3.37 (Class 15) Review More materials can be brazed than soldered • Higher temps • Greater strengths Thinner braze joints are stronger (in tension) due to contact strengthening Today Fusion Welding Processes Interested in heating the material to melt it Diagram on the board (W/cm^2) • Sun’s rays approx 1/10 W/cm^2 • Two limits for heating surface of metals o Below approx 300 W/cm^2 can’t melt most metals, thermal conductivity of metal will conduct heat away faster than can store it on the surface o Above approx 3x10^6 W/cm^2 start vaporizing the metal (laser and electron beam hole welding approx 10^7, laser weapon at approx 10^8 shatters the material) o 10^3 oxy-fuel (typically acetylene) approx 10^3 o Air-fuel flame (propane torch) below 10^2, also semiconductor chip o Open flame, just above 10 o Arcs approx 10^4 o Resistance welding approx 10^5 o Aside on electron beam weapons, propagate beam for 30 miles, but have hose instability, generate very high densities, too high to be useful for welding • Increasing heat efficiency as power density goes up o 10^3, may be about 0.1 efficiency, have to preheat more material o 10^6, at about 0.99 efficiency o Middle, from about 0.3-0.7 • Decreasing Heat Affected Zone (HAZ) size as power density goes up o Approx 1 – 10 cm at 10^3, controlled by heating time o Approx 0.1 – 0.5 cm at 10^4 o Approx 0.1 – 0.5 cm, at 10^6 no smaller after above 10^4, not putting lots of extra heat in during the heating cycle, controlled by how long it takes to cool o Common fallacy is to try to eliminated the heat affected zone by using electron beam weldingo Only time saw weld with virtually no heat affected zone, was plutonium at LLNL, like a ceramic, has very low thermal conductivity • Increasing travel speed as power density goes up o Approx 0.01 – 0.1 cm/s at 10^3 o 0.1 – 1.1 cm/s at 10^4 o 100 cm/s at 10^6 o Controlling size of the weld pool o Human reaction time on the order of 150-200ms, bill drop game on the order of 1/10th of sec o Time to control weld pools 10’s of ms at high energy densities, can’t control manually o Manual welding training often starts with oxyacetylene, slower so that can watch the weld pool and carefully control it • Increasing need to automate as power density goes up o Lasers and electron beam need to be automated to use them • Increasing equipment cost as power density goes up o Can approximately change W/cm^2 to $/capital equipment o Oxyacetylene kit can be had for about $1000 o Arc welding setup for production welding approx. $10,000 o Laser electron beam with automation, safety equipment, approx $0.5 - $1million, bigger systems can be $10million o Resistance welding is the only process that doesn’t fit this, $10,000 for equipment that gives the equivalent power density, HAZ, need to automate, travel speeds, etc., no surprise that this is heavily used in the automotive industry • Increasing production volume requirements as power density goes up o Oxyacetylene approx 10 joints/day (say for plumber who needs to do just a few welds) o Arc welding approx 100 joints/day o Laborer’s time, efficiency is very low o Automotive (only know how to make 50,000+ per day)  Run almost all the time o Aerospace (precision and high value added), few expensive parts  On-time is about 1-2%  Care more about making the perfect weld • Increasing depth/width ratio o Diagrams on board o 0.1 o 0.5 o 10/1 o 200/2 (too thin, traps porosities during non-homogenous cooling) Flames • Enthalpy of the reaction o Acetylene (C2H2, triple bonded carbon with hydrogen on either side, lots of energy released when break this bond)o Propylene and other complex hydrocarbons start to average out to about the same o Gasoline not much different than polyethylene, or say tar o Some things have higher enthalpies than acetylene  C2N2, Cyanagen, used a rocket propellant, poison gas, and welding gas (never seen it used)  H2N2, Hydrazine, used in nuclear reactors • Stoichiometry of the oxygen to fuel ration o From the Welding Handbook o Stochiometric mixture is near the peak o Rich or lean will drop the temperature considerably, have lots of unburned fuel, create extra baggage since not all atoms participate in the reaction (like society and welfare) • Presence of inerts (for example Nitrogen in