Certification of welder for tungsten inert gas welding in unmanned aerial vehicle systems

Aluminum and its alloys is one of the extensively used non-ferrous metal in aerospace engineering applications due to its excellent properties like higher strength to weight ratio, non-magnetic property, high thermal conductivity, good machinability, recyclability, and excellent formability. However, welding of aluminum and its alloys has always been a challenge due to drastic reduction in strength near heat affected zone (HAZ). Due to softness, ductility, lower melting point of aluminum and its alloys impose additional challenges to weld aluminum alloy components. Due to this, the quality and quantum of skill set needed to weld aluminum alloy joints for flight critical components is much more than that of welding of ferrous materials. The qualification and certification of welder is one of the prime requirements in welding applications especially in aerospace industry. This paper mainly discusses the various aspects involved in qualifying and certifying a welder for tungsten inert gas (TIG) welding process of aluminum alloys, which is treated as one of the special processes. The welder certification is a written verification that weld joints produced by welder meets the prescribed standard of welder performance. It is to verify the ability of an individual to execute a qualified welding procedure specification to produce a sound weld. Aluminum alloy 6xxx series is considered for base metal for sample preparation and weld joint assessment studies, as it is one of the prevalently used aluminum alloy material in unmanned aerial vehicle (UAV) applications. Weld samples are prepared as per internationally practiced standards and welding procedure specification (WPS) is recorded. Various mechanical tests carried out on weld samples during certification process are discussed in the paper. Weld joints are subjected to tensile testing, bending test and macro test. The certifying agencies are involved at every stage to ensure consistency of welder performance.


Introduction to Welding
Metal joining by welding process has been extensively used in almost all sectors of manufacturing industries.Welding has become very dependable, efficient method in aerospace industry in recent years.Due to its cost effectiveness, ease of maintenance, lesser weight penalty, structural soundness advantages, welds joints are replacing riveted joints and bolted joints since world war-2 and hence, become one of the favorite for military, ship building and aerospace industry.Welding is typically defined as joining o two similar or dissimilar metals which are melted at the joining interface and usually a filler material is added at the molten metal pool with application of pressure and with / without application of heat.Welded joints are permanent, portable, and most economical m method of joining metals in terms of fabrication cost and material usage.Based on type of materials to be welded, 1291 (2023) 012036 IOP Publishing doi:10.1088/1757-899X/1291/1/012036 2 type of shielding gas use, the manner in which heat and pressure are applied, welding techniques are classified as liquid state (fusion) welding and solid state welding.Liquid state welding melts the metal (by external heating) at the interface and uses an external filler rod to fill the molten metal pool.In solid state welding surfaces to be joined are brought into close proximity by heating the material (without melting the interface) and applying normal pressure.Arc welding, gas welding and resistance welding are few types of arc welding process.Forge welding, friction welding, explosion welding, diffusion welding and ultrasonic welding are few types of solid state welding method.However, dissimilar metals can also be welded by using advanced welding techniques like laser welding, electron beam welding, Plasma Arc welding etc. Due to minimum distortion, high impact strength, high speed, high quality of weld, low maintenance cost and strong weld joints, further arc welding is majorly classified as Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW) (Also called Metal Inert Gas welding (MIG)), Shielded Metal Arc Welding (SMAW), Flux Coated Arc Welding (FCAW), Gas Tungsten Arc Welding (GTAW) (also called Tungsten Inert Gas welding (TIG)).

Welding of Aluminum and its Alloys
The existence of aluminum was first discovered by Sir Humphrey Davy in 19th century [1].Usage of aluminum in aircraft structures became popular and extensive during the second world war.Aluminum and its alloys is one of the extensively used non-ferrous metals in aerospace engineering applications due to its excellent properties like higher strength to weight ratio, non-magnetic property, high thermal conductivity, good machinability, recyclability, and excellent formability.However, welding of aluminum and its alloys has always been a challenge due to drastic reduction in strength near heat affected zone (HAZ).Due to softness, ductility, lower melting point of aluminum and its alloys impose additional challenges to weld aluminum alloy components.Due to this, the quality and quantum of skill set needed to weld aluminum alloy joints for flight critical components is much more than that of welding of ferrous materials.Aluminum alloys are classified based on the key ingredients like copper, silicon, manganese, magnesium etc. Figure 1 shows the overall classification of aluminum alloy [2].Hardenability, machinability, and weld-ability of aluminum alloys are driven by the key constituent alloy component.Non-hardenable alloy groups are those materials whose strength cannot be increased through heat treatment, in contrast to that of hardenable alloys which are capable of increasing strength through heat treatment.Percentage usage of aluminum alloys used in some of the commercial airlines is shown in figure 2 [3].Welding of aluminum and its alloys is a critical and one of the challenging areas in the domain of welding engineering.Aluminum welding is a discipline that needs to be understood and learned.Correctly applied, welders find that an aluminum welded product is quicker to manufacture than an equivalent performing steel welding and offers about 40 % mass advantage [4].

Qualification of Weld Joints and Welder Certification
In welding terminology, weld joint qualification refers to qualification of joints envisaged by the designer to evaluate its fitness to use for the intended function [5,6,18].It is a design parameter.
During weld joint qualification studies samples are prepared as per internationally approved industry practices, Welding Procedure Specification (WPS) and PQR (Procedure Qualification Records) [19] prepared, and joints are subjected to various mechanical and metallurgical testing to assess the quality of weld joint [7].However, Welder qualification (or certification) is a manufacturing aspect.Here welder or welding operator is qualified to operate particular type of welding technique for specific kinds of applications.It is an index representing the ability of the operator to generate high quality defect free welds.In other words, a qualified welder or welding operator is the one who meets ASME Section IX requirements [5].The qualification and certification of welder is one of the prime requirements in welding applications especially in aerospace industry.This paper mainly discusses the various aspects involved in qualifying and certifying a welder for Tungsten Inert Gas (TIG) welding process of aluminum alloys, which is treated as one of the special processes [20,21].The welder certification is a written verification that weld joints produced by welder meets the prescribed standard of welder performance.It is to verify the ability of an individual to execute a qualified welding procedure specification to produce a sound weld.Aluminum alloy 6xxx series is considered for base metal for sample preparation and weld joint assessment studies, as it is one of the prevalently used aluminum alloy material in unmanned aerial vehicle (UAV) applications [22].Weld samples are prepared as per internationally practiced standards [5] and welding procedure specification (WPS) is recorded.Various mechanical tests carried out on weld samples during certification process are discussed in the paper.Weld joints are subjected to tensile testing, bending test and macro test.The certifying agencies are involved at every stage to ensure consistency of welder performance.analyzer machine [11,12,13].Tungsten Inert Gas welding is carried out using the TIG welding machine (Make : Miller Dynasty 400 model), which has a unique auto-line technology.Nonconsumable tungsten electrode [14] was used along with recommended AWS ER 5356 filler rod [16] and argon shielding gases as per MIL-A-18455C [15] were used to generate high quality weld specimens.Alloy deposit of ER 5356 filer rod gives good resistance to sea water corrosion, good anodizing property [4,24].Typical Mechanical properties of ER 5356 filler rod are shown in table 1.

Test Sample Preparation by Welder Under
All the activities were done in presence of certifying agency.The reinforcement and penetration of weld bead should be machined so that the whole welded coupon shall be of same thickness as that of base metal, 3mm.

Experimental Investigation
The welded test samples were subjected to mechanical destructive test namely tensile test and 180 degree bend test.Samples were also subjected to a macroscopic test to evidence quality of weld throughout the thickness.
Tensile testing of welded specimen : The tensile test specimen were tested in direct tension in Universal Testing Machine (UTM) (Machine : MTS 810 Material Test System).The test specimen mounted on the UTM machine is shown in figure 5. 0.2 % proof stress, ultimate tensile stress (UTS), percentage elongation and failure location were recorded in PQR.180 0 Bend test : The test specimen is to be bend so that the weld is along the axis of the bend (as shown in figure 6) and is bent round a former of the required radius with the weld face on the outer side (that is to say, the base of the 'V' is to be on the inner side of the specimen after bending).To facilitate close contact with former, bend test specimens may be dressed on both sides by filing, grinding or machining until the weld metal is in level with the parent metal.The edges of the samples in the vicinity of the bend should be given a reasonable radius.The specimen must be bent through 1800 over a radius equal to five times the nominal thickness of the parent metal (As shown in figure 7).The bending strength and position of the break are recorded Macro examination : Specimen for the examination to be cut from the middle of the sample as shown in figure 8.The cross section of the sample to be examined utilizing micro-scope.In case if no facilities ae available for mounting the micro-section for polishing, a convenient method is to bend the two ends of the micro sample to form a 'U' or 'V' section.Specimen shall be examined in etched and unetched condition.Width of the wed, height of the weld, root height are measured and recorded

Results and Analysis
The tensile testing of the specimen confirmed that the failure occurred at the heat affected zone (HAZ) [23] as expected and is shown in figure 9 and 10.In welded aluminum alloy materials, weld bead is stronger than base material and the failure is expected to occur at HAZ region.This infers that the weld samples prepared by the welder is of sound quality.The bend test resulted in failure of the specimen at the weld bead and the same is showed in figure 10 and 12.The macro examination revealed no cracks, porosity, indicated good quality weld.

Conclusions
The welder qualification for TIG welding of aluminum alloys for aerospace applications is satisfactorily carried out.Weld samples are prepared as per the internationally approved standards, and subjected to various tests to evaluate the fitness of the welder to produce sound welds with a particular machine type and welding parameters.Test results are satisfactorily and failure locations indicated that the weld joints were prepared as per the designers requirements.It is observed that the skill set required to TIG weld aluminum and its alloys is high due to low melting temperature, HAZ and reduction in strength of the material.The welding filler rod, number of passes, purging parameters and most importantly operator skill dictates the weld quality.Defect free good quality weld joints ensure that the aircraft structures are manufactured as per designer requirements of zero defect.The weld specimens prepared by welder met all the requirements towards welder qualifications.The inspection agency issued a certificate for the qualified TIG welder.However, in general, welder qualifies for a particular type of material combination, machine and welding parameters.The welder examination body (WEB) recommends that the qualification certificate needs to renewed at a frequency of one year.A typical sample of welder qualification certificate format is shown in figure 13 [17].

Figure 2 .
Figure 1.Classification of Aluminum AlloysFigure 2. % by wt.usage of aluminum for aircrafts Certification 3mm thick silicon and manganese based aluminum alloy in 6xxx series is considered for base metal for sample preparation.Standard samples are prepared under suitable inspection supervision as per the standard[9, 10].The dimensions of the tensile test specimen, bending test specimen and macro test specimen are depicted in figure3.Test specimen details are recorded in WPS document.After welding, the surface of the welds shall be sufficiently free from coarse ripples, grooves, overlaps, abrupt ridges and valleys.The welded test coupons shall be subjected to visual inspection, liquid dye penetrant inspection, and radiographic examination as per American Society for Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC)[5,8] is referred.Welded test samples are shown at figure 4.

Figure 3 . 4 .
Figure 3. Test specimen dimensional details Figure 4. Welded Test samples Initial Raw material was tested and conformed to specifications using handheld portable XRF material

Table 1 .
Properties of TIG 404 AWS ER 5356 Filler Rod