Importance of various shielding gases used in TIG welding for exotic materials in Aerospace Industry

398

Surendra Vaidya
Executive Vice President & Business Head,
Godrej Aerospace.  

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Ashok Kumar Sindhu Chief Technical Officer,
Godrej Aerospace.ashoka@godrej.com

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Hitesh Ashwin Narsia Deputy Manager Welding,
Godrej Aerospace. narsia@godrej.com

 

Abstract – Godrej Aerospace, the Business unit of the 120 year old Godrej & Boyce Mfg. Co. Ltd, has 30+ years of experience in precision manufacturing of metallic systems for Space, Defense and Aircrafts Applications. Over these years Godrej Aerospace has evolved to be trusted partners for the prime customers in these segments of Aerospace Business. It has established an integrated manufacturing facility by continually developing its competencies to provide a value-added product to the customer. Manufacturing of built-up systems for liquid engines for PSLV, GSLV & ULV rockets of ISRO, requires high quality welding of multiple exotic alloys wherein minor variation of less focussed parameters like welding gasses can play a very determent role. The Purpose of this paper is to describe the importance of different shielding gases used in aerospace industry to weld challenging materials and their effect on welding parameters, heat input, weld repairs, bead profiles, and their mechanical and metallurgical properties.

  1. Introduction

Aerospace designs are continually optimized in terms of the multiple exotic materials used and functional requirements in defense, space and aviation sectors. Joining of exotic materials is often more difficult than joining of common materials. For welding of aerospace materials, all variables (essential, supplementary essential and non-essential) are equally important to meet the desired requirements. It is more significant due to control of Weld discontinuities, Heat Input and Weld Distortion in Aerospace materials. Shielding gases plays an important role in number of aspects of welding, including arc characteristics and the microstructure of weldments. The application of different shielding gases can result in different penetration and weld bead profiles. Selection of appropriate shielding gas is important as it should not affect the mechanical, chemical and metallurgical properties of material. As for example, Small amount of H2 with Argon will be very helpful in welding of austenitic SS and Nickel base alloys, but if the same gas is used in CS and LAS, Hydrogen Embrittlement and failure will occur.

  1. Aerospace Standard Requirements

Aerospace is a generic term that includes commercial aircrafts, planes and helicopters, military and defense, ground equipments, and space. Producing aerospace parts needs a long supply chain, creating a major challenge for the industry.  As this chain is so long and complicated, it should come as no surprise that Aerospace standards are comes with stringent expectations from the manufacturers. Manufacturing sophisticated vehicles, such as airplanes or rockets, understandably needs special attention throughout the entire production process. It must ensure to demonstrate the integrity of the across the complete manufacturing process starting from raw material to finished part. The aerospace industry has a strong certification and compliance requirement, with consequences on development cost and technology solutions. Few of the mandatory certifications are listed below:

a) AS9100 ensures Aerospace manufacturers, being up-to-date with the quality standards, is essential to supply parts to the industries. System Structure is complex by disciplines, segment, component etc. by large system management combined with high precision. Data access and security is critical, especially on Defence and Space Programmes.

b) NADCAP ensures advancements in the aerospace industry continue, it is not only need to ensure they obtain the required certification but keeping up to date with revisions. The standard additionally focuses on aerospace manufacturers being able to improve their quality management system between their audits, which can make the certification quite difficult to maintain. NADCAP is important to all special processes like welding, Heat Treatment, Surface Treatment, Non- Destructive Testing.

c) Customer Approval is required to meet the design criteria depending upon the product. Different customers have different criteria depending upon the criticality of the product. Separate Procedures, Welders and Facilities need to qualify and maintained as per the customer requirements.

d) Product Specific Qualification and Acceptance after Meeting specification requirements, before commencing on actual hardwares. Mock up of the same size need to perform and shall be resulting positive in all destructive and Non-destructive tests.

3. Shielding Gases used in Aerospace Industry

Shielding gases fall into two categories – Inert or Semi-inert. Only two of the noble inert gaseshelium and argon, are used in welding. The latest developments in shielding gas technology have included semi-inert gases with new additions of Hydrogen, Nitrogen, and mixtures of these. Most of these gases, in large quantities, would damage the weld, but when used in small, controlled quantities, can improve weld characteristics.

3.1 Universally used Inert Gases

3.1.1 Argon – Argon is the most popular shielding gas and is often used for both gas metal arc and gas tungsten arc welding. Excellent welds are often produced using pure argon as a shielding gas. Argon gas produces a narrower penetration profile, which is useful for fillet and butt welds. All materials (except Cu of higher thickness) are easily weldable using Argon Gas. Aerospace industry uses a much higher purified form of Argon i.e. Grade5 UHP (Ultra High Purity) Gas. To ensure the gas purity and check the dew point of the gas, AWSA5.32 method is used.

                                          Fig (1) Commercial Argon Gas

                                Fig (2) Argon Grade-5 Ultra Pure Gas High

UHP Argon gas ensures quality weld and lessening the chances of porosities in the weld. Apart from mechanical and chemical properties understanding and maintaining dimensional requirements are equally important. First Time Right (FTR) results are necessary in aerospace industries as compare to all other sectors as rework cost is very high. Materials weld with this gas are Austenitic SS, Ni Alloys, Co Alloys, Titanium and its alloys, Aluminium, Maraging steel, Maraging steel to SAE4130 steel, 15CDV6 Steel, 25CD45 Steels used in different applications of Space, Defense and Aviation sectors.

3.1.2 Helium having high Thermal conductivity, is not easy to ionize hence requires higher voltage to start the arc. Due to higher ionization potential it produces hotter arc at higher voltage which helps produce wide deep bead, advantageous for welding of aluminum, magnesium, and copper alloys. In comparison with argon, helium provides more energy-rich but less stable arc. Helium is more expensive than argon and requires higher flow rates, so despite its advantages it may not be a cost-effective choice for higher-volume production. Pure helium is not used for steel, as it then

provides erratic arc and encourages spatter.

Apart from shielding gas, Helium gas is widely use in purging of difficult to access area in Copper to SS, SS to Nickel and Stellite Final Joints.

3.2 Improvised Unconventional Mixture Gases

3.2.1 Argon – Hydrogen Mixture – Hydrogen (H2) can be added to shielding gases for GTA welding of austenitic stainless steels, Nickel base alloys to reduce oxide formation. The addition also means more heat in the arc and a more constricted arc, which improves penetration. It also gives a smoother transition between weld bead and base metal. For root protection purposes, hydrogen addition is beneficial due to its reducing effect of oxygen which will improve the fluidity of weld molten pool and reduce the viscosity.

Experimental Procedure

Semi cryogenic engine – The second largest thrust generating engine produced till date, has one of its key material as high Strength Nickel Alloy having UTS of 930MPa. Weld thickness varies from 7mm to 21.5mm and joint accessibilities are also poor due to complex designs. Manual or Auto TIG mandate for this generates higher heat inputs, resulting in shrinkage and distortion. Secondly, porosities and linear indications are problem with the nickel alloys, the main culprit being nitrogen. As little as 0.025% of nitrogen will form pores in the solidifying weld of Nickel alloy. Oxygen is also a cause of porosity in certain circumstances when it combines with carbon in the weld pool to form carbon monoxide. A further characteristic of nickel alloys is that the amount of penetration is less compared to carbon or stainless steel. Increasing the welding current will not increase penetration proportionally.

To counter these conditions, addition of H2 seems to be the solution. Generally, reference case studies suggested addition of H2 with Ar within 2 to 10% range. Keeping low H2 content will lead to formation of the viscous and adherent scum on the surface of the weld pool. This sluggish weld pool does not flow freely, may result in lumpy bead and will not allow weld metal to wet the side walls to get fuse properly. On the other side, keeping higher H2 content will generate gas bubbles due to high turbulence, chances of tungsten inclusions will increase in weld. Hence, for above limitations, for this material addition of H2 with Ar gas is fixed to 5%.

Test Plate of 15mm thick welding carried out using Pure Ar (UHP) and Ar+5% H2 Gas. Comparison is shown below:

Weld Parameters Ar (UHP) Ar+5% H2
No. of Weld Passes 09 07
Current (A) 125 – 130 90 – 95
Voltage (V) 10.5-11.5 12.5 – 13.5
Travel Speed (mm/min) 45 55
Heat Input (KJ/mm) 1.99 1.39
Shrinkage (mm) 2.5 1.5
RT Defects (Out of 6 Segment) 2 0 (FTR)

 

Procedure Qualification Testing carried out having values listed below. Apart from regular testing, wet chemical test IS 228 carried out to ensure Hydrogen level in Weld and parent and results shown satisfactory.

Testing carried out Specified Achieved
Tensile Test UTS (MPa) 930 965 – 1024
Tensile Test 0.2%PS (MPa) 550 671 – 677
%Elongation 15 16.4 – 17.4
Side Bend Tests

(85mm Former Dia)

No Crack No Crack. Accepted
Micro/Macro Satisfactory Fusion.

Defect Free Weld and HAZ

Satisfactory Fusion.

Defect Free Weld and HAZ

H2 in weld and parent <0.5ppm <0.5ppm

 

  • Heat Input reduced by 30%, Zero defects and Controlled Shrinkage/Distortion. Mechanical properties Maintained.

3.2.2 Argon – Helium Mixture

Addition of Helium supplies more heat input to the base metal and causes an increase in the welding rate, penetration and weld puddle fluidity. Greater fluidity of the weld pool can facilitate the escape of hydrogen gas bubbles, which are the source of weld porosity in the weld pool. Addition of helium in the shielding atmosphere results in an increase in impact energy and decrease in the crack growth rate.

Experimental Procedure

  1. Welding of Aluminum (AA2219) Material

Welding of aluminum and its alloys encounters more difficulties than welding of steels. Major concerns are cracking and porosity, due to the high solubility of the hydrogen in the molten metal and the relatively high thermal expansion coefficient, which causes large changes in volume upon solidification. Due to surface oxide film formation during welding, it’s very difficult for the welder to identify the weld pool. Helium when added to Argon improves oxide breakdown performance and restricts hydrogen entrapment in the weld pool, thus resulting in better arc stability and weld quality.

AA2219 (Al-6%Cu) is a material used in VIKAS Engine. While using UHP argon as shielding gas, it was difficult to fuse the joint edges due to difficult joint configurations and lack of fusion defects was encountered. To overcome these, Mixture of Argon (80%) +Helium (20%) was used for shielding.

As shown in figure (3) Argon yield arc gives deeper penetration whereas pure He gives wider penetration. Appropriate Mixture of Argon and Helium gives penetration of correct depth and width. (Ar+O2/CO2 is not used in GTAW, it is useful in GMAW)

Figure (4) showing actual joint, welded using this mixture gas, having penetration of 0.5 to 1.0 mm as per the drawing requirements.

Details of welding trials & results are given in below table:

Weld Parameters Ar (UHP) Ar80 : He20
No. of Weld Passes 02 02
Current (A) 210-225 160-185
Voltage (V) 11-13 11-13
Travel Speed (mm/min) 70-80 75-85
Heat Input (KJ/mm) 2.50 1.92
Shrinkage (mm) NA NA
Defect % ~ 6-7% < 1%

 

With these parameters, Procedure qualification carried out having satisfactorily. This WPS and PQR is approved from LPSC, ISRO.

  • Heat Input reduced by 20 %, reduced defects and Controlled Distortion. Mechanical properties Maintained.
  1. b) Welding of Copper to SS.

One of the assemblies of Semi Cryo Engine has copper to SS joint before brazing. Thickness of the joint is 2mm. Qualification attempts done using pure Argon & Pure Helium were unsuccessful. When welded using Pure argon, the copper edges were not getting fused where using Helium very concentrated arc was generating which limited the weld pool visibility. Trails carried out using Ar80% + He20 % Gas and having satisfactory Depth of Fusion on both Copper and SS sides.

                      Fig (5) – Micro Photograph of Cu #SS Joint using Ar80:He20

Leg Length (Cu) Leg Length (SS) DOP(Cu) DOP(SS)
1.6mm 2.6mm 0.9mm 0.5mm

 

As shown in Fig (5) Fusion is satisfactory and weld and HAZ are free from defects. DOP and Leg Length Values are acceptable, and this procedure is also approved by LPSC, ISRO.

  1. Conclusion 
  • To meet the First-Time Right results in aerospace applications, shielding gas playing an important role along with other variables.
  • Desired requirements Physical, Chemical, Fusion and Dimensional are meeting with the selection of appropriate shielding gas against the material.
  • Ar+ H2 Gas has shown improved performance in High Strength Nickel alloys. In Future, ASS and Cobalt base alloys trails to be conducted to confirm the outcome.
  • Ar + He gas has shown better performance in Aluminium, Copper to SS materials in which fusion was very difficult.
  • Experimentation backed with metallurgical knowledge is must to utilise the individual characteristics of gases for applicability in exotic alloys of Aerospace Industry.

 

 

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