Day 2 MELE (Paolo's Version)

Adaptive and Physical Metallurgy, Mechanical Me

Adaptive metallurgy

the manufacture of metallic products for society.

Mechanical metallurgy

field concerned with the response of metals to forces or loads. A solid starting piece (workpiece, blank or stock) is plastically shaped. Includes rolling, forging, extrusion and drawing.

Foundry (casting)

Molten metal solidifies in a mold whose cavity sets the shape. Used for small to very large parts, intricate shapes, and metals too brittle to deform.

Powder metallurgy

Compaction of powder followed by a heat treatment (sintering). Used for metals with low ductility or very high melting points.

Metal joining

Mechanical fastening, adhesive bonding, soldering, brazing, welding.

Rolling

Thickness reduced by a pair of revolving rolls

Forging

Compressive force (successive or continuous) exceeds the flow stress

Extrusion

Block forced (pushed) through a die orifice

Drawing

Metal pulled through a die (tension applied at exit; die reaction compresses)

Deformation

a change in dimension or shape caused by an applied load.

elastic

deformation is temporary (low stress and strain).

plastic

deformation is permanent (high stress and strain).

stress

force over area.

strain

the % change in length or area.

load cell

applies the load and measures resistance

extensometer

measures elongation

software

converts raw data to the σ–ε plot

Young's modulus (modulus of elasticity), E

the slope of the linear part of stress-strain curve

Proportional limit

the point where the curve departs from linearity

Elastic limit

onset of plasticity (often equal to the yield strength)

Resilience

the capacity to absorb energy elastically and recover it on unloading

Yield strength

the limit of usable elastic behavior (also called the offset stress)

Flow stress

the instantaneous stress needed to keep deforming plastically

UTS

the maximum of the engineering curve; necking starts there

Fracture point

stress at breaking

Toughness

energy absorbed up to fracture

Elastic recovery

When the load is released after plastic deformation, the unloading line runs parallel to the initial elastic line. The elastic part of the strain is recovered, and the plastic part remains as permanent set.

Principal stresses

the normal stresses on the orientation where all shears are zero.

von Mises

maximum distortion energy

von Mises

yielding occurs when the distortion energy reaches that at yield in uniaxial tension. It does not depend on any particular normal or shear stress.

Tresca

maximum shear

Tresca

yielding occurs when the maximum shear stress reaches a critical value

Tresca

easier to use (only the largest and smallest stresses are needed), ignores σ2, and is more conservative.

Direct compression

Force applied ⊥ to the surface; metal flows ⊥ to the force

Indirect compression

Primary force (often tension) does not shape directly; the die's reaction produces high compressive stress

Direct compression

Forging, rolling are examples

Indirect compression

Wire/tube drawing, extrusion, deep drawing are examples

Tension

Metal pulled; directly shapes the work

Bending

Bending moments applied

Hot working

Tm − 50 > T > Trec. Recovery and recrystallization occur during deformation. Strain rates 0.5–500 s−1 with little or no strain hardening. First step from cast ingot to wrought product.

Cold working

Below Trec: the recrystallization rate is slower than the strain rate. Strength and hardness rise, ductility falls.

Hot working

product is: Oxidized, discolored surface; poorer tolerance; soft, low strength, high toughness and ductility; equiaxed recrystallized grains; isotropic

Cold working

product is: Bright, clean surface; good tolerance; high hardness and strength, low toughness; elongated deformed grains; anisotropic

Sticking friction

is the tendency of the surfaces to adhere rather than slide. It is prominent in rolling and is reduced by lubrication.

forging

shapes the workpiece with compressive forces. It is the oldest metalworking operation (≥ 4000 BC).

impact

stress application in forging that is quick

press

stress application in forging that is slow

open-die

die in forging: flat or simple dies, metal flows unconstrained laterally; large objects or few parts; simple shapes, usually machined after

Closed-die

die in forging: deformed between die halves carrying the impression; high pressure in a closed cavity; precision, near-net or net shape, often no machining

fiber flow lines

the control of grain flow is forging's principal advantage over all other metalworking processes. The grain orientation is designed to strengthen the part for its application.

Hydraulic press

Constant speed, load-limited (stops when capacity is exceeded); up to 125 MN open-die and 730 MN closed-die; slow, so guard against premature cooling

Mechanical press

Motor rotation converted to translation (crank); speed is maximum at mid-stroke and zero at the bottom; force is maximum at the end of the stroke; 2.7–107 MN; high rate, easy to automate

screw press

1.4–280 MN; small quantities, thin precision parts

Gravity drop hammer

Free-fall ram of 180–4500 kg; energy = weight × height; up to 120 kJ

Power drop hammer

Ram accelerated by steam, air or hydraulics; 225–22,500 kg; up to 1150 kJ

Counterblow hammer

Two rams approach each other; up to 1200 kJ

Smith forging

open die, hot, hammered manually

Drop forging

drop hammers replace the blacksmith

Upset forging / heading

enlarges the end cross-section; the die sets the head shape

Cogging / drawing out

thickness reduced incrementally with flat dies.

bite

The advance per step in forging

Swaging

reduces the diameter of a tube or rod by impacts around the circumference

Edging

concave dies; metal flows into an area from both sides.

Fullering

convex dies; metal flows out of an area to both sides

Coining

closed die; detail impressions on coins, medallions and jewelry

Barreling

forging defect: non-homogeneous deformation from die friction or a thermal gradient

Laps/folds

forging defect: metal folded over and forged into the surface; also caused by too little material. They act as stress raisers and cause machining or heat-treatment cracks.

Cold shuts

forging defect: defective metal flow; lower strength and act as stress raisers

Poor grain structure

forging defect: from overheating, wrong billet size or poor design. Properties are poor in critical directions and fatigue risk rises.

Bursts/cracks/tears

forging defect: from poor die design or practice, or inclusions.

Underfill

forging defect: not enough material

Flash

forging defect: excess metal squeezed out between the dies

Rolling

reduces thickness or changes the cross-section using compressive forces from two opposing rolls that rotate, pull in and squeeze the work.

Bloom

Square, ≥ 150 × 150 mm

Slab

Rectangular, t ≥ 40 mm, w ≥ 250 mm

Billet

Square, ≥ 40 × 40 mm

Plate, sheet

metal product that has: t > 6 mm; t < 6 mm

Spreading

(width increase) is a problem at low width/thickness ratios. Vertical edge rolls keep the width constant.

Two-high pullover / reversing

Stock returned to the entrance, or passed back and forth by reversing the rolls

Three-high

Upper and lower driven rolls, middle roll turned by friction; an elevator reverses the work each pass

Four-high

Small work rolls backed by large backup rolls: lower roll force and power, less spreading, cheaper to replace

Cluster (Sendzimir)

More backing rolls, so very small work rolls; cold-rolled high-strength sheet 0.66–1.5 m wide; foil

Tandem (continuous)

Series of stands; reduction and speed at each stand are critical and computer-controlled

Planetary

Heavy backing rolls ringed by planetary rolls. Each gives a near-constant small reduction, and the total is their sum. Needs feed rolls and planishing rolls. Hot-reduces slab to strip in one pass.

Flatness problems

the roll gap must be perfectly parallel. A 1 part in 10,000 difference in elongation causes waviness. Flatness depends strongly on speed.

camber/crown

Roll deflection is corrected by

Insufficient camber

thicker centre, edges elongate more. Residual stress is tension at the centreline and compression at the edges

centreline cracking, warping, edge wrinkling

manifestations of undercamber

Over-camber

thicker edges, centre elongates more. Tension at the edges, compression at the centre.

edge cracking, centerline splitting, centerline wrinkling

manifestations of overcamber

Alligatoring

non-uniform bulk deformation (or centreline weaknesses such as impurities) splits the work at the centre.

Surface defects

scratches, scale, cracks. Remove them before rolling

Extrusion

forces a billet through a die orifice under high pressure. The die sets the reduction and the cross-section, which may be solid or hollow and is uniform along the length.

Direct (forward)

extrusion type: Ram pushes the billet through the die. A dummy block (slightly smaller than the billet) protects the punch and leaves a skull. A butt end remains.

Indirect (backward)

extrusion type: Die on a hollow ram moves into the billet. Product flows opposite to the ram. No billet– container motion, so friction is only at the die.

Hydrostatic

extrusion type: Fluid between the ram and a smaller billet transmits the pressure. Triaxial compression improves formability.

Lateral

extrusion type: Vertical chamber, die on the side

Impact

extrusion type: Punch strikes a slug rapidly (mechanical press, short stroke). Thickness = punch–die clearance.

smaller die angle

in extrusion, increases friction (more billet–die contact).

larger die angle

in extrusion, gives more turbulent flow and bigger dead zones.