Outline: Absolute spot size effect on penetration depth in laser welding

Transcription

1 Absolute spot size effect on penetration depth in laser welding Wojciech Suder Stewart Williams aul Colegrove Welding Engineering Research Centre Cranfield University ILAS 9 Outline: Intensity-interaction time concept (material dependent parameters ) for laser welding Absolute spot size effect Characteristic time enetration efficiency Conclusions 1

2 ower, Travel Speed Traditional Approach 1 [mm] enetration Depth [ 1 1 kw 5kW kw Intuitive Easy to implement Difficult to transfer between different laser systems Travel Speed [m min -1 ] Intensity Interaction Time Concept Laser-material interactions J.C. Ion Laser processing of engineering materials.

3 Intensity, Interaction Time, rocess Energy Intensity I r [ Wm ] Interaction Time t i r [ s ] v r Constant welding speed rocess Energy E I t A i r r v J laser power; v welding speed; r beam radius MW/cm and ms instead of kw and m/min e enetration Depth [mm] I r [ Wm ] 17.3 ms mmin ms.5 mmin -1 3 ms 1 mmin ms 7. ms mmin -1.7 ms 5 mmin ms mmin -1.5 ms 15 mmin -1 r t i [ s ] v Intensity ower [MW [kw] cm -1 ] enetration Dep Dep pth pth [mm] [mm] MW cm MW cm -. MW cm High speed or small spot size Travel Interaction Speed Time [m min [ms] -1 ] Low speed or large spot size Allow comparison of different laser sources kw 5kW kw 3

4 ] How to Keep Intensity and Interaction Time Constant 1 I r ower [kw] 1 1.MW cm 1.MW cm MW cm Beam radius [mm] Travel Speed [m min -1 ] ms ms r v t i Constant intensity of.mw cm -1 and interaction time of ms Beam radius [mm] Beam Radius Effect If we set the intensity and interaction time to be the same If we set the intensity and interaction time to be the same with different beam radius what do we get?

5 Beam Radius Effect 1 1 Beam radius.39mm Beam radius.19mm mm] enetration Depth [ Interaction Time [ms] Constant intensity of 1.7MW cm - Beam Radius Effect enetration Depth [mm] Interaction Time 3ms Interaction Time 19ms Interaction Time 7.ms Interaction Time.5ms Beam Radius [mm] Why does the penetration go up with beam radius? rocess energy increases as the beam area increases E I t A But why does it depend on interaction time? i constant J Constant Intensity of 1. MW cm - ; mild steel 5

6 Thermal Diffusion Effect Determined by the characteristic thermal time Interaction ms time below interaction threshold time value ms interaction ti time Welding direction reheated front Welding direction reheated front Side losses Negligible conduction Side losses There is less time for conduction as interaction time decreases Characteristic Thermal Time T t T l [s] T l characteristic thermal length [m] κ thermal diffusivity [m s -1 ] Characteristic thermal time is in effect the thermal time constant for a particular material Characteristic thermal length defines the distance that heat conducts within characteristic thermal time

7 Beam Radius Effect enetration Depth [mm] Interaction Time 3ms Interaction Time 19ms Interaction Time 7.ms Interaction Time.5ms If interaction time is shorter than characteristic time heat conduction transfer is negligible and there is no influence of process energy on penetration Beam Radius [mm] Constant Intensity of 1. MW cm - ; mild steel Characteristic Thermal Time vs. Welding Conditions Characteristic Time [m ms] tion Depth [mm] enetrat Interaction Time 3ms Interaction Time 19ms Interaction Time 7.ms Interaction Time.5ms Beam Radius [mm] Tl Tt [ s] 7.ms spot size starts affecting the penetration ms no spot size effect on Characteristic Length [mm] penetration Characteristic thermal time in function of characteristic length for mild steel, thermal diffusivity 1.17* -5 m s -1 7

8 What does this mean for Efficiency? enetration efficiency is defined as the energy required per unit depth of weld The optimum penetration efficiency corresponds to minimum heat input for a particular weld depth eff D I Ti A D [ m J E p 1 ] D - penetration depth I - intensity Ti - Interaction time A - spot area E p - process energy enetration Efficiency vs. Interaction Time for Different Intensities e enetration Efficiency [m mm kj -1 ] mm mm.mwcm - 1.MWcm - 1mm 1 Log Interaction Time [ms] Constant spot radius.3mm

9 enetration Efficiency for Different Beam Radii enetration Efficiency [ mm kj -1 ] Ep=7.7J Ep=3J Ep=9J 1 Beam radius.39mm Beam 1 radius.19mmbeam radius.19mm Beam radius.3mm Depth [mm] enetration Ep=J ms 1 Constant intensity of 1.7MW cm - ms 5ms Log Interaction Time [ms] 5ms Log Interaction Time Time [ms] [ms] eff D [ m J E p 1 ] Keyhole Welding Regimes Long Interaction Time Constant Intensity Short Interaction Time Low Efficiency High Efficiency Conduction No Conduction Thermal Losses reheating Small Thermal Losses Insignificant reheating 9

10 Laser System arameter Selection Operating beam diameter Defects Weld width System limitations Depth of focus/working distance Focus shift? Maximum efficiency/minimum heat input Maximum interaction time Travel speed enetration depth requirement Intensity Laser power Conclusions By studying keyhole welding with constant intensity and interaction time two distinct regimes were identified : Long interaction time Lateral thermal diffusion i effects Large effect of process energy (beam radius) due to pre-heating on penetration depth Low process efficiency due to lateral thermal losses Short interaction time No lateral thermal diffusion effects No effect of process energy (beam radius) on penetration depth High process efficiency i as no lateral l thermal losses Transition is determined by the characteristic thermal time of the process

11 Thank You for Attention 11