

Draft

The angle or taper that allows for easy removal of the cast from the mold.

Core

Solid piece inserted in the mold that results in a hollow cavity in the final cast.

Sand casting

Sand is the matrix in which the molten metal will be cast. Most common and versatile, and is cost effective.

Chaplets

Solid pieces of the material that will be cast that are inserted into the mold to hold a core in place. Are not removed from the final cast, integrated into the part.

Investment casting

Wax or plastic inserted into a mold matrix, then are melted away before the molten metal is introduced. The cavity left behind results in the shape of the final cast.

Lost-foam processes

Foam inserted into a mold matrix, then melted away before the molten metal is introduced. The cavity left behind results in the shape of the final cast.

Die casting

Molten metal forced into a mold under high pressure. Produces complex, high-quality parts, with a high-volume production potential. Little to no post-casting processing required.

Centrifugal casting

Molten metal forced into a mold by centrifugal force. Best for parts with a round outer profile.

Stages of powder metallurgy

1. Powder manufacturing

2. Mixing / blending

3. Compacting

4. Sintering

Sintering

Compacted powder is heated to a temperature below its melting point to create a uniform solid.

Welding

Consolidation of two metals by means of temperature and / or pressure. The metals melt / diffuse at the joint.