WHI Formula as a New Criterion in Automatic Pipeline GMAW Process

Pipeline welding is one of the most significant applications of GMAW process. Automatic welding for pipelines has been developed from early 1970’s. In these systems the welding robot moves around the two pipe's seam and welds the pipes by arc welding machine. Depending on the pipe thickness, weld process is repeated in several passes while the seam is filled of weld mass. The automatic pipeline welding systems has been recently paid more attention [1, 2]. In order to achieve sufficient performance in the process, the input parameters must be chosen correctly [3]. Welding parameter designing is a complicated step in the GMAW process, because of the large number of parameters and complexity of dynamic behavior. This complexity is particularly intensified in automatic pipeline systems, because of the complex seam geometry, wide range of the angle variations and strict quality requirements [1]. The most important input parameters in the automatic pipeline GMAW process are: welding current, arc voltage, travel speed, wire feeding speed, Contact Tube to Workpiece Distance (CTWD), welding position (angle), gas type, pipe type/thickness and seam geometry [4, 5]. The output parameters of the process are usually defined as either mechanical properties or weld bead geometry [6]. Weld bead geometry method considers the relationships between the input parameters and weld bead dimensions (penetration, width, reinforcement height, and width to penetration ratio and dilution [3, 7, 8]. Appropriate melting of the seam walls is certainly one of the most important conditions to achieve a proper dimension in fusion zone. A fusion zone with a sufficient width is necessary to prevent from some defects like lack of Fusion (LOF) [9, 10]. Having a direct contact between the arc and seam walls and receiving enough energy to the walls led to suitable wall melting and appropriate fusion zone [11, 12]. Some criteria such as heat input are related to the total energy which is given to the weld region without considering the amount of energy required to melt the wire. Principal parameters to calculate the heat input value are: welding current, arc voltage, travel speed and welding efficiency [11, 13]. During the welding process, part of the arc energy is spent to melt the wire [12]. Seam geometry also plays an important role in the amount of arc energy that directly reaches the walls. However a more general formula is not yet introduced.


Introduction
Pipeline welding is one of the most significant applications of GMAW process.Automatic welding for pipelines has been developed from early 1970's.In these systems the welding robot moves around the two pipe's seam and welds the pipes by arc welding machine.Depending on the pipe thickness, weld process is repeated in several passes while the seam is filled of weld mass.The automatic pipeline welding systems has been recently paid more attention [1,2].In order to achieve sufficient performance in the process, the input parameters must be chosen correctly [3].Welding parameter designing is a complicated step in the GMAW process, because of the large number of parameters and complexity of dynamic behavior.This complexity is particularly intensified in automatic pipeline systems, because of the complex seam geometry, wide range of the angle variations and strict quality requirements [1].The most important input parameters in the automatic pipeline GMAW process are: welding current, arc voltage, travel speed, wire feeding speed, Contact Tube to Workpiece Distance (CTWD), welding position (angle), gas type, pipe type/thickness and seam geometry [4,5].The output parameters of the process are usually defined as either mechanical properties or weld bead geometry [6].Weld bead geometry method considers the relationships between the input parameters and weld bead dimensions (penetration, width, reinforcement height, and width to penetration ratio and dilution [3,7,8].Appropriate melting of the seam walls is certainly one of the most important conditions to achieve a proper dimension in fusion zone.A fusion zone with a sufficient width is necessary to prevent from some defects like lack of Fusion (LOF) [9,10].Having a direct contact between the arc and seam walls and receiving enough energy to the walls led to suitable wall melting and appropriate fusion zone [11,12].Some criteria such as heat input are related to the total energy which is given to the weld region without considering the amount of energy required to melt the wire.Principal parameters to calculate the heat input value are: welding current, arc voltage, travel speed and welding efficiency [11,13].During the welding process, part of the arc energy is spent to melt the wire [12].Seam geometry also plays an important role in the amount of arc energy that directly reaches the walls.However a more general formula is not yet introduced.
In this paper WHI introduced as a new criterion, which is related to the arc energy that directly reaches the walls considering the both required energy to melt the wire and seam geometry.This criterion has the capability to be used for designing the welding parameters and for welding analysis applications.Section 2 contains theoretical parts to achieve WHI criterion, which includes wire melting energy (section 2.1), WHI formula (section 2.2), and wall geometry calculations (section 2.3).In section 3 two experimental tests are performed using the fabricated automatic system [15] in order to validate the obtained results from the presented formula.

WHI theory
In this section, firstly the required energy for melting the wire is calculated and the wall length of the groove face in contact with the arc is computed as well, remaining arc energy on the seam walls is named WHI.

Wire melting energy
Weld metal area (cross section, in front of view) is a function of wire radius, wire feeding speed and travel speed.The deposition rate (w s /T s ) is usually considered to be fixed for designing of welding parameters.Therefore the molten metal cross section (A) is counted as an assumed parameter in design procedure.
The amount of heat input over the length unit of travel axis is computed considering the radiation energy [11].
The mass of 1mm 3 steel is equal to The volume of the molten wire poured down inside the seam along l t mm of the travel axis is obtained using eqn.( 4) The value of the required Energy for melting the wire poured inside the seam (versus travel axis unit (J/mm)) can be computed as below:

WHI formula
Arc energy is spent to melt both the filler wire and the walls (eqn.6).Therefore the remaining energy which directly contacts and melts the walls is named "wall energy" (E wall ).
Energy density with respect to the seam wall length is computable through dividing the energy of the wall by the seam wall length (l) (front view).Therefore WHI can be defined as the below equation.

Wall geometry
The front view of the arc and bevel for the welding of the first pass in a U-type bevel can be seen in fig. 1.
Fig. 1.A schematic view of the arc, melting wire area and seam walls The arc width (Fig. 1) is calculated using eqn.(10) [14].
2t a n () The wall length involving with the arc edges is calculated by eqn.(10-13).

sin( ) 2
For the other passes (except for the root pass), front view of the arc and seam (for U-type seam) is like Fig. 2. Seam walls length (front view) involving with the arc edges is computable using eqn.(14)(15).Fig. 2. A schematic view for melting area in the second pass 2sin( ) If there is torch oscillation amplitude (see Fig. 3) the effective arc width can be computable by eqn.( 16) [15].If the effective arc width is too much compared to the melting wire area, some of the wall energy will be lost over the seam without any involving with the molten metal.Furthermore if the center of the oscillation and the seam centerline are not identical (see Fig. 4(left)), the fusion area in the both sides, will not be same.
A schematic view and a real test result are shown in Fig. 4. In this figure the center of the oscillation and the seam centerline do not coincide, additionally, the oscillation amplitude is too much, hence Fig. 4(right) has been resulted in the experiment.

Setup
The automatic pipeline welding system used in the experiments [15] has been shown in Fig. 5.The welding progresses downward semi-circularly from top (0°) to the bottom (180°) of the pipe on each side.The solid wire was ER70S-6(SG3), having diameter of 1 mm, the shielding gas is the mixture of Argon and CO 2 by 82/18 proportion.The pipe material is API 5L x65 HSLA steel with the thickness of 20.6 mm and the outside diameter of 32 inches.A U-type joint design is used according to fig6.Seam area is about 130mm 2 , that is filled with several weld passes.The molten base metal area is about 66mm 2 .This area is highlighted in Fig. 9 (a).this area has been calculated by computer and image processing algorithms using MATLAB.Base metal molten area over to total molten area (summing of base metal and melting wire area) is defined as relative molten area, is about 34% for this example (Ex.1).

Conclusion
In this paper WHI was introduced as a new criterion for designing of the welding parameters and welding analysis.This criterion calculates some of the heat input that is directly given to the walls from the arc.This formula considers the effects of the both required wire melting energy and seam geometry on the input energy.It was shown that WHI has a more correlation with fusion zone area compared to the heat input formula.In the other view the obtained results can be extended to the other welding processes and other applications.However WHI has a good feature in welding parameters designing to achieve some appropriate welding properties like the fusion zone.

Fig. 4 .
Fig. 4. Unsymmetrical and extra amplitude of arc width compared to molten wire area, schematic view (left), real test result (right)

Table 2 .
Welding parameters for two experimental tests