Lecture 19Welding

ME 330 Engineering Materials

Terminology Metallurgy Weldability Defects Processes Variables

• Most products, machines and structures are made of parts• Fastening and joining are essential parts of design and manufacturing• Welding, brazing, and soldering are common metal joining processes• Welding is most adaptable and provides strongest connection• Good examples of:

– Solidification microstructures and properties – Heat treatment of metals

Metal Joining Processes

Weldsteelsteel

Pb-Sn solder

Cu

Cu

Typical Weldment

Types of Joints

Weldment = Joint type + Weld type

Types of Weld

Types of Groove Welds

Square

V-Groove

Bevel

Double V-Groove

J-Groove

U-Groove

Flare Bevel Flare V

Weld Microstructures

Columnar grains Mixed dendriteswith grains

EquiaxedRecrystallized, no GG

Weld MetalFusion Zone Base MetalFusion Line HAZ

Max

Tem

p D

urin

g W

eldi

ngLiquidus temperature

Solidus temperature

Recrystallization temperature

Base metal

Heat affected zoneFusion zone

Cold-Worked Welded Metals

“as cast” “weakest part”

Max

Tem

p.

Liquidus temperatureSolidus temperature

Pearlite

LStructure atmax temp

Highhardenabilitysteel

Lowhardenabilitysteel

Martensite

Welded Steel Microstructure

Austenite temperatureEutectoid temperature

Weldability• Weldability: Ability of a particular alloy to be welded without

substantial embrittlement due to martensite formation– Generally the opposite of hardenability– Of particular concern for high strength steels!– High strength steels are heavily alloyed, shifting TTT curves– Some alloys, under severe thermal cycling, will recrystallize, grow

grains, and even age in heat-affected zone

• Concept of effective carbon content:– Higher carbon content harder to weld– Carbon equivalent = %C+%Mn/6 + %Ni/15 + %Cr/5 + %Mo/4 +%V/5

Welding of Aluminum

• Not all metal alloys are “weldable”• Many alloys in the 1xxx - 5xxx series can be welded• Heat treatable high strength alloys (7xxx) are not

weldable due to heat affected zone (HAZ) embrittlement• Often weld underaged alloys

– Artificially aged in HAZ• Difficult issues:

– Oxide layer usually forms– Thermal conductivity very high

• Very large HAZ– Melting point very low

• No visual indication

Max

Tem

p

Liquidus temperature

Solidus temperatureSolvus temperature

Overaging temperature

L

+L ’ ”

’ ”

Base metalOveraged zone

Fusion zone

Partial fusion zoneSolution treated zone

At peak temperature

After cooling

Precipitation Hardened Metals

• Huge thermal cycling in welding leads to shrinkage and residual stress:

• In nearly all cases, weld metal and HAZ end up under residual tension

Shrinkage in a butt weld

Shrinkage in a fillet weld

Residual Stress and Warping

• Common problem: Cracking of weld or surrounding material due to residual stress, embrittlement of HAZ

• Related problem: Dramatically reduced fatigue life, weld typically is under surface tension

• One solution - shot peen to induce residual compression

Stre

ss ra

nge,

MPa

1E+06 1E+07 1E+08Cycles

1E+05

10

100

1000 Typical case

Base plate

Weld

Flaws and Properties of Welds

• Welding, brazing, and soldering are common metal joining processes

• Welding: highest temperature melt workpiece and add filler material

• Brazing: Medium temperature diffusion bond between molten filler and solid workpiece

• Soldering: Low temperature molten filler used as “adhesive”

Weldsteelsteel

Metal Joining Processes

Pb-Sn solder

Cu

Cu

Welding processes may be grouped into three categories: 1.the method used to supply heat to the weld; 2.the cleaning and preparation of the joint faces; and 3.the prevention of contamination from the air during welding.

Typical classifications for generating heat are:Mechanical: Heat is generated by friction between the workpieces or from rapid local deformation of the material. An example of the former is friction welding. One of the workpieces is rotated at high rate and then the workpieces are forced together. The friction between the two surfaces generates enough heat to liquefy the metal and fuse the components.Thermochemical: Heat is supplied by an exothermic chemical reaction. Two examples are flame welding and plasma arc welding.Electrical Resistance: Heat is generated by the resistance of the workpiece to a large electric current. An example is spot-welding.Electric Arc: Heat is supplied by an arc between an electrode and the workpiece. This process is typically performed at low voltages and high currents. A common example is gas-tungsten arc welding.Radiation: Heat is supplied by a suitably focused beam of radiation. Examples include electron beam welding and laser welding.

Metal Joining Processes

Welding Processes• Arc Welding

– Shielded Metal Arc (SMAW)• “Stick or Arc”

– Gas Metal Arc (GMAW)• “MIG” - metal inert gas

– Gas Tungsten Arc (GTAW)• “TIG” - tungsten inert

gas

• Resistance welding– Spot welds– Friction Welds

• Solid-state welding – No melting – direct

joining of pieces using pressure, plastic deformation, friction, etc.

• Diffusion Welding

• Gas Welding– Oxy-acetylene

• C2H2 - very explosive!– MAPP

• Methylacetylene propadiene

• Not as common– Propane

• Not enough heat to melt most metals

• Often used for soldering

• E-beam, laser welding:– Electron or photons

accelerated at surface and melt workpiece to form weld.

– Expensive, precise.

Arc Welding (SMAW)

• Very cheap apparatus• Simple to switch materials• Use in many

environments

• Dirty• Slow• Slag is a pain

Arc Welding (SMAW)

• Current heat– Thickness– Material– Rod size– Rod type

• Arc length– ~1/16” typical

• Travel speed• Electrode angle

Need to control:

Electrode Polarity

• electrons generate 70% of heat• DCEP

– Most of heat on electrode– Burn off electrode faster– Concentrate heat– Better penetration

• DCEN– Most of heat on base metal– Better for thin metals– Low penetration in thick metals

• AC Welding– Heat 50-50 - mid penetration– Easier to use– Need special electrodes

++++ - -

- --

-

+

DCEP

Current

EP

EN

AC Welding

A Few More Electrode Points

• Protective gas– Keeps out O, N– Prevent voids,

embrittlement• Slag

– Scavenges oxides– Controls cooling

• Usually run different currents for different positions

• Overhead is by far most difficult, painful

Electrode Definitions• E XXYZ

• E = Electrode• XX = Tensile strength in ksi

– e.g. 60 ~60,000 psi– Slightly higher than base metal

• Y = Permitted positions– 1 = all– 2 = flat and horizontal– 3 = flat only

• Most common– 6011– 6013– 7016

• Z = Coating on rod – 0/1 - cellulose coating, DC/AC

• Little slag “fast freeze”• Great penetration, low deposition• Hard to operate (& remove slag)

– 2/3 - Rutile, DC/AC– Medium penetration, deposition

• Mainly cosmetic• Very easy

– 7016 - low hydrogen (prevent H cracking)– 7018 - low hydrogen with Fe powder

• Excellent deposition• Very good penetration• Fairly easy

MIG Welding (GMAW)

• High deposition rate• Uninterrupted weld• Low fumes, spatter• No slag!

• Higher skill (??)• More complex equipment• Need controlled

environment

EP

TIG Welding (GTAW)

• Very high quality welds!• Almost any metal/alloy• Little postweld cleaning

– No spatter – No slag

• Arc & weld pool clearly visible• Higher skill (!!)• Slow• Sensitive, costly equipment• Need controlled environment

EN

Oxy-Acetylene Welding

• Oxygen-Acetylene flame– C2H2+2.5O22CO2+H2O+heat– Up to ~3400 °C– CO2 and H2O “protect” weld

• Not as good as inert gas or slag

• Danger! C2H2 with Cu, Hg, Ag form acetylides - violently explosive

• Very cheap, versatile• Low heat, low penetration

Workpiece Workpiece

Before After

Brazing• Typically done at > 450 °C, so

filler metal melts, but workpiece doesn’t

• Furnace brazing, Torch brazing, Induction brazing, Resistance brazing

• Often done with steel/silver or steel/copper

• On melting, filler must “wet” workpiece– Cleanliness is key– Flux used to clean and protect

braze pool• Key: diffusion of solid into

liquid

San Jose State University / Cronos

Soldering• Typically done at T< 450 °C, so filler metal

simply acts as an adhesive• Lead-Tin alloy is a common solder material

– Microelectronics packaging– Plumbing

New Concepts & Terms• Welding microstructure

– Material differences– Relationship to casting processes

• Joining processes– Understand basic differences between

welding, brazing, & soldering– General understanding of +/- of welding

techniques

Next Lecture ...• Polymers

– Mer structure– Molecular structure & shape– Isomerism– Copolymers– Crystallinity

• Please read chapters 14 & 15Please read chapters 14 & 15• Beginning of Nonmetallic Material coverageBeginning of Nonmetallic Material coverage