GMAW welding and brazing of thin sheet metal using modern power sources

Transcription

1 Schweißtechnische Lehr- und Versuchsanstalt SLV München M - 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 TECHNOLOGIES FOR JOINING ADVANCED MATERIALS June Timisoara

2 Short arc GMAW welding and soldering of thin sheet metal 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 2

3 Short arc GMAW welding and soldering of thin sheet metal User requests - examples aluminium - optimized welding start without spatter and lack of fusion - possibility to join material mixtures coated steel - back of coated steel without burning of the zinc content to avoid corrosion 3

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 and optimized weld interface Thickness of sheet: 1 mm and 2,5 mm 4

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,... 5

6 Suitable metal transfer forms with GMAW welding of thin sheet metal 6 Short arc cycle (with electric arc interruptions) Pulsed arc cycle (without electric arc interruptions) Working ranges short arc

7 Short arc diagram metal 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-ignition should be carried out with limited power to avoid spatter. 7

8 Short arc Possibilities to influence the power peak during re-ignition large Throttling effect Characteristic curve slope small Limited short-circuit current Spatter due to current and power being too high during arc re-ignition 8

9 Conventional short arc Excessive change of the short-circuit frequency and power peaks (with spatter) due to interference problems due to: irregular wire speed in contact tube long hose package inside diameter of a to large wire guide tube flawed wire surface quality greater torch distance changes 9

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 10

11 ColdArc-process according to EWM arc on Phase 1 Lichtbogen brennt short circuit Phase 2 Kurzschluß re-ignition arc on Phase 3 Auflösung des Kurzschlusses und erneute Brennphase coldarc process coldarc convent. short arc Us Is Us Is t t t A new high dynamic inverter switching in 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 power reduction during re-ignition. Thin sheet can be welded or brazed with low heat input. 11

12 coldarc-process according to EWM current voltage arc power 12

13 coldarc-process according to EWM arc power conventional short arc arc power coldarc Power in time of re-ignition 13

14 GMAW soldering with triggered short arc ColdArc process Company EWM Basic material: H260+Z100 t = 1.0 / 2.5 mm v L = 50 cm/min 14

15 Triggered short arc STT process (Surface-Tension-Transfer) Arc power [kw] voltage [V] current int. [A] Company Lincoln Quick current reduction with electric arc reignition wire electrode: , 1.2 mm shielding gas: 2.5 % CO 2, residual Ar 15

16 CMT process (Cold Metal Transfer) by papers from Fronius t = 0 ms t = 4.59 ms t = 6.21 ms t = 7.56 ms t = ms t = ms t = ms t = ms The withdrawal movement of the wire supports drop separation and stabilization of the electric arc length. Special push-pull wire feed and wire buffer needed. 16

17 CMT process (Cold Metal Transfer) by papers from Fronius current int. [A] voltage [V] Electric arc interruption 17

18 CMT process (Cold Metal Transfer) by papers from Fronius welding plant with special push-pull wire feed and wire buffer 18

19 CMT process Aluminum 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 19

20 CMT process used for combining steel and aluminum Lap joint Sheet thickness 1 mm Welding speed 70 cm/min (27.6 /min) Principle: Aluminum-side welding Steel-side soldering Requirement: Galvanized sheet steel (> 10 µm) 20

21 GMAW pulsed arc Material transfer without electric arc interruption Current intensitiy pulse current (causes quick drop forming and drop transfer) background current (avoids arc instabilities and forms the drop) time 21

22 GMAW pulsed arc current forms DC AC conventional pulsed arc pulsed arc with small interpulses to stabilize the process during the long background current time 22

23 GMAW AC pulsed arc parameter diagram company ELMATECH 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. 23

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 24

25 GMAW AC pulsed arc High-speed pictures shows the differences between negative and positiv polarity during a period 25

26 GMAW AC pulsed arc + + Stromstärke 14 % 45 % 0 Zeit Stromstärke 0 Zeit Part of negative electrode low Part of negative electrode high - more heat input - less heat input - more penetration - less penetration GMAW AC pulsed arc: t = 1,5 mm, v D = 3,8 m/min, v S = 1 m/min 26

27 Rhythmical change between short arc and pulsed arc Superpuls welding, company ESAB 3,2 27

28 Summary Compared with step-switched welding rectifiers, transistorized power 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,... 28

29 Summary (continued) The combination of pulsed and short arcs especially facilitates adapting the energy input per unit length and the optimization of 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. 29