Ehsan Khan
Altec Expert – W&C (Middle East & India)
Air Liquide India
New developments in gas shielded arc welding of aluminium alloys using gas mixtures containing minor gaseous additions
Introduction
In shielded gas metal arc welding, especially in TIG or MIG processes, various gaseous additions had been used in order to improve overall performances.
View from the past, the main additions are;
- Hydrogen -for the welding of common austenitic stainless steels – in the range of 1 to 15%
- Helium – for the welding of aluminum and alloys for example – in the range of 5 up to 100%.
Basically, all these additions used to show inert effects, as the Tungsten electrode in the TIG process- had to be protected against rapid erosion. These inert gases shield in an effective way the tungsten electrode against oxygen and nitrogen contamination, provided that these gases or mixtures with argon are strictly controlled- as delivered- in humidity, O2 and N2.
These inert gas processes had been enlarged in the more recent past by mixtures containing slightly reactive gas additions as Nitrogen. The main application here is still the assembly of Duplex and Superduplex stainless steels which request a specific shielding mixture containing generally between 1 and 5% N2 into argon or argon-helium.
In aluminum welding with fusible electrode, the use of a specific argon-oxygen mixture, between 1 and 2% of O2 appeared some few years ago. Benefit was an extremely high penetration rate at constant mechanical properties – compared to the classic argon-helium mixtures. Drawback here is the surface aspect properties that show an important deposit of dark oxides. These oxides can be removed easily and in a cost effective way, but are not acceptable in all applications.
Micro-additions of gaseous elements go back to the late 70’s when the first mixtures with micro-additions appeared on the European market.
A micro-addition is an addition that is generally smaller than 0,5% . The first of these additions were applied for the TIG welding of aluminum to reduce the O3 emission level. The additions are here CO (carbon monoxide) and NO (Nitrous monoxide).The level of addition was between 150 and 300ppm or 0,015 to 0,03 %.
The weld performances are equivalent to straight argon, when Helium additions (or H2 additions for austenitic stainless steels) give the possibility to achieve higher performances (productivity, cost reduction per piece) at reduced emission level.
In the 80’s a first shielding mixture for the MIG/MAG process was introduced for the welding of Nickel base alloys. This type of mixture containing CO2 as a micro-addition in the range of 500-1000 ppm (0,05 to 0,1%) shows excellent properties regarding spatter reduction , wetting and weld aspect compared to the standard argon or argon- helium mixture that was normally used.
More recently and mainly for TIG welding of aluminum micro-additions in the range of 50 to 250 ppm appeared and concern various active but also low reactive gas additions as oxygen, carbon dioxide, nitrogen or their various mixtures with or without nitrous monoxide.
Target of the development was to reach basically a better arc stability during TIG welding of aluminum alloys, eventually avoiding the necessary pre-work that consists in removing the isolating aluminum oxide layer from the joint area.
Influence of gas additions to the weld properties
In the case of micro-additions, the physical or chemical property of the added gas component does not play any role. Thermal conductivity, enthalpy or electrical conductivity and ionization potential show now influence on the arc behavior.
The added gas properties are so to be investigated regarding its behavior regarding the interaction between gas component and molten metal.
The known mono atomic gases are of low interest, as being chemically inert to the weld metallurgy and interaction.
Multi-atomic gases show the particular interest, that the molecule is split inside the welding arc into their individual elements that may combine either with the molten pool or recombine with the free radicals. The liberated energy during splitting and recombination is negligible for the improvements of performances.
From the multi-atomic range of possible gases the following are of interest regarding the pool metallurgy:
CO2, O2, N2, NO, other NOx, CO, H2, H2O
CO2 – carbon dioxide- and O2 – oxygen- show a significant influence on the weld performance when being in additions above 1000 ppm or 0,1% .
Regarding arc welding of aluminum, CO2 shows a higher tendency to porosity formation, when an oxygen addition –in MIG/MAG process- can reach 2% without showing porosities.
Drawback of both gaseous additions is the extremely high sensitivity of the Tungsten electrode in the TIG process to O2 contaminations. In fact above a level of 50 ppm of O2 ( 0,005%), the lifetime of the electrode is shortened, or to be precise the frequency of re-shaping the electrode is increased.
H2 and H20 are of no effect as micro-additions when welding standard austenitic stainless steels. In aluminum welding the presence of porosities in the solidified weld pool is mainly due to a moisture contamination. The moisture level of the shielding atmosphere should be below or far below 40 ppm –depending on the application.
NO, CO or other NOx are toxic elements that show a toxic level (TLV or threshold value) that is close or below the addition level. Any Freon, fluor containing gases, chlorine …are highly toxic and/or corrosive and are excluded in the use for industrial applications;
Nitrogen is a diatomic gas that already was in use as an addition for TIG welding of Duplex steels. The atomic nitrogen shows a certain chemical reactivity with aluminum as it shows a reactivity on austenite formation when stainless steel welding.
What is the effect of nitrogen to the weld properties?
A minor addition range of N2 to argon had been investigated in our R&D laboratory in Paris in order to determine the maximum admissible level of nitrogen into argon for MIG and for TIG welding, but showing:
– improvements in penetration rate versus straight argon
– improvements in arc stability
– limit of bead aspect after welding.
The additions ranged from 0 up to 1200 ppm, knowing that for nitrogen contamination of the shielding atmosphere as delivered may range from lower than 5 ppm up to 200 ppm, depending on the shielding gas type and quality (shielding gases for MAG welding of stainless steel for example are highly tolerant to N2, when the welding of Titanium requires less than 10 ppm of N2).
For the latter point, it seemed important to reach a weld surface aspect similar/identical to the one obtained with straight argon or an argon- helium mixture with 20 to 30% helium additions.
Welding conditions:
The MIG and the TIG were used to weld various aluminum alloys, especially the aluminum alloy type 5086.
Bending test, tensile strength test, radiographic testing and high speed video recording had been performed for the development.
The tests were carried out on a water cooled backing with controlled cooling speed, in order to obtain comparable results.
Gas additions had been obtained by multiple-dilution, controlled by gas chromatography.
Results
Penetration rate versus argon
It can be seen that 300 ppm of nitrogen additions (or 0,03%) show the same penetration effect than 10-15% of Helium.
A mixture containing 600 ppm or 0,06% of N2 has an equivalent performance than the classical mixture of 20 to 30% He. Mechanical properties, X-ray quality up to 1000 ppm are similar to the results obtained with argon, 4.8 quality with moisture and O2 content below 5 ppm and N2 content below 10 ppm.
The conventional 180° bending tests on top and reverse side showed no cracking sensitivity for the considered range of N2 additions.
Improvement in arc stability
The arc stability for the MIG and TIG welding process are recorded by analysis of the electrical signal.
No major difference in the arc signal could be seen between argon or the argon mixture with various micro-additions.
On the other side, the welder had the feeling that the arc stability showed up as better.
We will give the explanation below.
Bead aspect after welding
The bead aspect, having cooled down to ambient air temperature, showed no difference to the weld performed with straight argon.
Above 1000 ppm (0.1%) of N2 addition the weld aspect is changing and leads to a reject of the performance.
Explanation of the phenomena
The addition of N2 into the shielding gas atmosphere leads to a very quick reaction of the mono-atomic nitrogen with the molten aluminum pool. Highly electron emissive nitrides are formed is which establish a highly “stable” arc column.
In fact, this arc column shows a rather high pressure onto the molten pool, is effectively pushing the weld pool down and favors a certain pool rotation, top-down.
This leads to a significant improvement of penetration and the excellent x-ray quality.
(a short high speed video comparison will be shown)
Tests performed on stainless steels and carbon steels could put in evidence this phenomena, but in those cases the existing argon hydrogen mixtures for austenitic stainless steels or ar-helium mixtures for the other cases (marginal) are already well performing.
Conclusions:
Aluminum welding in TIG or MIG processes needs the use of Argon Helium mixtures when high performances are requested.
It is possible today to replace the standard helium mixture by a micro-addition of nitrogen to argon in keeping all the mechanical properties as well as the weld aspect.
