Structural MIG / GMAW welds in steel fabrication

To disambiguate which welding process is being applied:
synonyms for this welding process are: MIG; MAG; GMAW; "wire-feed"; ISO4063 process-reference-number 135
(active gas is used; so "MAG" (Metal Active Gas) is correct, whereas the more commonly used term "MIG" (Metal Inert Gas) is somewhat incorrect (and would be ISO4063 process-reference-number 131))

This page is about real welding
- a weld developed by the author.
Read on...!

(above picture has nick-break and macro-etch weld-tests shown - immediately see more weld photos: "8t8f1r"; "10t6f1r")

Richard Smith is experienced in engineering, welding and metals.

The central, guiding, principle is: every weld in a product is proven and evidence is available to verify that.

As a welder, the freedoms and the responsibilities go together like this:

you apply general welder skills to identify how to make each type of weld <--> every type of weld is proven in tests before use on product and evidence is recorded

Even if no "specification" is written for a weld, a welder should always have a "specification" clearly in their mind defining how a satisfactory weld meeting engineering requirements is made.

MIG welding needs special treatment - why?

It's about the correlation between the appearance of a weld and how good the weld is.

TIG / GTAW and SMAW / "stick" - generally a good-looking weld is a good weld
MIG / GMAW - very stylised good-looking welds are preferentially produced by welding conditions with a propensity to cause serious weld defects

If a TIG or SMA weld has a good appearance as evaluated by shape, weld bead surface texture, etc, it is, in general, well-executed and dependable.

The same is not true for MIG welds.
Conditions which give the most reliable welds cause appearance features held to be less desirable.
The corollary is a systematic pressure on welders to weld with conditions favouring serious defects which cannot be seen, in applying welding conditions optimising appearance features evaluated on inspection.
A welder can be readily dismissed for welds with high quality but insignificant sub-idealised visible appearance. Whereas serious defects not detected before in-service failures happen are too late to affect the employment success of a welder producing "pretty" welds accepted on visual inspection.
Hence a systematic tendency to prevalence of MIG weld lack-of-fusion defects.

There is a further technical dimension to the issue

TIG and SMAW are "constant-current" welding processes
MIG is a "constant-voltage" welding process

With TIG and SMAW the welder sets only the Current (Amps), with the Voltage dictated by the physics of the welding system.
With MIG, the welder sets both the Current, through the wire-feed-speed, and the Voltage, though the voltage setting. MIG can produce excellent welds and is very adaptable over a wide range of conditions; however the opportunity for error is unfortunately much increased.

A reality with MIG welds is that excessively pretty stylised MIG welds can be a well-founded alert forewarning of the ultimately serious weld quality issue of lack-of-fusion (there is weld-metal but there is no weld!).

Systematically getting good MIG welds

Strategy...

Actively choose your welding conditions - specify them, if only in your mind, to produce welds which fulfil requirements. Do not passively drift buffeted by Standards, historical traditions and other "environmental" influences.

With your weld, "know where you are going". Have a strategy. Hold in your mind a "map" of the "terrain" you are in - what the welding outcomes are across the current / wire-feed-speed and voltage map, where the third "vertical" axis is the quality of the weld resulting.

Get good training in developing your vocation as a welder.

Testing...

Measurement and testing will very much optimise the application of experience and judgement.

Measurement:

What are your Amps and Volts? Those are the fundamental physical units. Think in terms of them for interoperability in transferring between welding machines. Have a calibrated reliable meter available. The author has one...
Measure your weld deposit sizes - especially for fillet welds. How well are you doing? Could your welds be more economical? Simple "plate" fillet-gauges with a "cut-out" of given height are the reliable, consistent, accurate and repeatable ones to use.

Weld destructive testing:
Your production welds are consistent. So what you find from cutting-open and/or breaking a representative sample applies to the product!
Standards describe and therefore validate "workshop" weld tests which are commonly known and very cheap and rapid to perform, using only familiar workshop tools.
These tests are very informative. Do not overlook them.
You can know how you are doing before you spend any money on accredited testing services. The client may appraise your systematic approach with recorded evidence and be satisfied.

macro-etches of weld sections show the fusing power of your weld - are you fusing by a good margin? There are barely hazardous etchants - the author uses an iodine etchant
"break" tests "search" for any defects - the line-of-fracture will track through any gross defects - and a length of the weld is tested - so these tests are very revealing. One adage is "don't make welds if you don't break welds".

Non-destructive weld testing:

keep sight of the most important "NDT" - "VT"; Visual Testing. Is there impartial analysis of weld appearance? What does analysis of weld appearance inform you about the welding strategy? Are the engineering requirements met well? How does the Company work together to benefit its shared future?

By the way; it is possible to "calibrate" a copper-and-iron welding machine with voltage-taps and wire-feed-speed, both with totally arbitrary markings. Only some welding conditions are good - you have to be able to identify those anyway by qualitative judgement. There is a "contour" of good conditions with increasing corresponding Amps-with-Volts for increasing thickness of metal. You can establish the Amps-and-Volts for those fundamental good conditions at increments of weldment thickness, and that calibrates your welding machine.
The author has a calibrated clamp-meter by which he can do this welding-machine calibration.

Weld in pictures

My practical credentials in pictures - work-a-day fillet welds I developed for a customer, applying the methods and approaches described here:

(R. Smith, 26Nov2014, 15May2017, 17May2017, 26May2017)