SchweiSchweißßtechnische Lehrtechnische Lehr-- und Versuchsanstalt und Versuchsanstalt SLV MSLV Müünchen nchen -- Niederlassung der GSI mbH Niederlassung der GSI mbH

GMAW welding and brazing of thin sheet metal

using modern power sources

F. Zech, Dr.-Ing. H. Cramer, L. Baum

INNOVATIVE TECHNOLOGIESFOR JOINING ADVANCED

MATERIALS07 – 08 June

Timisoara

Timisoara 2007 2

User requests when applying GMAW:Avoidance of weld spatter and weld penetration flaws (lack of fusion) at start of bead

Limiting power peaks during arc re-ignitions to minimize weld spatter in spite of inhomogenities

Exact control of the heat input and the consumption rate for - joining very thin sheet metal (t ≤ 1,0 mm)- controlling component tolerances- material mixture (steel-aluminium)

Stable process under different conditions of the component surfaces and high working speeds

Easy way to set parameters

Short arc GMAW welding and soldering of thin sheet metal

Timisoara 2007 3

Short arc GMAW welding and soldering of thin sheet metal

User requests - examples

- optimized welding startwithout spatter and lack of fusion

- possibility to join material mixtures

coated steel

aluminium

- back of coated steel without burning of the zinc content to avoid corrosion

Timisoara 2007 4

Short arc GMAW welding and soldering of thin sheet metal

User requests - examples

GMAW welding:controlling component tolerances on a three-member weld with different thickness(gap bridging on high alloy steel)

GMAW brazing:gap bridging without fusion of the coated steel andoptimized weld interface

Thickness of sheet: 1 mm and 2,5 mm

Timisoara 2007 5

GMAW welding and soldering of thin sheet metal

Solutions for short arc welding:• Optimizing the conventional short arc• Short arc with trigger points during process flow

CMT, CP, ColdArc, DIP-pulsed, FastROOT, STT process, ...• Short arc with controlled pulsed current

Solutions for pulsed arc welding:• GMAW DC pulsed electric arc welding• GMAW AC pulsed electric arc welding

Solutions for periodically changing electric arc output:• Alu pulse, double pulse, mixed pulse, Superpuls, ...

Timisoara 2007 6

Suitable metal transfer forms with GMAW welding of thin sheet metal

Shor

t arc

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ith e

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rc in

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uptio

ns)

Puls

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ycle

(with

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lect

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rc in

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ns)

Working ranges

short arc

Timisoara 2007 7

Short arc diagrammetal transfer with arc interruption

2520 W

In the short-circuit phase, the liquid wire end is absorbed by the weld pool and pinched off by the pinch force.The electric arc re-ignitionshould be carried out with limited power to avoid spatter.

Timisoara 2007 8

Short arc

Spatter due to current and power being too high during arc re-ignition

large smallThrottling effect Throttling effect

Characteristic curve slopeCharacteristic curve slope

Limited short-circuit current

Possibilities to influence the power peak during re-ignition

Timisoara 2007 9

Conventional short arcExcessive change of the short-circuit frequency andpower peaks (with spatter) due to interference

problems due to:

• irregular wire speed in contact tube

• long hose package

• inside diameter of ato large wire guide tube

• flawed wire surface quality

• greater torch distance changes

Timisoara 2007 10

Advantages and disadvantages of short arc used with conventional welding plants

Advantages:• robust switching concepts• small number of parameters• limited costs

Disadvantages:• the electric arc ignition cannot be sufficiently optimized• due to costs, the dynamic properties of the power sources are

usually not variable at all or only to a limited extent• the electric arc power cannot be changed at the start and end of

bead

Timisoara 2007 11

coldArc – processA new high dynamic inverter switchingin connection with a very quick digital current regulation decreases the power peak significantly during re-ignition.

The result is a spatter-free, heat- and arc force-reduced joining process.

The arc is colder due to the powerreduction during re-ignition. Thin sheet can be welded or brazed with low heatinput.

t

t

Phase 1Lichtbogen brennt

Phase 2Kurzschluß

Phase 3Auflösung des Kurzschlussesund erneute Brennphase

t

Us

Us

Is

Is

ColdArc-processaccording to EWM

cold

Arc

co

nven

t. sh

ort a

rc

arc on short circuit

re-ignitionarc on

Timisoara 2007 12

current

voltage

arc power

coldArc-processaccording to EWM

Timisoara 2007 13

coldArc-processaccording to EWM

arc power

conventional short arc

arc power

coldArc

Power in time of re-ignition

Timisoara 2007 14

GMAW soldering with triggered short arc

ColdArc processCompany EWM

Basic material: H260+Z100

t = 1.0 / 2.5 mm

vL = 50 cm/min

Timisoara 2007 15

Triggered short arcSTT process (Surface-Tension-Transfer)

wire electrode:1.4316, 1.2 mmshielding gas:2.5 % CO2, residual Ar

Company LincolnQuick current reduction with electric arc re-ignition

Arc

pow

er [k

W]

volta

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] cu

rren

tint

. [A

]

Timisoara 2007 16

t = 0 ms t = 4.59 ms t = 6.21 ms t = 7.56 ms t = 11.34 ms t = 13.23 ms t = 13.77 ms t = 14.31 ms

The withdrawal movement of the wire supports dropseparation and stabilization of the electric arc length.

Special push-pull wire feed and wire buffer needed.

CMT process (Cold Metal Transfer)by papers from Fronius

Timisoara 2007 17

CMT process (Cold Metal Transfer)by papers from Fronius

curr

ent

int.

[A]

v

olta

ge [V

]

Electric arcinterruption

Timisoara 2007 18

CMT process (Cold Metal Transfer)by papers from Fronius

welding plant with special push-pull wire feed and wire buffer

Timisoara 2007 19

CMT processAluminum sheet metal welding

• Aluminum butt weld• Sheet thickness 0.8 mm• Welding speed 3 m/min• Welding without weld pool support• Secure root penetration without weld

seam flattening

Timisoara 2007 20

CMT process used for combining steel and aluminum

Lap jointSheet thickness 1 mmWelding speed 70 cm/min (27.6“/min)

Principle:Aluminum-side weldingSteel-side soldering

Requirement: Galvanized sheet steel(> 10 µm)

Timisoara 2007 21

GMAW pulsed arcMaterial transfer without electric arc interruption

pulse current (causes quick dropforming and droptransfer)

background current (avoids arc instabilities andforms the drop)

Cur

rent

inte

nsiti

y

time

Timisoara 2007 22

GMAW pulsed arccurrent forms

DC AC

conventional pulsed arc

pulsed arc with small interpulses to stabilize the process during the long background current time

Timisoara 2007 23

GMAW AC pulsed arcparameter diagram

Depending on software version, 4 to 8 time segments can be set within one period (15 to 40 parameters per period).

Parameter changes can be initiated for trigger points of short arc applications.

company

ELMATECH

Timisoara 2007 24

Polarity effects during GMAW welding

Electrode negative (negative polarity)- larger surface of arc attachment spot on

base material and on wire electrode end- colder wire electrode end - higher surface tension of the drop (bigger drops)

- less pinch effect caused by a lower current densitiy

- less penetration - higher deposition rate

Electrode positive (positive polarity)- smaller surface of arc attachment spot on base material and wire electrode end

- hoter wire electrode end - lower surface tension of the drop (smaller drops)

- higher pinch effect caused by higher current densitiy

- more penetration - lower deposition rate

Timisoara 2007 25

GMAW AC pulsed arc

High-speed pictures shows the differences between negative and positiv polarityduring a period

Timisoara 2007 26

Stro

mst

ärke

0Zeit

+

Stro

mst

ärke

0Zeit

+

14 % 45 %

GMAW AC pulsed arc: t = 1,5 mm , vD = 3,8 m/min, vS = 1 m/min

GMAW AC pulsed arc

Part of negative electrode low- more heat input- more penetration

Part of negative electrode high- less heat input- less penetration

Timisoara 2007 27

Rhythmical change between short arc and pulsed arcSuperpuls welding, company ESAB

3,23,2

Timisoara 2007 28

Summary

• Compared with step-switched welding rectifiers, transistorizedpower sources make it easier to optimize the electric arc ignition and the bead start and end.

• Spatter can be avoided with any type of short arc welding only if the power peaks are sufficiently low during the electric arc re-ignition.

• The quality of the available characteristic synergy curve should be considered when selecting a welding facility or line.

• The following special short arc types are currently ready for practical welding applications:CMT, ColdArc, dip transfer, SCW with pulsed current, SCW with minus polarity, STT, FastROOT, ...

Timisoara 2007 29

Summary (continued)

• The combination of pulsed and short arcs especially facilitates adapting the energy input per unit length and the optimizationof the bead profile in the upper metal sheet range.

• AC and DC pulsed arcs with intermediate pulses expand the application scope of the electric arc in the lower power range.

• Electric arc interferences due to flawed or insufficient wire electrodes and contact pipes as well as excessively long supply pipes can be avoided by the control functions of the systems only to a limited extent.

• Minimizing the electric arc power (energy input per unit length) increases the risk of weld seam flaws and joining defects. This means that processes and quality assurance inspections must usually be carried out more carefully.