Common weld defects and their causes: porosity

Topics: Laser welding

Laser welding

Alright, now that you’ve read the “Misconceptions About Welding” blogs, you want to know what else you might not know about welding, huh?  So, let’s talk defects.  In this blog we’ll discuss porosity.

I like to talk about porosity more than other weld defects because even though there are many contributing factors to it, at the end of the day it is always (did I use an absolute?) trapped gas in the solidifying weld.

-How do you trap gas in a solid material?  Well, start by making that solid a liquid.

-How do I do that?  Melt it.

-With what?  Doesn’t matter.  Porosity is a common problem across all types of welding, and other processes that involve molten metal as well – think casting.

-Where does the gas come from?  Lots of different places. Let’s list some:

  • The parent material. We already touched briefly on this in blog #2 (If it's metal, I can weld it).  Some elements evaporate at the melting point of other elements.  These light elements turn into gas bubbles in the weld pool when they rapidly evaporate under intense heat.
  • The parent material. Yes, I wrote it twice.  During high energy density welding, i.e. laser welding or electron beam welding, some of the parent material goes beyond melting and actually evaporates.  Yes, you really can make metal vapor.
  • Dissolved gasses. Some alloys have gasses deliberately dissolved into them to add beneficial characteristics.  Heating these up is like shaking a bottle of soda then popping the top, except instead of a shower of sweet, syrupy liquid, there will be a shower of glowing, molten metal spraying in your direction.
  • Trapped atmosphere. During very high power welding, there can actually be enough of a crater in the weld that molten metal can “slosh over” and trap some of the local atmosphere in the weld.
  • Contamination. This is a topic in and of itself, but this could be oil, dirt, water, paper, tape, plastic -- you name it, I’ve seen it.

-What does all this mean to me?  Well, that depends on what you want to accomplish.  It might be harmless, or it might be the initiation point of a catastrophic failure.  This is another case where you want to collaborate with someone in the know.  Put your design brains in the same room with the welding brains and talk through your options.

-What are my options?  That’s a topic for another blog, or series of blogs.  There are LOTS of ways to deal with common weld problems.  Stay tuned, we’ll touch on many of them in upcoming blogs.


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