Thursday, February 12, 2015

Maximizing durability of MIG welded joints


Welded joints


MIG welding (aka, wire welding, gas metal arc welding) makes it possible for craftsmen who are part time welders to make high quality, good looking welds.  The equipment and consumables are relatively inexpensive.  The process is quick.  The technique required is within the reach of most people who have the ability to tie their shoes or can reliably hit the bristles of their toothbrush with a  bead of toothpaste.

I want to share a few tips that will help welders produce more durable welds.

The three Ps:  Penetration, penetration penetration.


The cardinal sin of MIG welding is inadequate penetration.  It is possible to produce a cosmetically beautiful weld with unacceptable amounts of cold lap.  Errors of excessive penetration are self announcing and self correcting.

Usually, the best solution to excessive penetration is to back-up the work with a heat sink.  A heat sink needs to be no fanciner than a very rusty piece of steel plate that is held 0.10 inches (2.5mm) away from the back of the work piece.

To increase penetration:
  • Increase wire speed.  Penetration on mild steel increases up to a wire speed of about 200 inches per minute.
  • Decrease voltage.  Decreasing the voltage reduces the "fanning" diameter.  That is, the weld bead is narrower.  The power is deposited over a smaller area and digs more deeply.
  • Reduce stick-out.  Choke up on the puddle.  Conversely, at the end of the weld bead when you are trying to fill the crater without blowing through the metal, you probably want to increase stick-out. 
  • Increasing the wire diameter.  Your equipment will have limitations but increasing wire diameter to the largest your equipment will handle will increase penetration.
  • Reorient the weld so it is vertical and the direction of travel is upward.  Discussed later in this essay.
One of the best welders I know claims to be able to weld anything with 18V and 180 inches per minute.  I do not have his expertise but it is still a great place to start.

Fillet welds:

Fillet welds are probably the most common type of weld for the home hobbiest.  They are typically used to attached "hard points" to structures.  These hardpoints are places where concentrated loads are applied.  Brackets, shackles, hooks, cleats and  pads are typical "hard points"  They are usually made of materials that are thicker than the base material.

The jargon.  Find the toe of the weld.  We will be talking about it later.
I apologize for the change in color schemes and the reversal of orientation.  The orange triangle represents the arc weld.  Three areas are identified: A, B and C.

Fusion typically first occurs at area A because there is less metal mass absorbing the heat of the arc.  It heats up more quickly and the corner melts.  An additional factor is that sharp edges attract arc.  That is why lightening rods are pointy and the business end of spark plugs have crisp corners.

Red arrows illustrating that 90 degrees of metal are wicking heat away from corner.  Fusion (penetration) first occurs at area A


Area B is more vulnerable to lack of fusion than area A because there is 180 degrees of metal wicking the heat away from the weld.

Area C is the most difficult place to get fusion to the base metal because there is 270 degrees of metal sucking the heat away.  Vertical-up welding works well when full penetration welds are required because the puddle of weld metal sags away from the parent metal rather than freezing to it.

The start and end are the parts of the weld where structural failures, cracks, are most likely to originate.  There are several reasons for this.  One reason is that the stresses are highest here even if the welds were perfect.  High stresses are the result of load funneling through a limited cross section of material.  All of the load from the area where the weld "skips" tries to crowd into the hard point through the start and end of the weld.

In fact, the weld quality at the start and end of the weld are rarely perfect.  The start of the weld is susceptible to cold lapping, inadequate leg length and sharp toe angles.  The end of the weld is vulnerable to lack of metal fill.  The weld technique to ensure good crater fill at the end of the weld is easy to master, so this essay will focus on the start of the weld.  Think of the start of the weld as the slider of a zipper.  The zipper will not unzip if you lock the slider.


One of the first things I was taught in driver's ed was to separate hazards.  Suppose you are driving and there is a car parked on your side of a narrow street.  Further, suppose you have oncoming traffic.  The prudent thing to do is to either speed up or slow down so you do not need to deal with both hazards at the same time, in the same space.

Most weld fab drawings have no "penalty" for extra length of weld.  Starting off-the-joint puts heat into the base metal ensuring good toe angles and no cold lapping where it counts.  I saw this technique used on the trailing-twist axle of a Nissan product.

Reflowing


Another technique is to reflow the weld by hitting select portions of the weld with a heat source like a TIG, plasma arc or oxy-acetylene torch.

Reflowing is the best (in many cases the only economical) way to smooth out sharp toe angles which look like blunt crack tips to the structure.    The beauty of reflowing is that 90% of the benefit can be attained by reflowing the start of the weld.  That is, the first 5/8" (15mm) of a typical weld bead.

Reflowing addresses the sharp toe angles that come with the high, humped up beads characteristic of weld starts.  Reflowing also addresses potential cold lapping (area B) and insufficient leg length by driving fusion into area C. 

Reflowing the start of the weld bead is one way of "locking the slider of the zipper" after the fact.  A handy thing to know if you job the MIG welding out and just don't have a good feeling about the quality of the job that was done.

6 comments:

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  6. Great post! Thank you for the helpful tips on MIG welding.

    - Long Peter

    ReplyDelete