1. With the assistance of a diagram, illustrate the equipment for GMAW shown during the research lab session. Indicate its chief operating variables and explicate how they affect weld formation.
Figure – GMAW equipment -source: hypertext transfer protocol: //www.g-w.com/PDF/SampChap/56637_9878_CH07.pdf
Gas metal discharge welding ( GMAW ) is a procedure in which the welding occurs by an electric discharge that forms between the electrode and the base metal. The electrode used for this procedure is a consumable one and these vary depending on the base metal used. The one shown in the lab session was made up of mild steel coated with Cu. This type of electrode eases the procedure flow and it does non corrode. The electrode is fed through the nose or welding gun by reels or membranophones and a thrust unit ( 1 ) . The electric discharge on the workpiece is formed between the consumable wire and the base metal where a weld pool is formed ( 2 ) . The dyer’s rocket is formed by the hardening of the liquefied weld pool by the motion of the discharge in the coveted way whilst the electrode is continuously fed through the nose to maintain the electric discharge alive onto the base metal ( workpiece ) ( 3 ) . The screening gas ( 5 ) is besides present in the nose together with the electrode. The intent of the shielding gas is to protect the discharge and weld pool from any contaminations that may impede a nice and smooth dyer’s rocket. The discharge is supplied by electric energy throughout the procedure ( 4 ) .
Main runing parametric quantities for GMAW:
For GMAW a reversed mutual opposition direct current is used. This type of current offers a stable discharge, smooth transportation with good incursion and fewer splatters on the welding country. Although the deposition rate is significantly low to that compared with direct current direct mutual opposition, but despite this fact it is still used over the latter 1. A changeless electromotive force is besides applied to the GMAW. This will help in keeping a changeless discharge throughout the procedure. This is done by the ability to self adjust the arc length. When the welding gun or nose is near to the base metal, the machine will present high current so that the inordinate discharge length is burnt so that it will be stable once more. On the other manus if the welding gun or nose is farther off from the base metal the machine will present low current supply so the discharge length will recover its length.
The velocity of the dyer’s rocket determines the quality of the terminal consequence every bit good as the possibilities of future defects in the dyer’s rocket. If the velocity is excessively low so there is incursion and merger in surplus. If overheating occurs this may take to pores inside the dyer’s rocket and overheating of the heat affected zone will ensue into coarse grains which in bend will impede some mechanical belongingss. The weld pool formed will be larger than the usual one therefore it will get down to organize in front of the electrode. On the other manus if the velocity is fast a thin and tall dyer’s rocket will be formed caused by deficient merger and incursion.
The shielding gas, as the name implies protects the discharge and the weld pool from any contaminations that are present in the air such as O, N, H2O vapor and C dioxide. Its flow rate must be right calibrated so at that place would non be high or low flows. Shielding gases that were mentioned during the lab session where Helium, Argon and a mix of both, but these depend on the base metal being used together with the electrode chosen. The shielding gases are inert gases therefore they do non chemically respond with the base metal that is why inert gases are chosen as shielding gases.
2. Sketch and label an ideal filet dyer’s rocket. Indicate common filet dyer’s rocket defects and sketch how the defects are normally caused.
An ideal filet dyer’s rocket should be somewhat bulging as shown in the image underneath.
hypertext transfer protocol: //www.weldguru.com/images/fig6-13.gif
Figure – Ideal filet dyer’s rocket, Beginning: hypertext transfer protocol: //www.weldguru.com
Figure – Slag inclusions, Beginning: hypertext transfer protocol: //www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/TechCalibrations/RadiographInterp.htmSlag inclusion – occurs when solid stuff gets entrapped between weld base on ballss or in the dyer’s rocket itself. This happens when the dyer’s rocket is non cleaned decently after weld passes.http: //www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Graphics/RadiographInterp/SlagInclusion.jpg
Figure – Insufficient merger, Beginning: hypertext transfer protocol: //www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/TechCalibrations/RadiographInterp.htm
Insufficient merger – occurs when a dyer’s rocket is discontinued throughout the cross subdivision of the joint and therefore merger does non decently occur between the base metal and the dyer’s rocket metal.http: //www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Graphics/RadiographInterp/LackofFusion.jpg
Insufficient incursion – occurs when merger has non penetrated deep plenty into the root of the joint, comparative to its joint designs ‘standard demands. This may ensue from deficient heat and control of the electrode.
Figure – Insufficient incursion, Beginning: hypertext transfer protocol: //www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/TechCalibrations/RadiographInterp.htm
Undercut – it is a channel that has been melted off. This occurs between the weld-and-base metal junction and normally happens at the toe of the filet weld or at the root of the weld.http: //www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Graphics/RadiographInterp/ExternalUndercut.jpg
Figure – Undercut at toe ( above ) , undercut at root ( below ) , Source: hypertext transfer protocol: //www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/TechCalibrations/RadiographInterp.htm
Figure – Porosity, Beginning: hypertext transfer protocol: //www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/TechCalibrations/RadiographInterp.htmPorosity – occurs when dyer’s rocket is discontinued, hence bubbled gas is entrapped during the hardening of the dyer’s rocket metal. This entrapped gas will organize pores inside the dyer’s rocket which will take to clefts. Porosity chiefly occurs due to the improper control of electrode or because of the taint of the base metal.http: //www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Graphics/RadiographInterp/Porosity.jpg
Figure – Spatter, Beginning: hypertext transfer protocol: //www.scribd.com/doc/29867411/Welding-MetallurgySpattering – are little liquefied weld droplets that are formed along the weld country. These droplets are caused by the perturbation in the liquefied pool during the welding itself. Low electromotive force or high currents are 2 parametric quantities that affect splatter, so does the instability of the electrode during the welding procedure. hypertext transfer protocol: //htmlimg2.scribdassets.com/j86oy3351es2fuo/images/38-f25fa57832/000.jpg
3. Describe the microstructure of the heat affected zone for individual stage work hardened metals and allotropic stuffs following commercial manual metal discharge welding.
The heat affected zone is an country on the base metal that experiences high welding temperatures but non high plenty to run the whole country. The metallurgical alteration and belongings change of the base metal in the heat affected zone depend on a figure of factors such as the base stuff itself, the heat input and clip of warming every bit good as the filler metal being used. hypertext transfer protocol: //htmlimg4.scribdassets.com/j86oy3351es2fuo/images/12-4d39bc7739/000.jpg
Figure – Fusion weld joint, Source: Fundamentalss of modern fabrication. Mikell P. Groover
Microstructure for individual stage work hardened metals:
The heat affected zone of a work hardened alloy will change depending on the distance to the merger zone. Since the metal has been antecedently work hardened it has many disruptions and residuary emphasiss. The heat that is generated by the welding procedure will be plenty to take the residuary emphasiss that were antecedently formed. Besides at high temperatures the disruptions present will be largely removed by the new grains that form at the cell boundaries. This process is called recrystallization and it largely occurs closest to the merger line and reduces as the recrystallization temperature is decreased. Subsequently grain growing will follow this procedure as the temperature is still above the recrystallization temperature. Therefore, coarse grains will develop following to the merger line followed by refined grains as we go closer to the unaffected base metal country.
Microstructure for allotropic stuffs:
Allotropic stuffs are different from individual stage work hardened metals as these will exhibit a stage alteration due to the heat supplied by the welding procedure. Taking into consideration field C steel, at first the stage is all ferrite and pearlite. After the dyer’s rocket is deposited, the heat affected zone ‘s stage alterations to austenite. Grains which are closer to the merger line will be coarser due to grain growing. Whereas grains which are farther off from the merger line are finer because there was n’t adequate clip for the grains to undergo a full grain growing.
4. Write notes on the hardening construction, infrastructure and microstructure of the merger zone for metals that experience solid province transmutation during chilling. Give particular mention to multi-pass manual metal arc welding of unalloyed field C steel and individual base on balls autogenously laser welded AISI 316 chromium steel steel.
The merger zone is that country where the filler stuff has been deposited during the welding procedure. The belongingss of the merger zone depend chiefly on the filler stuff used and the basal stuff.
The base metal of such metals that experience solid province transmutation will move as a nucleating substrate ; grains will nucleate on already bing grains due to the heat applied. During hardening, these grains tend to solidify in an epitaxial growing and perpendicular to the dyer’s rocket pool. This epitaxial growing will depend on the hardening of the dyer’s rocket pool because this will besides command the grain sizes and their form of how they are traveling to be solidified.
The hardening and chilling rate will coerce the infrastructure of the merger zone to change from planar to cellular to columnar dentritic and eventually to equiaxed dentritic. This hardening of such infrastructures will depend on the temperature gradient of the merger zone, chiefly along its centreline. The faster the chilling rate is will depend on the formation of equiaxed and columniform dendrites near the merger centreline following by cellular and two-dimensional infrastructures next to the merger boundary line.
Multi-pass manual metal discharge:
Figure – multi base on balls welding Beginning: hypertext transfer protocol: //mewelding.com/welding/ ? p=51Taking into consideration unalloyed field C steels and using multi-pass welding will ensue in grain polish with every welding base on balls. If we look at the image on the side ; the first welding tally was made ( 1 ) , its heat affected zone would be on either sides of the dyer’s rocket. When the 2nd welding tally is deposited ( 2 ) , its heat affected zone would be on its right side every bit good as some of the old first dyer’s rocket ( 1 ) deposited. The procedure of every dyer’s rocket tally will hold a antecedently weld tally as its heat affected zone. Therefore that is why grain polish occurs at every dyer’s rocket tally that is made. As already mentioned before, the high temperature of the dyer’s rocket will turn the ferritic and pearlitic microstructure into austenite and upon chilling, the microstructure will turn once more into ferrite and pearlite. Shot peening is a method that is frequently used after every dyer’s rocket run to take any residuary stressed caused. Multi base on balls Welds welding techniques
Single base on balls autogenously laser welded AISI 316 chromium steel steel:
When autogenously laser welding is applied to AISI 316 chromium steel steel, the temperature of this procedure compared to manual metal discharge is higher. Therefore delta ferrite is formed upon the welding procedure. Upon hardening of the dyer’s rocket, at the merger Centre line, delta ferrite dendrites will organize. Upon chilling, the microstructure of ferrite will turn wholly to austenite.