Lecture 19 Welding 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 Weld steel Pb-Sn solder 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 BevelFlare V

Weld Microstructures Columnar grains Mixed dendrites with grains Equiaxed Recrystallized, no GG Weld Metal Fusion Zone Base Metal Fusion Line HAZ

Max Temp During Welding Liquidus temperature Solidus temperature Recrystallization temperature Base metal Heat affected zone Fusion zone Cold-Worked Welded Metals “as cast” “weakest part”

Max Temp. Liquidus temperature Solidus temperature Pearlite   L Structure at max temp High hardenability steel Low hardenability steel Martensite Welded Steel Microstructure Austenite temperature Eutectoid 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 Temp Liquidus temperature Solidus temperature Solvus temperature Overaging temperature L  +L  ’  ”   ’   ” Base metal Overaged zone Fusion zone Partial fusion zone Solution 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 Stress range, MPa 1E+061E+071E+08 Cycles 1E 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” Weld steel Metal Joining Processes Pb-Sn solder 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 C 2 H 2 - 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 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 – low hydrogen (prevent H cracking) – 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 –C 2 H O 2  2CO 2 +H 2 O+heat –Up to ~3400 °C –CO 2 and H 2 O “protect” weld Not as good as inert gas or slag Danger! C 2 H 2 with Cu, Hg, Ag form acetylides - violently explosive Very cheap, versatile Low heat, low penetration

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