porosity, underfill and magnesium loss during continuous wave...
TRANSCRIPT
ABSTRACT. The influence of variouswelding parameters on porosity and un-derfill formation and magnesium lossduring continuous wave Nd:YAG laserbeam welding of thin plates of alu-minum-magnesium Alloys 5182 and5754 was investigated. The porositywithin the welds was characterized byradiography, optical microscopy andSEM. The compositional change in thewelds was measured by electron micro-probe analysis.
The experimental results showed thatthe instability of the keyhole was thedominant cause of macro-porosity for-mation during laser welding of thin platesof aluminum Alloys 5182 and 5754. Hy-drogen did not play a significant role inporosity formation. Although underfillwas commonly observed at the root offull-penetration welds, sharp or deepnotches, which are harmful to the me-chanical properties of the welds, werenot present. Reduction in magnesiumconcentration was more pronouncedduring conduction mode welding. Weld-ing in keyhole mode resulted in muchlarger weld pool and less pronouncedcomposition change. The extent of defo-cusing of the laser beam greatly affectedthe stability of the keyhole, weld poolgeometry, pore formation and composi-tion change.
Introduction
The U.S. automotive industry is cur-rently facing increased demands to si-multaneously increase its fleet averagefuel economy and reduce greenhousegas emissions. In order to meet these newstandards, the industry is increasinglymoving toward decreasing the weight ofthe vehicles through the use of new ma-terials, especially lightweight aluminum
alloys (Ref. 1). One of the major factorsin their implementation involves the abil-ity to fabricate, easily and reproducibly,structurally sound and defect-free welds.Laser beam welding is particularly criti-cal to reduce the weight of the bodystructure through increased use of alu-minum alloys and tailor welded blanks.
Porosity, loss of alloying elementsand, for some heat treatable aluminumalloys, solidification cracking are themost common problems encountered inthe laser welding of these alloys. Thepoor coupling between aluminum alloysand the laser beam is another major con-cern. Aluminum alloys absorb the lasermore efficiently as the laser wavelengthdecreases (Ref. 2). Duley (Ref. 3) reportedthat the Nd:YAG laser with a characteris-tic wavelength of 1.06 µm provided bet-ter coupling with aluminum than theCO2 laser, which has a characteristicwavelength of 10.6 µm. Furthermore, theabsorption of the laser beam increasesdrastically when a keyhole is formed dueto multiple reflections of the beam in thekeyhole (Ref. 4). A Nd:YAG laser is there-fore more attractive for the welding ofaluminum alloys.
The detrimental effect of porosity onthe mechanical properties of aluminumwelds has been documented in the liter-ature (Refs. 5–7). However, the mecha-nism of porosity formation during laser
beam welding is less well understood.Pore formation has been linked to hy-drogen rejection from the solid phaseduring solidification (Refs. 8–11), imper-fect collapse of the keyhole (Refs. 8,12–14) and turbulent flow in the weldpool (Ref. 15). Sources of hydrogen in theweld metal include the filler metal and,to a lesser extent, the shielding gas andthe base metal (Ref. 16). Woods (Ref. 11)showed that porosity does not form whenthe hydrogen content in aluminum alloysis lower than a threshold level. Thethreshold level varies for different alu-minum alloys due to their differences inhydrogen solubility. Therefore, control-ling the hydrogen content in the metal tobelow the threshold level can effectivelycontrol hydrogen porosity. However, se-vere porosity has been observed consis-tently (Ref. 17) during autogenous laserwelding of aluminum alloys even whenhydrogen contamination was minimizedfrom the three known sources. Therefore,imperfect collapse of the keyhole and/orturbulent flow in the weld pool are im-portant causes of porosity during laserwelding of aluminum alloys.
Loss of volatile alloying elements,such as magnesium and zinc, due to se-lective vaporization is a common occur-rence in the laser welding of aluminumalloys (Refs. 18–22). Automotive alu-minum alloys are either solid-solutionstrengthened, such as the Al-5xxx alloyscontaining magnesium, or precipitationstrengthened, such as Al-6xxx alloyscontaining Mg2Si. Loss of magnesiumduring the laser welding of Al-5xxx andAl-6xxx alloys may affect the degree ofstrengthening and cause degradation ofthe mechanical properties of these al-loys. The change in weld metal compo-sition depends on the vaporization rateand the volume of the weld pool (Refs.23, 24). Although the rate of vaporizationincreases with laser power, the change incomposition is most pronounced at lowpowers because of the small size and,consequently, the high surface-to-vol-
WELDING RESEARCH SUPPLEMENT | 207-s
RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
/RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
Porosity, Underfill and Magnesium Loss duringContinuous Wave Nd:YAG Laser Welding of
Thin Plates of Aluminum Alloys 5182 and 5754
BY M. PASTOR, H. ZHAO, R. P. MARTUKANITZ AND T. DEBROY
Keyhole stability is found to play a major role in porosity formation
KEY WORDS
Nd:YAGLasersWeldingPorosityUnderfillAluminumKeyhole ModeThin PlateM. PASTOR, H. ZHAO, R. P. MARTUKANITZ
and T. DEBROY are with the Department ofMaterials Science and Engineering, The Penn-sylvania State University, University Park, Pa.
ume ratio of the weld pool (Ref. 24). Mostof the previous research on the quantita-tive understanding of vaporization rateduring laser welding has focused on thewelding of pure metals and steels (Refs.25–28). A realistic calculation of vapor-ization rates involves solution of theequations of conservation of mass, mo-mentum and kinetic energy. These rates
and the geometry of theweld pool can then beused to calculate thecomposition changeduring laser welding(Refs. 29–32). How-ever, for high-powerlaser welding of auto-motive aluminum al-loys, a comprehensivetheoretical model tocalculate weld poolgeometry, temperaturedistribution, vaporiza-tion rate and composi-tion change still re-mains to be developed.
Solidification crack-ing is a major concernduring laser welding ofaluminum alloys (Refs.21, 33–37), especiallypulsed laser welding(Refs. 21, 33, 34). Opti-mization of pulsing pa-
rameters is required to avoid solidifica-tion cracking (Ref. 34). The Al-2xxx andAl-6xxx heat treatable aluminum alloysare more prone to solidification crackingthan the Al-5xxx non-heat-treatable al-loys (Refs. 35, 36). Laser welding of Al-2xxx and Al-6xxx alloys requires theproper selection of filler metals to avoidsolidification cracking, while most of the
Al-5xxx alloys can be welded autoge-nously using a continuous-wave laserbeam with no solidification cracking(Ref. 37).
The purpose of this research is to in-vestigate the influence of welding para-meters on porosity and underfill forma-tion and compositional change duringcontinuous-wave Nd:YAG laser weldingof thin plates of aluminum Alloys 5182and 5754 and identify the cause of poros-ity formation. The influence of powerdensity was evaluated by welding at sev-eral positive and negative defocus valuesand the influence of the heat input wasexamined by welding at several speeds.The change in the concentration of mag-nesium in the weldment was measuredusing electron microprobe analysis.
Experimental
Bead-on-plate autogenous weldswere produced using a 3.0-kW continu-ous-wave Nd:YAG laser on thin plates of5182 and 5754 aluminum alloys withthicknesses of 1.0 mm and 1.45 mm, re-spectively. Both alloys were in the an-nealed condition prior to welding. Thechemical compositions of the alloys areshown in Table 1. The welding arrange-ment is shown in Fig. 1. The outputpower of the laser source was set at 3.0kW, which resulted in 2.6 kW being de-
208-s | JUNE 1999
RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
/RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
Fig. 1 — Nd:YAG laser welding arrangement.
Fig. 2 — A — Cross sections of Alloy 5182 welds for defocus values in the range of –2.0 mm to +2.0 mm. Nominal power 3 kW, welding speed250 in./min (105.8 mm/s) and shielding gas flow rate 200 ft3/h (5.66 m3/h) of helium; B — cross sections of Alloy 5754 welds for defocus valuesin the range of –2.0 mm to +2.0 mm. Nominal power 3 kW, welding speed 150 in./min (63.5 mm/s) and shielding gas flow rate 200 ft3/h (5.66m3/h) of helium.
A B
livered to the workpiece. The beam wasdelivered using a 600-µm diameter fiberof fused silica to a f2 focus optics manip-ulated through a micropositioning stagemounted on a linear translation device.The focal length of the f2 optics forNd:YAG laser is 77.7 mm. The beam ra-dius at the focal point is 300 µm. Thebeam was provided at a 75-deg forwardangle relative to the workpiece to preventdamage to the optics due to back reflec-tion. An ancillary copper nozzle havinga 8.0-mm inside diameter was utilized toprovide shielding gas. This gas nozzlewas directed opposite to the direction oftravel at an angle of 30 deg with theworkpiece. During welding, the alu-minum plates were placed horizontallyon a copper back plate. The back platehad a U-shaped groove of 2.0-mm widthand 1.5-mm depth under the weld re-gion. Therefore, the liquid metal was notsupported by the back plate. Helium wasused as the shielding gas. Because of itshigh thermal conductivity, helium caneasily conduct heat away from theplasma plume and keep the plasma vol-ume small.
The effect of laser beam defocusingwas evaluated by altering the distance be-tween the workpiece and the optics to ob-tain defocusing distances ranging from–2.0 mm to +2.0 mm. The variation ofbeam radius and power density with dis-tance from the focal point is given in theappendix. The focal point position of theNd:YAG laser was determined with thehelp of a He-Ne red diode focusing laserwhose focal length was 0.104 in. (2.64mm) shorter than that of the Nd:YAGlaser. A stainless steel plate of 0.104-in.thickness was placed on the specimentable and its elevation was adjusted tofocus the He-Ne laser on the stainless
steel plate surface. The original specimentable surface without the stainless steelplate was taken as the focal point for theNd:YAG laser. A nomenclature of positivedefocusing to represent the focal point tobe above the top surface of the workpieceand negative defocusing to represent thefocal point to be below the top surfacewill be used throughout this paper. The ef-fect of travel speed on weld characteris-tics was also investigated by varying thetravel speed of the laser beam from 125to 300 in./min (52.9 to 127 mm/s).
To examine the effect of hydrogen onporosity formation, both dry and wet he-lium were used. The wet helium was ob-tained by bubbling dry helium throughwater prior to its use.The partial pressure ofwater vapor at equilib-rium with pure water is0.03 atm at 298 K.However, the actualpartial pressure ofmoisture in the shield-ing gas due to bubblingwas found to be 0.008atm based on theweight loss of the waterfrom the bubbler. Theflow rate of the shield-ing gas was set to be200 ft3/h (5.66 m3/h)for both dry and wethelium.
The weld geometrywas determined usingoptical microscopyand computer-assistedimage analysis. Poros-ity was characterizedby radiography utiliz-ing X-ray source, opti-cal microscopy and
SEM. The radiographs of the weldedsamples were obtained using 30 kV and2.5 mA with 38 s exposure.
The concentration profiles of magne-sium were determined using a CamebaxSX50 electron microprobe, operated at15 kV and with about 12 mA beam cur-rent. In order to obtain the spatial varia-
WELDING RESEARCH SUPPLEMENT | 209-s
RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
/RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
Fig. 3 — Depth of penetration in laser welded aluminum alloys at several defocus values. A — 5182; B — 5754. Nominal power 3 kW, weldingspeeds 250 in./min (105.8 mm/s) and 150 in./min (63.5 mm/s) for Alloys 5182 and 5754, respectively, and shielding gas flow rate 200 ft3/h (5.66m3/h) of helium.
Table 1 — Chemical Composition ofAluminum Alloys 5182 and 5754
Material Mg Si Mn Zn
5182 4.44 0.20 0.35 0.07
5754 2.82 0.30 0.45 0.02
A B
Fig. 4 — Schematic diagram showing interaction of the laser beamwith the liquid surface at positive and negative defocusing posi-tions. At positive defocusing, the beam power density decreaseswith the deepening of the cavity, restricting the growth of the cav-ity. At negative defocusing, the beam power density increases withthe deepening of the cavity, resulting in a deeper keyhole.
tion of concentration in the bulk weldmetal and eliminate the effect of inter-dendritic segregation, the signal for eachdata point was averaged from a 45 x 55µm area. The overall change in magne-sium concentration owing to weldingwas calculated from the average compo-sition of the fusion zone.
Results and Discussion
Weld Pool Geometry and Underfill
The fusion zone cross sections pro-duced by laser welding of Alloys 5182
and 5754 at different defocusing areshown in Fig. 2. No crack was observedin the welds under the optical micro-scope. The depth of penetration as afunction of defocusing for the welds isgiven in Fig. 3. It is observed from Figs. 2and 3 that when welding was performedusing a focused beam, a high depth ofpenetration, characteristic of the keyholemode of welding, was obtained. On theother hand, when highly defocusedbeams were used, the depth of penetra-tion was low and the weld pool shapeswere characteristic of the conductionmode of welding. When the power den-
sity was near the threshold for keyholeformation, the mode of welding was notreproducible and pool shapes character-istic of either the keyhole or the conduc-tion mode were observed in various crosssections of the same welded sample. Inthis range, the keyhole was believed tobe unstable due to the occurrences ofsmall disturbances within the system,which led to collapse of the keyhole. Forexample, this behavior was observed forAlloy 5182 when the extent of defocus-ing was in the range of –1.0 to –2.0 mmand +0.5 to +1.5 mm, as shown in Fig. 3.This figure also shows similar behavior
210-s | JUNE 1999
RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
/RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
Fig. 5 — Underfill at the root of the weld pool of Alloy 5182 for sev-eral defocus values. Nominal power 3 kW, welding speed 250 in./min(105.8 mm/s) and shielding gas flow rate 200 ft3/h (5.66 m3/h) of he-lium.
Fig. 6 — Underfill at the root of the weld pool of Alloy 5754 for sev-eral defocus values. Nominal power 3 kW, welding speed 150 in./min(63.5 mm/s) and shielding gas flow rate 200 ft3/h (5.66 m3/h) of he-lium.
Fig. 7 — A — Cross sections of Alloy 5182 welds obtained with a focused beam for various welding speeds. Nominal power 3 kW and shieldinggas flow rate 200 ft3/h (5.66 m3/h) of helium; B — cross sections of Alloy 5754 welds obtained with a focused beam for various welding speeds.Nominal power 3 kW and shielding gas flow rate 200 ft3/h (5.66 m3/h) of helium.
A B
for the welding of Alloy 5754 in the rangeof –1.5 to –2.0 mm and +0.5 to +1.5 mmdefocusing. The corresponding esti-mated laser power densities on the spec-imen surface were 8.4 x 105, 6.7 x 105,5.0 x 105 and 3.7 x 105 W/cm2 for ±0.5,±1.0, ±1.5 and ±2.0 mm defocusing, re-spectively.
Figures 2 and 3 show the weld poolsize at negative defocusing is larger thanthat for the same extent of positive defo-cusing. Arata and coworkers (Ref. 38) alsoobserved larger pool size at negative de-focusing. The pool size difference can beexplained by considering the interactionof the beam with the liquid weld pool sur-face (Ref. 38). Near the threshold powerdensity for keyhole formation, the liquidsurface is depressed by the recoil force ofevaporation, and a shallow depression isproduced. As shown in Fig. 4, for positivedefocusing, the beam is divergent and thedepression moves the liquid surface awayfrom the focal point. As a result, thepower density decreases as the liquid sur-face moves from position 1 to position 2,as shown in this figure. Since the beampower density is near the threshold value,it would decrease below that value if thecavity were to grow any further. Thus,positive defocusing restricts the cavityfrom further growth. In contrast, at nega-tive defocusing, the beam is convergentand the depression of the liquid surfaceexposes the bottom surface to a higherpower density. Exposure of the liquid sur-face to progressively higher power den-sity allows the cavity to grow deeper(from position 3 to position 4 in Fig. 4).Thus, once a shallow keyhole is producedat negative defocusing, it has a tendencyto grow into a deep keyhole and conse-quently produce a large weld pool.
The extent of defocusing had pro-nounced influence on underfill forma-
tion, which was measured by themaximum linear depth of depres-sion at the root of the weld. The in-fluence of defocusing on underfillformation was measured from thecross sections of the welds con-ducted at several defocus valuesfor three sets of experiments, andthe results are presented in Figs. 5and 6 for Alloys 5182 and 5754, re-spectively. The underfill was ob-served in the range from –2.0 mmto +0.5 mm of defocusing in Alloy5182 and in the range from –1.5mm to +0.5 mm of defocusing inAlloy 5754. Because of the com-plexity of the physical phenomenainvolved, a quantitative under-standing of the evolution of the un-derfill at the bottom of the weldpool does not exist at the presenttime. In welds with underfill, spat-tered metal particles were col-lected in the back-plate groovebelow the weld region. This be-havior indicates that the underfillwas caused by expulsion of themolten metal. When the vaporpressure in the keyhole increasesabove a certain level, it may opena conduit at the bottom of the weldand the flow of gas can carry someliquid metal. As the laser beammoves forward, underfill may formif the molten metal can not refill allthe depressions at the bottom of theweld.
The control of the laser power densityby changing the extent of laser beam de-focusing did not result in welds com-pletely free of underfill. However, sharpor deep notches, which are harmful tothe mechanical properties of the weld,were not observed in the cross section ofthe welds as a result of underfill.
Figure 7 depicts the fusion zone crosssections at different welding speeds. It isobserved that the keyhole mode of weld-ing with complete penetration wasachieved at welding speeds up to 150in./min (63.5 mm/s) for Alloy 5754 andup to 250 in./min (105.8 mm/s) for Alloy5182. At higher speeds, nonuniform pen-etration was observed, and the keyhole
WELDING RESEARCH SUPPLEMENT | 211-s
RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
/RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
Fig. 8 — Porosity produced at several defocus values. A — 5182; B — 5754. Nominal power 3 kW, welding speeds 250 in./min (105.8 mm/s) and150 in./min (63.5 mm/s) for Alloys 5182 and 5754, respectively, and shielding gas flow rate 200 ft3/h (5.66 m3/h) of helium.
Fig. 9 — Cross sections of the welds of Alloy 5754showing characteristic macroporosity produced bythe instability of the keyhole. A — Spherical porosity;B— irregular-shaped porosity. Nominal power 3 kW,welding speed 150 in./min (63.5 mm/s) and shieldinggas flow rate 200 in./min (5.66 m3/h) of helium.
A
A
B
B
was unstable. Melt through was observedat welding speeds of less than 200in./min (84.7 mm/s) for Alloy 5182 dueto excessive heat input.
Porosity
Two types of porosity are common inthe welding of aluminum alloys. Poreswith diameters larger than 0.2 mm,which can be observed by radiography,are called macropores. On the other
hand, pores with diametersof several micrometers,which can only be ob-served by optical mi-croscopy or SEM, are calledmicropores. Therefore,both radiography and mi-croscopy were used tocharacterize porosity in thewelds.
Macroporosity
It was found that themacropores, which arereadily detected by radiog-raphy, were the main formof porosity in both alloys.Figure 8 shows the amountof macroporosity in thewelds. Three sets of experi-ments are presented for
both the alloys. The data indicate thatporosity is minimum when welding isconducted in either keyhole or conduc-tion mode. In contrast, high porosity inthe weld metal is observed in the transi-tion region where the keyhole is not sta-ble. Therefore, the formation of macrop-orosity seems to be linked to theinstability of the keyhole. Figure 9 showstwo types of macropores in the welds.These pores did not form due to hydro-
gen rejection because, as will be dis-cussed below, the high cooling rate dur-ing the laser welding would not allowgrowth of hydrogen-induced pores toreach this size. Therefore, these pores areconsidered to be entrapped gas bubbles,which were formed due to the unstablecollapse of the keyhole. The main con-stituent of the pores is believed to be theshielding gas. The spherical pore in Fig.9A was formed as the liquid metal sur-rounding it solidified uniformly. The ir-regular-shaped pore in Fig. 9B wasformed as the trapped gas bubble waspushed against the solid-liquid interfaceby the flow of liquid metal.
Kim, et al. (Ref. 39), found moreporosity to be formed at negative defo-cusing than at positive defocusing inlaser spot welded 6061 aluminum alloyplates. The fusion zone cross sectionsshown in Fig. 2 indicate for positive de-focusing there is a gradual transition fromconduction mode to keyhole mode.Here, the weld penetration increasesgradually with a change in defocusing.For negative defocusing, the instability ofthe keyhole is more pronounced in thetransition region.
The difference in the keyhole behav-ior between positive and negative defo-cusing in the two transition regions canbe explained by considering the stabilityof the keyhole. The keyhole is inherentlymore stable at positive defocusing, asshown in Fig. 10. In this arrangement, thebeam-material interaction area increasesas the depth of the keyhole increases. If adisturbance causes the keyhole to ex-pand, the beam-material interaction areaalso expands, and the resulting lowerpower density tends to shrink the key-hole. Similarly, any disturbance thattends to shrink the keyhole causes expo-sure of the keyhole to a higher powerdensity, which, again, opposes theshrinking of the keyhole. Therefore, apositively defocused laser beam tends tooppose any changes from the stable key-hole size. This explains the smooth tran-sition from conduction mode to keyholemode for a positively defocused laserbeam. On the other hand, since the tran-sition from keyhole to conduction modeoccurs in the range of –1.5 to –2.0 mm,the focal point is below the bottom of theplate. In this configuration, the keyhole isinherently unstable because the powerdensity increases all the way from the topto the bottom. Therefore, when a distur-bance increases the depth of the keyhole,the material is exposed to a even higherpower density that, in turn, further in-creases the keyhole depth. Thus, the ob-served variation in the weld metal geom-etry is consistent with instability of thekeyhole during the transition from con-
212-s | JUNE 1999
RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
/RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
Fig. 10 — Self-stabilizing effect of positively defocused laserbeam. The graph shows if the keyhole expands due to distur-bance in the system, the laser power density decreases due tothe increased beam-keyhole interaction area at the bottom ofthe keyhole. As a result, the keyhole returns back to its stablesize. Similarly, if the keyhole shrinks due to disturbance, thelaser power density increases. This increase opposes shrink-ing of the keyhole.
Fig. 11 — Influence of welding speed on pore formation during laser welding of aluminum Al-loys 5182 and 5754 using a focused beam. Nominal power 3 kW and shielding gas flow rate200 ft3/h (5.66 m3/h) of helium.
duction mode to keyhole mode when thefocal point of the beam is below the bot-tom of the plate.
Figure 11 shows the influence of thewelding speed on the extent of porosity.At a given defocusing, as the weldingspeed is increased, the mode of weldingchanges from keyhole to conductionregime. During this transition, the key-hole is unstable. It is observed that thehighest level of porosity is obtained in re-gions where an unstable keyhole isformed, whereas porosity is minimizedwhen the welding is conducted in eitherthe stable keyhole or conduction mode.For example, porosity in Alloy 5754welds was most pronounced between150 and 200 in./min (63.5 to 84.7 mm/s).At a welding speed of 300 in./min (127mm/s), the welding takes place in con-duction mode and porosity-free weldswere obtained. These results, again, in-dicate that macroporosity is formed dueto instability of the keyhole.
Since hydrogen is generally consid-ered to be the main cause of porosity inthe welding and casting of aluminum al-loys, the role of hydrogen on porositywas examined by using both dry and wethelium as the shielding gas. The volumepercent macroporosity in the weldsusing dry and wet helium as the shield-ing gas is given in Fig. 12. Comparing thecurves for dry and bubbled helium, it isobserved that the volume of macrop-orosity does not significantly change be-yond the experimental uncertainty withthe addition of moisture to the heliumshielding gas. In both cases, the volumepercent of macroporosity was the high-est when the welding mode was unsta-ble and alternated between keyhole andconduction modes. These experimentsprovide further evidence that the insta-bility of the keyhole is the main cause ofmacro-porosity in the welding of Alloys
5182 and 5754. Segregation of hy-drogen did not play any significantrole in the formation of macrop-orosity in these welds.
Microporosity
In this investigation, very fewmicropores were observed in thewelds. Figure 13 shows three typesof micropores in the welds. TypesA and B have a size range between10 and 30 µm. However, Type A isirregular in shape and Type B isspherical in shape. These two typesof porosity are most likely causedby shrinkage or unstable keyholecollapse. Type C is a cluster of ran-domly distributed small microp-ores with less than 1 µm size.Among the micropores, Types Aand B were more frequently ob-served than Type C. Type C was ob-served only in a few samples thatwere welded using wet heliumshielding gas. These pores, withsizes less than 1 µm, are most prob-ably caused by hydrogen rejectionfrom the solid metal.
Hydrogen porosity can beformed by the rejection of hydro-gen from the solid due to significanthydrogen solubility difference inthe liquid and solid alloy (Ref. 40).Two types of hydrogen porosity arepossible in aluminum alloys (Ref.41). When the hydrogen content inthe metal is so high that rejection ofthe gas from the growing solidraises the equilibrium gas pressurein the liquid to greater than 1 atm(Refs. 42, 43), interdendritic poros-ity with irregular shape and largesize, usually visible to the unaidedeye, may form. These features showsurface tension forces do not re-strain its development. On the
WELDING RESEARCH SUPPLEMENT | 213-s
RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
/RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
Fig. 12 — Porosity produced at several defocus values with dry and wet helium as the shielding gas. A — Alloy 5182; B— Alloy 5754 . Nominalpower 3 kW, welding speeds 250 in./min (105.8 mm/s) and 150 in./min (63.5 mm/s) for Alloys 5182 and 5754, respectively, and shielding gas flowrate 200 ft3/h (5.66 m3/h) of helium.
Fig. 13 — Three types of microporosity in Alloy 5754weld. A — Irregular-shaped porosity; B sphericalporosity; C — randomly distributed porosity havingspherical or interdendritic shape.
A
A
B
B
C
other hand, secondaryporosity with sphericalshape and 1–2 µm diame-ter can form even when thehydrogen content of themetal is low. For example,0.2 cc (STP)/100 g of hy-drogen content in pure alu-minum is sufficient tocause the formation of sec-ondary porosity (Ref. 41).Generally, aluminum al-loys contain a low-hydro-gen content. Therefore,secondary porosity is amore common form of hy-drogen induced porosity inaluminum alloys.
Talbot and Granger(Ref. 41) showed in theircalculations homogeneousnucleation of hydrogenpores in aluminum is ex-
tremely difficult. There-fore, heterogeneous nu-cleation is the onlypossibility in the pres-ence of imperfectionsor minute inclusions inthe metal. The growthof hydrogen porosity isa diffusion controlledprocess. Fang, et al.(Ref. 44), showed theaverage size of the hy-drogen porosity in Al-4.7% Mg casting wasreduced as the coolingrate was increased.High cooling rate re-sults in less time for hy-drogen to diffuse, caus-ing reduction in poresize, as shown in Fig.14. The cooling ratestudied by Fang, et al.(Ref. 44), was in therange of 1–10°C/s andthe observed pore sizewas in the range of1–10 µm. The rate ofnucleation and growthof hydrogen-inducedpores is reduced as thecooling rate increases(Refs. 45–48). Duringlaser welding of alu-minum, the cooling rateis much higher than thatin casting. Therefore, amuch smaller size ofhydrogen porosity andnumber density wouldbe expected duringlaser welding. Due tothe rare occurrence ofthe clusters of randomly
distributed micropores smaller than 1 µm(type C porosity in Fig. 13), it is believedthat the role of hydrogen on porosity for-mation is insignificant during laser weld-ing of thin plates of aluminum Alloys5182 and 5754.
Magnesium Loss
Magnesium concentration profileswere obtained across the welds. Figure15 shows the typical magnesium con-centration profiles in the weld pool crosssections. It is observed that magnesiumconcentration in the bulk of the weldmetal is essentially uniform, indicating avigorous convective mixing in themolten weld metal.
The changes in the magnesium con-centration in Alloys 5182 and 5754under different welding conditions aregiven in Tables 2 and 3, respectively. Inmany aluminum-magnesium alloys, theyield strength depends on the concentra-tion of magnesium in the alloy, as shownin Fig. 16 (Ref. 49). The data in this figureshow that the yield strength of alu-minum-magnesium alloys increases lin-early with the concentration of magne-sium. Although the data in Fig. 16 are foralloys in the annealed condition, theyprovide some guidance for the loss ofstrength resulting from the depletion ofmagnesium. Welding in the conductionmode, the reduction in the magnesiumconcentration, ∆%Mg, was about 1.11 to1.30 wt-% in Alloy 5182 and about 0.48to 0.62 wt-% in Alloy 5754. If the data inFig. 16 are used to estimate the degrada-tion of strength resulting from the deple-tion of magnesium, the estimated reduc-tion in yield strength is approximately 22to 25% for Alloy 5182 and 14 to 18% forAlloy 5754. Welding in the keyholemode, ∆%Mg of 0.74 wt-% in Alloy 5182and 0.22 wt-% in Alloy 5754 were ob-served. Consequently, the estimated re-duction in yield strength is approxi-mately 14% and 6% for Alloys 5182 and5754, respectively.
The reduction in magnesium concen-tration is much less pronounced in thekeyhole mode of welding. The cross sec-tion of the weld pool in the keyholemode of welding is significantly largerthan in the conduction mode of welding,as observed from the data presented inFig. 2. Consequently, the loss of magne-sium in the keyhole mode of welding isdistributed in a much larger volume ofmetal than that in the conduction modeof welding. This fact can be illustratedfrom the results of welding Al-5182 witha laser power of 3.0 kW and speed of 250in./min (105.8 mm/s). As can be ob-served from Fig. 2, a focused beam pro-duced a keyhole, while a beam with +2.0
214-s | JUNE 1999
RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
/RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
Fig. 14 — Pore size as a function of the cooling rate in Al-4.7%Mg alloy (Ref. 44).
Fig. 15 — Typical magnesium concentration profiles in the fusionzone of Alloy 5754. Nominal power 3 kW, welding speed 150in./min (63.5 mm/s), shielding gas flow rate 200 ft3/h (5.66 m3/h)of helium, and beam defocusing –1.75 mm.
mm defocusing resulted in conductionmode welding. The volumes of metalmelted per unit time were 90.1 mm3/sand 50.2 mm3/s for focused and +2.0mm defocused beam, respectively. Thecorresponding magnesium vaporizationrates, calculated from the compositionchange data, were 1.8 and 1.7 mg/s, re-spectively. Thus, the volume of themolten metal was significantly larger for
focused beam than for +2 mm defo-cused, while the magnesium vaporiza-tion rates were similar in two conditions.As a result, the composition change inthe weld with focused beam was lessthan that with +2 mm defocused beam.Therefore, the keyhole mode of weldingresults in minimizing changes in magne-sium concentration during laser welding.
Welding speed may affect mode of
welding and, consequently, the extent ofcompositional change in the weld metal.However, when the welding speeds werechosen to maintain the keyhole mode ofwelding, the compositional change didnot vary significantly as shown in Fig. 17.For the welding conditions shown in Fig.17, the size of the weld pool did not varysignificantly to cause major changes inmagnesium concentration. At very highwelding speeds, the welding modechanges to conduction mode, whichleads to more pronounced changes in themagnesium concentration due to muchsmaller volume of the weld pool. Thischange occurs, for example, at weldingspeeds above 275 in./min (116 mm/s) forAlloy 5754. Therefore, a welding speedshould be selected to achieve the key-hole mode of welding where possible tominimize the change in magnesium con-centration.
Summary and Conclusions
Porosity and underfill formation andmagnesium concentration change duringNd:YAG laser welding of aluminum al-loys 5182 and 5754 were studied. Themain conclusions are as follows:
1) When the welding parameters wereclose to those for the transition betweenthe keyhole and the conduction modes,pores with diameters larger than 0.20mm were commonly observed in theweld metal. The macroporosity in thewelds resulted from the instability of thekeyhole.
2) The instability of the keyhole andpore formation can be minimized bycontrolling the laser beam defocusingand welding speed. The keyhole is more
WELDING RESEARCH SUPPLEMENT | 215-s
RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
/RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
Fig. 16 — Correlation between yield strength, elongation and magne-sium concentration for aluminum-magnesium alloys (Ref. 49).
Fig. 17 — Influence of welding speed on the reduction in magnesiumconcentration during laser welding of aluminum Alloys 5182 and5754 using a focused beam. Nominal power 3 kW and shielding gasflow rate 200 ft3/h (5.66 m3/h) of helium.
Table 2 — Reduction in Magnesium Concentration in 5182 Aluminum Alloy Welds
Nominal laser power (kW) 3.0Welding speed (mm/s) 105.8Mode of welding Conduction KeyholeDefocusing (mm) +2.0 –2.0 +1.75 –1.75 0Reduction in magnesium 1.30 1.20 1.21 1.11 0.74
concentration (∆% Mg)Changes in magnesium 29.3 27.0 27.3 25.0 16.7
concentration relative tooriginal composition (%)
Table 3 — Reduction in Magnesium Concentration in 5754 Aluminum Alloy Welds
Nominal laser power (kW) 3.0Welding speed (mm/s) 63.5Mode of welding Conduction KeyholeDefocusing (mm) +2.0 –2.0 +1.75 –1.75 0Reduction in magnesium 0.62 0.59 0.51 0.48 0.22
concentration (∆ % Mg)Changes in magnesium 22.0 20.9 18.1 17.0 7.8
concentration relative tooriginal composition (%)
stable when the focal point of the laserbeam is above the top surface of theworkpiece.
3) Hydrogen induced microporositywas not a significant problem in the laserwelding of 5182 and 5754 aluminum al-loys.
4) Underfill at the root of full penetra-tion welds was a recurrent defect. How-ever, sharp or deep notches that areharmful to the mechanical properties ofthe weld were not observed in the weldcross sections.
5) Significant depletion of magnesiumfrom the weld metal was observed. Themagnesium depletion problem was morepronounced for welding in the conduc-tion mode than in the keyhole mode.
Acknowledgments
This study was supported by a grantfrom the U.S. Department of Energy, Of-fice of Basic Energy Sciences, Division ofMaterials Sciences, under Grant No. DE-FG02-96ER45620. The authors thank T.A. Palmer for helpful discussions. Thematerial utilized for this investigationwas supplied by Alcan International.
References
1. Brown, K. R., Venie, M. S., and Woods,R. A. 1995. Journal of Metals, July, pp. 20–23.
2. Bramson, M. A. 1968. Infrared radia-tion: A Handbook for Applications, p. 127,Plenum Press, New York, N. Y.
3. Duley, W. W. 1983. Laser Processingand Analysis of Materials, p. 71, Plenum Press,New York, N. Y.
4. Arata, Y. and Miyamoto, I. 1978. Tech-nocrat 11(5): 33.
5. Lawrence, F. V., Jr. 1973. WRC Bulletin181: 7.
6. Ashton, R. F., and Wesley, R. P. 1975.Welding Journal 54(3): 95-s.
7. Katoh, M. 1996. Journal of Light MetalWelding and Construction 34(4): 42.
8. Katayama, S. 1996. Journal of LightMetal Welding and Construction 34(4): 199.
9. Masumoto, I., and Kutsuna, M. 1991. In-ternational Institute of Welding, IIW Doc. IV-566-91.
10. Kutsuna, M. 1993. Welding in theWorld IIW 31: 126–135.
11. Woods, R. A. 1974. Hydrogen in Met-als. eds. I. M. Bernstein and A. W. Thompson,pp. 713–725. ASM International, MaterialsPark, Ohio.
12. Schauer, D. A., and Giedt, W. H. 1978.Welding Journal 57(7): 189-s to 195-s.
13. Whitaker, I. R., McCartney, D. G., andSteen, W. M. 1992. ICALEO ‘92, pp. 565–573.
14. Martukanitz, R. P., and Smith, D. J.1995. Proceedings of the Sixth InternationalConference on Aluminum Weldments, p. 309,AWS Conference, Cleveland, Ohio.
15. Kim, J. S., Watanabe, T., and Yoshida,Y. 1995. Journal of Laser Applications (7):38–46.
16. Martukanitz, R. P., and Michnuk, P. R.
1982. Aluminum 58(5): 276–279.17. Marsico, T. 1988. Master’s thesis, The
Pennsylvania State University.18. Moon, D. W., and Metzbower, E. A.
1983. Welding Journal 62(2): 53-s to 58-s.19. Block-Bolten, A., and Eagar, T. W.
1984. Metallurgical Transactions 15B:461–469.
20. Blake, A., and Mazumder, J. 1985.Journal of Engineering for Industry 107(8):275.
21. Cieslak, M. J., and Fuerschbach, P. W.1988. Metallurgical Transactions 19B:319–329.
22. Xijing, W., Katayama, S., and Mat-sunawa, A. 1997. Welding Journal 76(2): 70-sto 73-s.
23. Khan, P. A. A., DebRoy, T., and David,S. A. 1988. Welding Journal 67(1): 1-s to 7-s.
24. David, S. A., and DebRoy, T. 1992. Sci-ence 257: 497–502.
25. Khan, P. A. A., and DebRoy, T. 1984.Metallurgical Transactions 15B: 641–644.
26. Collur, M. M., Paul, A., and DebRoy,T. 1987. Metallurgical Transactions 18B:733–740.
27. Sahoo, P., Collur, M. M., and DebRoy,T. 1988. Metallurgical Transactions 19B:967–972.
28. Sahoo, P. and DebRoy, T. 1988. Mate-rials Letters 6(11): 406.
29. Chan, C. L., and Mazumder, J. 1987.Journal of Applied Physics 62(11):4579–4586.
30. DebRoy, T., Basu, S., and Mundra, K.1991. Journal of Applied Physics 70(3):1313–1319.
31. Mundra, K., and DebRoy, T. 1993.Metallurgical Transactions 24B: 145–155.
32. Mundra, K., and DebRoy, T. 1993.Welding Journal 72(1): 1-s to 9-s.
33. Katayama, S., and Lundin, C. D. 1991.Journal of Light Metal Welding and Construc-tion 29(9): 1–13.
34. Milewski, J. O., Lewis, G. K., and Wit-tig, J. E. 1993. Welding Journal 72(7): 341-s to346-s.
35. Hilton, P. A. 1995. Optical and Quan-tum Electronics 27: 1127–1147.
36. Rapp, J., Glumann, C., Dausinger, F.,and Hugel, H. 1995. Optical and QuantumElectronics 27: 1203–1211.
37. Martukanitz, R. P., Armao, F. G., Alt-shuller, B., and Pickering, E. R. 1996. Pro-ceedings of SAE International Congress andExposition, SAE International.
38. Arata, Y. 1986. Plasma, Electron andLaser Beam Technology, p. 395, ASM Interna-tional, Materials Park, Ohio.
39. Kim, J. S., Watanabe, T., and Yoshida,Y. 1995. Journal of Materials Science Letters14: 1624–1626.
40. Hatch, J. E. 1984. Aluminum: Proper-ties and Physical Metallurgy, p. 85, ASM In-ternational, Materials Park, Ohio.
41. Talbot, D. E. J., and Granger, D. A.1963-64. Journal of Institute of Metals 92:290–297.
42. Ransley, C. E., and Talbot, D. E. J. 1955.Z. Metallkunde 46: 328.
43. Jordan, M. F., Denyer, G. D., andTurner, A. N. 1962-63. Journal of Institute ofMetals 91: 48.
44. Fang, Q. T., Anyalebechi, P. N., andGranger, D. A. 1988. Proceedings of TMS An-nual Meeting of Light Metals, pp. 477–485.
45. Thomas, P. M., and Gruzleski, J. E.
1978. Metallurgical Transactions 9B: 139.46. Vaccari, J. H. 1991. American Ma-
chinist, February, p. 139.47. Metcalfe, G. E. 1945. Journal of Insti-
tute of Metals 71: 618.48. Ransley, C. E., and Neufeld, H. 1948.
Journal of Institute of Metals 74: 599.49. Hatch, J. E. 1984. Aluminum: Proper-
ties and Physical Metallurgy, p. 209, ASM In-ternational, Materials Park, Ohio.
50. Mouroulis, P., and Macdonald, J. 1997.Geometrical Optics and Optical Design, p.302, Oxford University Press, Cambridge,U.K.
Appendix
Calculation of Laser Beam Radiusand Power Density
The variation of beam radius r alongthe beam axis z is given by (Ref. 50)
(1)where r0 is the beam radius at the focalpoint, z is the distance from the focalpoint and zR is the Rayleigh length de-fined by
zR = ± 2r0F (2)where F is focusing number defined by
(3)where f is the focal length and db is thebeam diameter impinging on the lens.The beam power density I at defocusingposition z is calculated from
(4)where p is the beam power. The com-puted beam radius and power density atvarious defocusing values are given as: r0= 0.3 mm, f = 77.7 mm, db = 28.28 andp = 2.6 kW.
Ip
r=
π 2
Ff
db=
r rz
zR
= +
0
2
1
1
2
216-s | JUNE 1999
RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
/RE
SE
AR
CH
/DE
VE
LO
PM
EN
T/R
ES
EA
RC
H/D
EV
EL
OP
ME
NT
To Order Custom Reprints of Articles in the
Welding Journal
Call Denis Mulliganat (800) 259-0470
REPRINTS REPRINTS
REPRINTS REPRINTS