What factors affect the penetration depth and weld deposition rate in arc welding?
How does polarity affect the penetration depth and weld deposition rate?
My lecturer said that penetration depth is higher when using straight polarity when compared to reverse polarity.
But I've read in a textbook that it's the opposite.
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3$\begingroup$ Adding your own understanding of the process and any research you've done on the topic will help this from getting closed or put on hold. We generally frown upon "go find me answers" type of questions without first seeing the effort you've put into the topic. $\endgroup$– grfrazeeCommented Sep 14, 2015 at 18:48
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2 Answers
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Quick answer, you are both right and wrong. Arc Welding has many types and for Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), Flux Cored Arc Welding (FCAW) and SAW processes electrode positive polarity increases depth but for Gas Tungsten Arc Welding (GTAW) electrode negative polarity increases penetration depth. I'm not at the office so can't give reference from books but here is a page from Lincoln Electric .
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$\begingroup$ Can you please explain how polarity influences penetration depth? $\endgroup$– M.TarunCommented Sep 16, 2015 at 6:00
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1$\begingroup$ More heat is generated at cathode than anode. Heat increases fusion depth. For GTAW more heat is needed at torch. Please check TWI documentation for example this page twi-global.com/technical-knowledge/faqs/process-faqs/… $\endgroup$ Commented Sep 16, 2015 at 7:52
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The practical effects of polarity of quite complex and depend on the process.
In a sense TIG is where the 'pure' effects of polarity are clearest as the arc creation is entirely separate from the filler feed. Here DC- (electrode -ve) is used for most metals except for aluminium and magnesium alloys. This polarity puts the majority of the energy into the weld puddle which is both more efficient and helps prevent the tungsten electrode from overheating. For aluminium the problem is that the thick, stable oxide layer needs to be cleaned off as welding progresses lest it contaminate the weld. Here reverse polarity is used as the electron flow tends to rip the oxide off the surface. You can actually see this happening in a TIG weld and a fresh weld will have a 'frosted' area around the weld bead.
In practice most modern machines now use a modified AC current for aluminium as this allows more adjustment of the arc properties. Specifically the balance between +ve and -ve current flow balances cleaning effect with electrode temperature and adjusting the AC frequency controls the width of the arc (which in turn affects penetration). The most sophisticated machines also a range of different wave-forms (square, sine, trapezium, triangular etc) which also affect penetration and heat distribution. Note also that in AC TIG the end of the electrode will melt and form a ball slightly larger than the electrode diameter the fast cycling AC stops it from falling off into the weld. In DC welding the tungsten should not melt at all.
In MIG welding the fact that the filler wire is also the electrode means that heat is transferred to the puddle in a different way and the shape and shape and size of the metal droplets formed is very important. This favours putting more heat into the electrode as it is constantly melting and physically transferring that energy to the weld puddle in any case. So solid wire MIG welders conventionally run with reverse polarity.
In MMA things are more complex again as the composition of the electrode coating interacts with the arc polarity and the results are really a function of the particular electrode type and most electrodes will have a recommended polarity.
This subject is also further confused by the fact that conventional current is actually backwards in its sign convention compared to the rest of physics (ie the actual flow of electrons is from -ve to +ve) which doesn't make this any easier to understand.
answered Aug 9, 2016 at 11:42