Grain Refinement of AZ31 Magnesium Alloy .pdf

Grain Refinement of AZ31 Magnesium Alloy Weldmentsby AC Pulsing Technique

N. KISHORE BABU and C.E. CROSS

The current study has investigated the influence of alternating current pulsing on the structureand mechanical properties of AZ31 magnesium alloy gas tungsten arc (GTA) weldments.Autogenous full penetration bead-on-plate GTA welds were made under a variety of conditionsincluding variable polarity (VP), variable polarity mixed (VPM), alternating current (AC), andalternating current pulsing (ACPC). AC pulsing resulted in significant refinement of weld metalwhen compared with the unpulsed conditions. AC pulsing leads to relatively finer and moreequiaxed grain structure in GTA welds. In contrast, VP, VPM, and AC welding resulted inpredominantly columnar grain structures. The reason for this grain refinement may be attrib-uted to the periodic variations in temperature gradient and solidification rate associated withpulsing as well as weld pool oscillation observed in the ACPC welds. The observed grainrefinement was shown to result in an appreciable increase in fusion zone hardness, tensilestrength, and ductility.

DOI: 10.1007/s11661-012-1241-2� The Minerals, Metals & Materials Society and ASM International 2012

I. INTRODUCTION

MAGNESIUM alloys have exceptional specificstrength, stiffness, damping capacity, machinability,castability, and weldability, making it attractive foruse in the automobile, aerospace, and electronic indus-tries.[1] However, magnesium alloys have poor form-ability at room temperature because of the limited slipsystems in the hexagonal close-packed lattice. Mostmagnesium alloys are readily weldable using gas tung-sten arc (GTA), gas metal arc (GMA), plasma arc,electron beam, laser beam, friction, explosion, stud,ultrasonic, and spot welding processes.[2] GTA weldingwas selected for use in this study because it producesvery high quality welds as opposed to other methods(e.g., GMA or laser), thus minimizing complicatingeffects arising from weld defects such as undercuts,porosity, and weld spatter.[3]

The main problems associated with welding of mag-nesium alloys are caused by the high oxidizing proper-ties of magnesium during heating to high temperaturesand formation of oxide film. A significant difference inthe melting temperatures of magnesium oxide andmagnesium itself about 2273 K (2000 �C) results in thesurface of molten pool being covered by an oxide filmduring welding.[4] The density of magnesium oxide filmis approximately 3.2 g/cm3, whereas that of magnesiumitself in liquid state is 1.6 g/cm3. As a result, the film canimmerse into the molten metal and form an incomplete

fusion of welded edges. The loose magnesium oxide filmdoes not protect the metal from subsequent oxidation,especially within the range of welding temperatures.Thus, during the welding process, the oxygen andnitrogen present in the air can actively dissolve intothe molten metal and decreases the mechanical proper-ties. Hence, alternating current (AC) or variable polarity(VP) are preferred GTA welding techniques for magne-sium alloys because they destroy the oxide layer presenton the surface of base metal during the electrode-positive cycle. VP and AC GTA welding have beensuccessfully applied to various aluminum and magne-sium alloys.[5,6]

VP is similar to AC in that it alternates betweenpositive and negative electrode polarities. It is differentfrom AC in that the balance of two polarities and thecurrent at each polarity can be varied independently(e.g., compare Figures 1 (a) and (c)). The VPMmode is avariation of VP welding that allows programming asegment of VP welding followed by a segment of straightdirect current (DC) welding (Figure 1(b)). This abilityincreases penetration while obtaining the required clean-ing action of VP. AC pulsing (ACPC) is a variation ofAC welding that involves varying between two differentAC parameter sets (peak and base currents) at a selectedregular frequency (Figures 1(d) through (f)). The highcurrent set is generally selected to give adequate pene-tration and bead contour, whereas the low current sethelps maintain a stable arc. This permits arc energy to beused efficiently to fuse a spot of controlled dimensions ina short time, thereby producing the weld as a series ofoverlapping nuggets and limiting the wastage of heat byconduction into the adjacent parent material in normalconstant current welding.The main objective of the current work is to study the

influence of VP, VPM, AC, and AC pulsing on the solid-ification structure of a GTA-welded AZ31 magnesium

N. KISHORE BABU, Scientist, is with the Joining TechnologyGroup, Singapore Institute of Manufacturing Technology (SIMTech),Singapore 638075 Singapore. Contact e-mails: [email protected]; [email protected] C.E. CROSS, Staff Scientist,is with the Los Alamos National Laboratory, Los Alamos, NM 87545.

Manuscript submitted October 13, 2011.

METALLURGICAL AND MATERIALS TRANSACTIONS A

alloy. The effect of the observed grain refinement ontensile behavior has also been investigated.

II. BACKGROUND

Weld fusion zones typically exhibit coarse columnargrains because of the prevailing thermal conditions duringweld metal solidification, which often results in inferiorweld mechanical properties and poor resistance to hot

cracking. Although it is thus highly desirable to controlsolidification structure in welds, such control is often verydifficult because of the higher thermal gradients in welds inrelation to castings and the tendency for columnar growth.Nevertheless, several methods for refining weld fusionzones have been tried with some success in the past:inoculation with heterogeneous nucleants, microcooleradditions, surface nucleation induced by gas impingement,and introduction of physical disturbance through tech-niques suchaspulsedcurrentweldingand torchvibration.[5]

Fig. 1—Welding current vs time relationship for GTA welds (a) VP; (b) VPM; (c) AC; (d) ACPC, 2 Hz pulse frequency; (e) ACPC, 4 Hz pulsefrequency; and (f) ACPC, 6 Hz pulse frequency.


Current pulsing has been used in the past, both inGTA and GMA welding, for obtaining grain refinementin weld fusion zones. Significant refinement of thesolidification structure has been reported in aluminumalloys,[7–10] austenitic stainless steels,[11–13] titaniumalloys[14,15] and tantalum.[16] Early work by Becker andAdams[17] showed that the use of current pulsing inGTA welding of titanium alloys did not result in anymeasurable grain refinement, nor did it affect theintragranular segregation or the tensile properties.However, current pulsing was later shown to have abeneficial influence on fusion zone grain structure in atitanium alloy weld.[14] Sundaresan et al.[15] have studiedthe use of current pulsing in GTA welding of two a-btitanium alloys (Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo).Current pulsing was found to result in significantrefinement of the fusion zone grain structure. Thereduction in prior-b grain size was found to result inappreciable improvement in weld room temperaturetensile ductility. The observed refinement was attributedto the effects of current pulsing on weld pool shape, fluidflow, and thermal gradients.

In the case of aluminum alloys, current pulsing hasbeen used for grain refinement both in GTA[7,18,19] andGMA welding.[9] Reddy et al.[8,9] reported significantrefinement of fusion zone solidification structure andreduction in segregation leading to substantial improve-ment in solidification cracking resistance and weldtensile properties in type 1441 Al-Li alloy GTA weldsusing pulsed alternating current. Ram et al.[19] alsoreported significant grain refinement and improvedtensile properties in type 2090 Al-Li alloy and type7020 Al-Zn-Mg alloy GTA welds using current pulsing.The combined use of current pulsing and heterogeneousnucleants (Zr) for reducing grain size and improvingsolidification cracking resistance in an aluminum alloyhas been studied by Matsuda et al.[20] It was reportedthat the combined use of these techniques resulted ingreater refinement than when these techniques were usedalone. Significant grain refinement was observed in mildsteel welds by Vishnu et al.,[21] which was attributed tomultiple cycling of the fusion zone through the austenitetransformation temperature range. Structural refine-ment by cycling through the d to c transformationrange in an austenitic stainless steel has also beenreported.[11] Kou[22] studied the influence of currentpulsing on 6061 aluminum alloy containing 0.04 wt pctTi. They found that heterogeneous nucleation aided bythermal undercooling resulting from the high coolingrates is responsible for grain refinement in the weld.McInerney et al.[23] reported that mechanical oscillationapplied in the direction of welding resulted in grainrefinement of aluminum 2219 alloy welds because ofdendrite fragmentation. Various combinations of fre-quency and amplitude of oscillation were examined todefine the conditions necessary for breaking up thecolumnar grain structure. They found that grain refine-ment can be achieved at any frequency, provided theamplitude is sufficiently high. More recently, Padman-aban and Balasubramanian[24] studied the influence ofpulse frequency on the tensile properties of a GTA-welded AZ31B magnesium alloy. They found that pulse

frequency has the greatest influence on tensile strength,followed by peak current, pulse-on time, and basecurrent. A maximum tensile strength of 188 MPa wasobtained at 6 Hz pulse frequency and 50 pct pulse-ontime.[24] Liu and Dong[25] studied the influence of AZ61filler wire on the microstructure and tensile properties ofGTA welded AZ31 alloy. The microstructure of AZ61filler welded joint revealed that the grain size in the heat-affected zone (HAZ) was reduced when compared withGTA welded joint without filler, which has changed thefracture location in the tensile test and improved theultimate tensile strength of welded joint. It was observedthat the filler wire reduced the degree of overheat in themolten pool, which has changed the thermal cycle in theHAZ.[25]

Thus, although several researchers have reported grainrefinement as a result of pulsed current, there has beenlittle direct examination of the mechanisms involved.However, it is suggested that current pulsing caninfluence many aspects of weld metal solidification. Theimportant effects are (1) fluid flow, (2) thermal gradients,(3) growth rates, and (4) continuous change in weld poolsize and shape. Several investigators attributed theobserved grain refinement in welds caused by currentpulsing to dendrite fragmentation and subsequent het-erogeneous nucleation[11,13] as well as by solidificationundercooling effects of current pulsing.[8,15]

Important parameters in pulsed current welding arepeak current, background current, pulse on-time, pulsefrequency, pulse amplitude, and welding speed. All theseparameters are interdependent to some extent and mustbe carefully selected to realize fully the benefits of currentpulsing. In most of the previous investigations on currentpulsing, it was observed that an optimum frequencyexisted at which the effects of current pulsing weremaximized. The optimum frequencies reported mostinvestigations were generally less than 10 Hz.[8,13,19]

The weld pool has a natural oscillation frequency thatdepends on factors such as its size and shape, surfacetension, viscosity, etc.[26] The natural frequency gener-ally rises with a smaller weld pool size, ranging betweenapproximately 10 and 200 Hz frequency. If a pulsedcurrent is applied near the natural oscillation frequency,then resonance phenomena arise. The molten poolvibration is more vigorous when the resonance fre-quency of molten pool is close to pulse frequency,resulting in significant grain refinement. Using high-speed photography in pulsed GMA welding, it has beenshown that these forces induce vibrations in the moltenpool.[10] The amplitude of these vibrations was shown topass through a maximum in the range of 10 to 30 Hzand drop again at frequencies greater than 30 Hz.

III. EXPERIMENTAL

Sheets of 3-mm-thick, continuous cast, and rolledAZ31 magnesium alloy were used in the current study.The chemical composition of the alloy is given inTable I. Automatic VP, VPM, AC, and ACPC-GTAwelding was used, and autogenous, full-penetration,bead-on-plate welds were made on coupons of AZ31


base material. An AMET VPC-450 (variable polaritycontrol 450-Amp) power source was used to make GTAwelds on AZ31 sheet. The power supply is digital signalprocessor controlled, resulting in extremely preciseswitching cycles, with little ripple and noise. Initially,welding trials were conducted to identify the conditionsthat can produce sound full-penetration welds withoutother undesirable effects like arc instability, excessivespatter, porosity, undercutting, etc. Current pulsingparameters important to grain refinement include bothfrequency and amplitude. For the case of ACPC,different pulse frequencies (2, 4, 6, and 8 Hz) wereselected to find out the most suitable frequency for grainrefinement. The amplitude was selected to give anappropriately shaped weld pool. Welds with VP,VPM, AC, and ACPC were made under the conditionslisted in Table II. Just prior to welding, the base materialcoupons were wire brushed and degreased with acetone.Welding was performed such that the beads wereoriented perpendicular to the rolling direction. Argonwas used as the shielding gas, and a mixture of argon

and helium (50 pct-50 pct) was used for auxiliarytrailing and backing gas shields. The flow rate of theshielding gas was 15 L/min and 20 L/min for the trailingand backing gas shields, respectively. During the weld-ing process, the arc images of GTA welding wereobtained using a high-speed camera and stored in acomputer with a frame grabber.The arc current and welding time were recorded

during the welding process. The welding current vs timerelationship for VP, VPM, AC, and ACPC-GTA areshown in Figure 1. The thermal history curves weremeasured by inserting a type-K thermocouple (0.2-mm-diameter wire) in the weld pool, and they were recordedby a personal computer-based, high-speed data-acquisitionunit. The samples for light microscopy were suitablysectioned, mounted, mechanically polished, and etched.For etching, a solution containing 10 mL acetic acid,4.2 g picric acid, 10 mL H2O, and 70 mL ethanol wasused. The microstructures and compositional profiles inthe weld metal were investigated using a scanningelectron microscope (SEM) with energy-dispersive spec-troscopy capabilities. Microhardness measurementswere done on the base metal, HAZ, and weld metal bya diamond pyramid indenter under a load of 1000 g for15 seconds. A microhardness traverse was made acrossthe weldments at 0.5-mm intervals. Tensile specimenswere prepared in accordance with ASTM E 8M fromas-welded coupons. For tensile tests, transverse weldspecimens with a gauge length of 25 mm, gauge width of

Table I. Measured Chemical Compositionof AZ31 Magnesium Alloy Sheet (Weight Percent)

Al Zn Mn Mg

2.70 0.81 0.30 balance

Table II. Welding Parameters for VP and VPPC GTA Welds

Variable Polarity GTA WeldsDCEN (Direct currentelectrode negative)

110 A, 16 ms

DCEP (Direct currentelectrode positive)

80 A, 4 ms

Frequency 50 HzArc voltage 11 VTravel speed 4.16 mm/s

Variable Polarity Mixed Mode GTA WeldsDCEN 100 A, 16 msDCEP 80 A, 4 msFrequency 50 HzVP time 87 msConstant current time 213 msArc voltage 11 VTravel speed 4.16 mm/s

Alternating Current GTA WeldsDCEN 110 A, 16 msDCEP 110 A, 4 msFrequency 50 HzArc voltage 11 VTravel speed 4.16 mm/s

Alternating Pulsed Current GTA WeldsAC I AC II

DCEN 150 A, 15 ms DCEN 50 A, 15 msDCEP 150 A, 4 ms DCEP 50 A, 4 msAC frequency 50 Hz AC frequency 50 HzPulse frequency 2, 4, 6, 8, 10 Hz Pulse frequency 2, 4, 6, 8, 10 HzPulse time 150, 75, 45, 30,

30 msPulse time 225, 105, 75, 60, 45 ms

Arc voltage 11.5 V Arc voltage 11.5 VTravel speed 4.16 mm/s Travel speed 4.16 mm/s


6 mm, and thickness of 3 mm were used. Tensile testswere carried out on base metal as well as weldments at aconstant displacement rate of 3 mm/min.

IV. RESULTS AND DISCUSSION

A. Weld Pool/Weld Metal Characterization

The optical microstructure of base metal, as shown inFigure 2, consisted of equiaxed a magnesium grains.The average grain size is 23 lm with some larger grains(140 lm) elongated in the rolling direction. A smallamount of fine Al8Mn5 particles was distributed homo-geneously throughout the wrought roll-cast mate-rial.The macrostructures of the as-welded fusion zone(top surface and cross section) and weld pool shapefrom the top surface for welds made using VP, VPM,AC, and ACPC are shown in Figure 3. The weld poolshape was approximately constant for different weld con-ditions, with distance ‘‘A’’ summarized in Table III.TheVP, VPM, and AC GTA weld fusion zone is composedof coarse columnar grains extending from the fusionboundaries to the weld center. Because of the presenceof steep thermal gradients and epitaxial nucleation infusion welds, weld metal solidification often takes placein a columnar mode.[27] Although nucleation occursepitaxially, grain coarsening in the heat-affected zoneand competitive growth in the fusion zone has resultedin a much larger grain size in the weld metal in relationto the base material. It is observed that AC pulsing hasresulted in producing not only fully equiaxed structuresbut also in reducing the grain size. These macrographswere taken from both top and transverse cross sectionsof the welds. Figure 4 shows a weld cross sectioncomparing base metal, HAZ, and weld metal micro-structures of an ACPC weld. Significant inhomogeneousgrain coarsening (~40 to 165 lm) was observed in theHAZ of all as-welded conditions. Within the grains,second-phase particles are present in fusion zone aswell as HAZ. A microchemical analysis in the SEMshows the interdendritic regions to be rich in Al, Zn, andMg. Second-phase particles have been identified asb-Mg17Al12 and s-Mg32(Al,Zn)49.

[28]

Figure 5 shows the fusion zone optical microstruc-tures for VP, VPM, and AC conditions. The opticalmicrostructures of ACPC welds were made at pulsefrequencies of 2, 4, 6, and 8 Hz are shown in Figure 6.The grain size measurements for the AC welds made atdifferent pulsing frequencies are listed in Table IV.Microstructural examination showed that the grain sizewas the least in the welds made at a pulse frequency of6 Hz. The grain aspect ratio (length/width ratio) ishighest in the VP welds and lowest in the ACPC welds.The measurement of grain size in VP, VPM, and ACwelds showed an average columnar grain diameter ~115,89, 83 lm, respectively and aspect ratios of between5 and 11. However, ACPC resulted in marked change ingrain structure, and columnar grains have been replacedby equiaxed grains (Figure 5).

B. Thermal History

Figure 7 compares the cooling rates for VP, VPM,AC, and ACPC conditions. Over the solidification rangefor AZ31 (903 K to 709 K [630 �C to 436 �C]), ACPC,6 Hz pulse frequency showed a faster cooling rate(383 K/s [110 �C/s]) compared with AC welds (339 K/s[66 �C/s]), VP welds (345 K/s [72 �C/s]), and VPM welds(348 K/s [75 �C/s]). With an increased cooling rate, thesolidification time is suppressed (ACPC: 1.7 seconds,AC: 2.9 seconds, VP: 2.6 seconds, and VPM: 2.5 seconds)and a finer weld metal microstructure is produced. Pulsingof the welding current increases the weld metal coolingrate because heat energy is more efficiently used.[29] Forexample, in the current study, the average DCENheat input for ACPC (241 J/mm) was lower comparedwith AC (290 J/mm), VP (290 J/mm), and VPM-GTA(264 J/mm). Second, the periodic nature of the energyinput allows better dissipation of heat during the periodsbetween peak current pulses, which also contributes toincreased weld metal cooling rates in pulsed currentwelding.[29] The thermal gradient (cooling rate/travelspeed) calculated for VP, VPM, AC, and ACPC condi-tions are 290 K/mm (17 �C/mm), 291 K/mm (18 �C/mm),288 K/mm (15 �C/mm), and 303 K/mm (30 �C/mm),respectively. In contrast to AC welding, the fact thatheat energy required to melt the base material is sup-plied only during peak current pulses for brief intervalsof time allows the heat to dissipate into the base materialduring the background current time and thus lowersheat buildup in the adjacent base material leading to anarrower HAZ in ACPC welds.

C. Grain Refinement Mechanism

Grain refinement in solidification is known to requirethe simultaneous occurrence of two things: (1) sufficientundercooling and (2) the presence of appropriatesubstrate particles (i.e., nucleant) ahead of the solid–liquid interface. For the first requirement, undercoolingahead of the solid–liquid interface is provided by bothdendrite curvature and partitioning of solute (i.e.,constitutional undercooling). Burden and Hunt[30]

derived a relationship relating undercooling DT to bothtemperature gradient G and solidification rate RFig. 2—Optical microstructure of AZ31 base metal.


DT ¼ DG=Rð Þ þ AR1=2 ½1�

where D is liquid diffusivity and A represents a materialconstant. When applied to welding, it is found that highundercooling is expected to occur in two locations,

along the fusion line and at the weld center.[31,32] Thethermal gradient calculated in the previous section is anaverage over the pulsing process whereby the thermalgradient and travel speed varied locally; in reality, thewelding arc continuously changes between two levels(high pulse and low pulse currents) as shown inFigure 1. Over a period of time, the weld pool reachesits maximum size during high pulse, thus making thesolidification rate (R) approach zero, and the left term inEq. [1] becomes very large, resulting in high underco-oling and grain refinement.Current pulsing also disrupts fluid motion, which

enhances the convective forces already existing in the

Fig. 3—Weld pool shape, top and transverse cross sections of the welds: A-width of top surface (a and b) VP, (c and d) VPM, (e and f) AC, and(g and h) ACPC, 6 Hz pulse frequency.

Table III. Weld Pool Measurements According to Fig. 3

Condition Top Width (A) (mm)

VP 10VPM 8AC 11ACPC 7


weld pool. The periodic variations in the arc currentresult in similar changes in the arc forces impinging onthe weld puddle, which are proportional to the square ofthe welding current.[11] In the current study, from thehigh-speed camera videos, it was clear that periodicvariations in the arc current induce greater vibrations inthe weld pool when compared with the unpulsed ACcondition. These vibrations might cause remelting andbreaking off of the growing dendrites, which is aided bythe mechanical action of the weld pool turbulence inbringing the dendrite fragments ahead of the solid–liquid interface. These fragments then become sites forheterogeneous nucleation, which eventually block thecolumnar growth process and produce a fine grain size.

Although the microstructural refinement observed inprevious investigations on pulsed welding has beenattributed to dendrite fragmentation,[11,13] some investi-gators doubt if whether will be truly effective under theconditions of weld pool solidification because of thesmall size of the fusion welds and the fine interdendriticspacing in the weld microstructures.[33,34] However,alloy AZ31 exhibits a long solidification range (morethan 453 K [180 �C]) that results in larger mushy zonesin the solidifying weld pool and can make dendritefragmentation mechanism more probable. Although theexact mechanism of refinement of solidification struc-ture in the pulsed current welds is not clear, therefinement observed in the current investigation maybe attributed to a combination of mechanisms involvingdendrite fragmentation and subsequent heterogeneousnucleation and growth, as well as other effects of currentpulsing on weld pool shape, fluid flow, thermal gradi-ents, and growth rates.

D. Hardness

Traverse microhardness measurements are shown inFigure 8, where a hardness reduction in the weld metalcompared with HAZ and base metal is evident in allconditions. The welds prepared using the ACPC tech-nique exhibited a slightly higher hardness comparedwith unpulsed welds, and was attributed to grain

refinement. Grain refinement plays a crucial role inmagnesium alloys, which are particularly sensitive tograin boundary strengthening.[35] Among the weldsprepared using VP, VPM, and AC, no difference wasfound in the hardness values.

E. Tensile Properties

The important objective of the current investigationwas to determine whether the observed grain refinement

Fig. 4—Weld cross section comparing base metal, HAZ, and weldmetal microstructures of an ACPC weld at 2 Hz pulse frequency.

Fig. 5—Optical microstructures for GTA-welded AZ31 magnesiumalloy: (a) VP weld metal, (b) VPM weld metal, and (c) AC weldmetal.


led to an improvement in fusion zone tensile properties.For this purpose, the unpulsed welds were comparedwith those in which ACPC at optimum frequency wasused. These results are listed in Table V. The basematerial tensile properties have also been included inthis table for comparison. The tensile data for eachcondition are an average of the measurements made inthree specimens.It can be observed from Table V that the AC pulsed

weldments exhibit high strength and ductility comparedwith unpulsed welds, and they are attributed to the grain

Table IV. Fusion Zone Grain Size Measurements

AC Pulse Frequency (Hz) Average Grain Size (lm)

2 774 606 528 74

Fig. 6—The optical microstructures of ACPC welds at different pulse frequencies: (a) 2 Hz, (b) 4 Hz, (c) 6 Hz, and (d) 8 Hz.

Fig. 8—Hardness profiles across VP, VPM, and AC- and ACPC-GTA welds in the as-welded condition.

Fig. 7—Temperature measurements during VP, VPM, and AC- andACPC-GTA weld conditions.


refinement. However, all weldments exhibited lowerstrength and ductility compared with the wrought basemetal. More importantly, the unpulsed welds (VP,VPM, and AC) exhibit a considerably reduced strengthand tensile elongation compared with AC pulsed weldsand base metal (Table V). This could be a result of thecoarse columnar grain structure. Among the unpulsedwelds, VPM exhibited slightly higher strength andductility, and it was attributed to a lower heat input.The tensile fracture surfaces of the base metal, ACPC,6 Hz pulse frequency, and AC GTA welds are shown inFigure 9, which is typical for an hcp metal with limitedslip systems. The ductile and very fine dimple fracturefeatures are observed in base metal, and ACPC GTAwelds are shown in Figures 9(a) and (b). Fine dimplesare characteristic features of ductile materials, andhence, pulsed current welds exhibit higher ductilitywhen compared with other conditions. Figure 9(c)shows the lower ductility fracture surface of GTA weldsmade with AC.

V. CONCLUSIONS

VP, VPM, AC, and ACPC-GTA welds are usefulmethods to join magnesium alloys, serving to removesurface oxidation. Pulsing of the welding current inACPC-GTA welds results in significant grain refinementin AZ31 magnesium alloy welds compared withunpulsed GTA welds. ACPC welds produced very finegrains (~52 lm) in the welds made at a pulse frequencyof 6 Hz. Part of the reason for this grain refinement maybe attributed to the periodic variation in temperaturegradient and solidification rate as a result of pulsing. Itis understood that the measured cooling rate andassociated temperature gradient represent an averagecontrolled by local variations in temperature gradientand solidification rate determined by pulsing. Pulsingalso contributes to weld pool oscillation and thepossibility for dendrite fragmentation. These fragmentsthen become sites for heterogeneous growth that even-tually block the columnar growth process. A notableimprovement in weld metal hardness, tensile strength,and ductility of ACPC welds is associated with grainrefinement. Ductile and very fine dimple fracture fea-tures are observed in the base metal and ACPC weld

Table V. Average Base Metal and Fusion Zone Tensile

Properties

Condition

YieldStrength(MPa)

UltimateTensile

Strength (MPa)Elongation

(pct)

Base metal 156 251 18VP 68 164 5.5VPM 88 215 8AC 64 163 6.75ACPC,6 Hz pulsefrequency

101(±3 to 5)

230(±2 to 7)

9.16(±0.7 to 2)

Fig. 9—Tensile fracture surfaces of (a) base metal; (b) ACPC, 6 Hzpulse frequency; and (c) AC GTA welds.


metal compared with the lower ductility fracture fea-tures noticed in the AC weld metal.

ACKNOWLEDGMENTS

The authors are grateful to the Federal Institutefor Materials Research and Testing (BAM), Berlin,Germany for internal support of this research wherethis work was conducted, and they greatly appreciatethe work of M. Marten for metallography, T. Michaelfor weld process development, and S. Brunow fortemperature measurement during welding.

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