WELDING RESEARCH

Effects of Laser Parameters on Porosity

Formation: Investigating Millimeter Scale

Continuous Wave Nd:YAG Laser Welds

An evaluation of the role continuous wave laser process parameters has on

pore formation in milliscale keyhole mode welds is presented

BY J. T. NORRIS, C. V. ROBINO, D. A. HIRSCHFELD, AND M. J. PERRICONE

KEYWORDS

Porosity

Milliscale

Nd:YAG Laser Keyhole

Mode Welds

Continuous Wave (CW)

Type 304L Stainless Steel

J. T. NORRIS

Introduction

Porosity in laser welds has largely been

attributed to laser keyhole instabilities in

the weld pool. Collapsing of the keyhole is

thought to trap a gas pocket creating a

void (pore) in the weldment — Fig. 1. The

mechanisms that lead to instability are not

fully understood as many factors are be-

lieved to contribute to the general condi-

tion of the keyhole. Factors including weld

size, laser output modes (e.g. pulsed, con-

tinuous wave [CW], and modulated), ma-

terial type, and weld joint geometry, all

impact laser keyhole formation and evolu-

tion. Given that many variables affect key-

hole stability, it is not surprising that many

researchers consider porosity formation

to be random (Refs. 1–3). Prescribed

methods for avoiding this defect are scarce

in the literature and tend to focus pre-

dominately on large-scale welds, com-

monly much greater than a 2-mm pene-

tration (Refs. 2, 4). Very little information

on mitigation tactics for smaller milliscale

(

The basic cause of porosity is metal vapour, which is also common in EBW. Pulse laser welding with proper parameters might be a good solution.

Among the possible causes are keyhole instability, improper gas shielding, surface contamination and inclusions in the steel. The first one creates small and very regular spaced porous near the weld root. The second could be seen by unstable very intense plasma during processing. The other two causes a lot of small bobbles everywhere in the molten zone.

The concentration of hydrogen in the open environment in stainless steel can arise to 5-10 weight ppm. The amounth is depend to: humidity, temperature, pressure, interaction with water (reaction with water solutions) and more. To decresed porosity you can applied backing treatment in higher temperatures ( like in vaccum) or heat the sample in vaccum (10 to -6) at 150 celsius degrees during 24 hours. Gases porosion process will be decreased. Paul

Particularly, in the laser welding is very common that the porosity is not specifically associated with the steel, but the welding parameters. Unless you're welding coated steel (galvanized) or pretreated steel…

Process instabilities related to travel speed vs solidification rate that generated entrepped gases

Various possible causes as following :

A. Improper shielding

B.improper edge preparation ; notches etch

C. Moistured plate

D. Laser keyhole and speed improper setting or not according to WPS/PQR instruction

E.oxydisation, paint coating, galvanization plate

F. Windy environment

I agree with Dr. Rethmeier. For example by welding of overlapp joints of Zn coated steel in car body production the noppen are made on the one part of joint, so that the thin area between two plates gives a possibility for Zn vapour to leave the welding place.

Pores (voids) formation inside the keyhole associates with the hydrodynamic fluctuations of capillary nature. It is confirmed by the capillary model of hydrodynamic processes which has correlation with empirical data. According to this model the dynamic fluctuations in keyhole completely disappear at some value of optimal technological speed VT . The fast growth of the pores volume up to maximum value should be observe with increase of welding speed from VW = 0 up to VW = 0.25 VT . And then with further increasing of welding speed the pores volume should be decrease to zero at the optimal speed VW = VT.

The frequency of voids formation should change somewhat otherwise. It reaches a maximum at VW = 0.81 VT, and then promptly reduces to zero at VW = VT. This dependence is also confirmed by the experiment (fig. 1).

Coincidence of calculated and empirical dependences of volume of voids, and also of frequency of voids formation in dependence on welding speed confirms the hydrodynamic model of keyhole formation and voids formation.

Falling character of both dependences (especially, of frequency) in the range close to technological speed VT imposes severe constraint on accuracy of definition of VT value.

This dependence of pores volume with welding speed changes is confirmed by the measurements.

Unfortunately, there is no application of capillary model to the TRIP steel. But I think the main relations of process remain of the capillary nature.

in laser welding porosity can form due to keyhole instability, specially when you are working at lower welidng speed. keyhole can not stay stable and when it collapse cuase the vapour trap inside and form porosity.

these porosities are mostely near to the root.

u can increase the welding speed or work in conduction mode if it is possible.

theother reason for porosity formation in laser welding can be the same as other welding process...