M.Sch?rghuber，M.Eberhard，K.Wimberger

(1.伏能士國(guó)際有限公司;2.伏能士中國(guó))

0 Introduction

GMAW processes for joining high strength steels have displaced other welding processes because of regulating the heat input，less threat of hydrogen induced cracks，but also more productivity.

High strength steel and filler material manufacturer give limits for cooling rates to guarantee the properties of high strength with sufficient toughness.Using the properties and correction parameters of welding processes will help to reach the designed seam thickness and sufficient penetration.For dynamically loaded constructions，a secure penetration depth and welding without undercuts are demanded attributes which are discussed in this paper.

Modern digital welding systems and further developed welding processes can be a valuable instrument for the welder and the quality supervizor to reach easier the efforts of a high strength steel welding joint.

1 Welding processes and heat input

Different welding processes varies in heat input and weld seam geometry.

1.1 Heat input per unit length

The heat input per unit length(1)is calculated by the electric parameters of welding current and voltage and welding speed.

The instantaneous power(2)of an arc has to be calculated with high sampling rates［1］.The voltage measurement is difficult and shall compensate the voltage drop in the hosepack.Modern power sources give the user an algorithm to calculate and display the compensated voltage.The efficiency factor is defined by the choice of welding process but also the arc length and resulting radiation losses.

The resulting cooling rate defines the soft zone enlargement or hardness increase.The cooling rate is also be interacted by thermal conduction，material thickness，preheat treatment，welding process and multilayer technique.

1.2 Pulsed arc to spray arc welding process

A pulse welding process uses the effect of droplet detachement by the pinch effect to obtain a spatter free welding process over the whole range of a synergic line.The instantaneous power of a pulse in compare to a spray arc is lower at same melting rate，stickout and arclength.Figure 1 shows this effect with 9，1kW for pulsed process to 10，1kW to spray arc process at the same melting rate of 6kg/h.This reduced average energy for the same melting rate is because of the preheating effect in the electrode stickout with the quadratic impact of the pulse current(3).

Fig.1 Spray arc to pulsed arc at same melting rate and arclength

The welding voltage determines the length of the arc and its process characteristics.A long arclength has a lot of instantaneous power and the risk of undercuts.A short arclength has to be preferred because of the less energy impact and a deep penetration because of a higher arc current density.

Therefore the arc length-or voltage-correction gives an important instrument to control a spray arc or a pulsed arc.

If the arc becomes too short，the droplet transfer has short circuits which can result in spatters and disrupted shielding gas coverage.The dynamic-correction will handle the short circuit current behavior for spray arc and the pulse dynamic correction the pulse energy for the droplet transfer.

1.3 Singlewire to doublewire welding process

Welding with two electrodes in one melting pool has significant advantages for pouring out of the weld seam because of the enlarged molten area.Therefore the welding speed can be raised at a good seam transition and the heat input is even reduced.The influence of the arcs is reduced by isolating both current paths=tandem welding process.

Table 1 and figure 2 give a comparison between a single wire and a tandem welding process for a 10mm high strength steel joint with and the resulting different properties.

Tab.1 Welding parameters of 3layer(single wire)and 2layer(double wire)

For single wire welding，the experience of 1/3 of the sheet thickness，gives three layers for a high strength steel joint.The higher deposition rate of a tandem welding process results in a two layer joint and much better seam transition for the top layer.It has to be balanced if the smaller softening zone of multilayer technique or better dynamic behavior of smooth seam transition hast to be preferred like in the result in figure2.

Fig.2 Welding and hardness profile［2］ of the same joint in tree layer sinlge wire welding and two layer tandem welding

New tandem welding processes like CMT-Twin combine the advantage of a deep penetration or fast welding speed of a short pulse process with the first electrode and the backing task of the stabile CMT-process for the second electrode［3］.

1.4 Laserhybrid welding process

The combination of the laser with a GMAW process gives good results with high strength steels because of the small heat affected zone and the deep penetration of the laser process.

Filler material with less strength than the base material and a resulting“undermachting”results in better toughness for the joint at the high cooling rate.With the additional heat input of the arc and the molten electrode，the cooling rate can be interacted.

Fig.3 Welding profile of slingle layer laserhybrid welding joint with differernt seam preperation and welding parameters

2 Advantages of constant penetration and a short arc length

Lack of fusion is one of the most dangerous threats for a welding joint and therefore sufficient penetration is necessary.The penetration depends of welding speed，torch angle，shielding gas，and especially the welding current or short arc lengths with high current densities［4］.

2.1 Penetration stabilizer

GMAW powersources have usually synergic lines with constant voltage(CV)behavior.The advantage of constant arc length at constant welding speed with the“inner regulation”of the arc length by varying the welding cur-rent is used.Synergic lines with constant current behavior(CC)need to adjust the wire speed drive for a constant arc length and its properties.

Modern synergic lines use both control strategies.The so called“penetration stabilizer”gives a CC behavior in adjustable limits.Therefore a change in the contact to workpiece distance(CTWD)will give the same penetration in the predefined range with a tolerable change in weld seam thickness.This effect is demonstrated in figure 4 with a cross section in length direction of al weld run with penetration stabilizer=0.5 m/min.

Fig.4 Penetration effect with constant current welding using a penetration stabilizer

2.2 Arclength stabilizer

As mentioned before，the properties of a short arclength are to be preferred.The arclength correction can only control a static voltage point for the welding path.If the control aim is the short circuit behavior，the length of the arc is always the same.Even at disturbances like CTWD changes or geometric changes of the arc because of different welding speed or seam geometry.This effect is illustrated in figure 5.The first two pictures describe the behavior at a static voltage point by changing from a butt weld to a fillet weld.The geometry of the arc enlarges at the same welding voltage and stickout and undercuts may occur.By using the arclength stabilizer the properties of the arclength keep the same.

The attributes of very small short circuits will not disrupt a welding process with large spatters or shieling gas disturbance.This optimum can only be adjusted by a controlled short circuit behavior like in figure 6 for a pulsed welding process which can be adjusted by the user with the arclength stabilizer.

3 Weld Seam documentation

For manual welding systems it is always difficult to define a welding speed.Therefore the heat input per unit length(4)can be calculated by the instantaneous energy(5)and the length of the weld run.

A modern welding machine adds up or integrates the product of the welding current and the compensated welding voltage and shows the instantaneous energy on the display.Automated welding processes have a defined welding speed and can use formula(1)and(2)for the heat input per unit length.An example for a manual instantaneour energy display is given in figure 7.

Fig.5 Example for geometric changes in arclength without and with arclength stabilizer

Fig.6 Droplet transfer with arclength stabilizer in pulsed welding process

Fig.7 Instantaneous energy on the display of a manual power source or a remote control RCU 500i and automated welding system.

For documentation of welded parts including several welding jobs and parameters have the powersource manufacturer created software solutions to observe the welding parameters in adjustable limits and document the heat input.Figure 8 shows a screenshot of a modern software solution.

Fig.8 Display and documentation of welding parameters and heat input with Fronius Xplorer

4 Conclusion

Welding high strength steels with GMAW and modern power source technology will help the manufacturer to use the benefits of the base and filler material.Welding processes，synergic lines and correction-and stabilizer-parameters were developed to make the joint as secure as possible and reduce imperfections and spatters and ensure penetration depth and proper heat input.

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