manu_135_06_061012

Fabrizia Caiazzoe-mail: [email protected]

Vittorio Alfierie-mail: [email protected]

Gaetano Corradoe-mail: [email protected]

Francesco Cardaropolie-mail: [email protected]

Vincenzo Sergie-mail: [email protected]

Department of Industrial Engineering,

University of Salerno,

Fisciano, Salerno 84084, Italy

Investigation and Optimizationof Laser Welding of Ti-6Al-4 VTitanium Alloy PlatesTitanium alloys are employed in a wide range of applications, from aerospace to medi-cine. In particular, Ti-6Al-4 V is the most common, thanks to an excellent combination oflow density, high specific strength, and corrosion resistance. Laser welding has beenincreasingly considered as an alternative to traditional techniques to join titanium alloys.An increase in penetration depth and a reduction of possible welding defects are indeedachieved; moreover, a smaller grain size in the fused zone (FZ) is benefited in compari-son to either tungsten inert gas (TIG) or plasma arc welding, thus improving the tensilestrength of the welded structures. This study was carried out on 3 mm thick Ti-6Al-4 Vplates in square butt welding configuration. The novelty element of the investigation isthe use of a disk-laser source, which allows a number of benefits thanks to better beamquality; furthermore, a proper device was developed for bead protection, as titanium isprone to oxidation when in fused state. A three-level factorial plan was arranged in face-centered cubic scheme. The regression models were found for a number of crucialresponses and the corresponding surfaces were discussed; then a numerical optimizationwas carried out. The suggested condition was evaluated to compare the actual responsesto the predicted values; X-ray inspections, Vickers micro hardness tests, and tensile testswere performed for the optimum. [DOI: 10.1115/1.4025578]

Introduction

Titanium alloys are among the most common in aerospace,thanks to high strength in combination with low density and goodtensile properties [1]. In particular, Ti-6Al-4 V accounts for morethan half of all titanium tonnage in the world and no other tita-nium alloy is deemed to threaten its dominant position [2].Ti-6Al-4 V is employed for turbine disks, compressor blades, air-frame and space capsule structural components, rings for jetengines, pressure vessels, rocket engine cases, helicopter rotorhubs, fasteners, and engine exhausts. Medical and surgical devicesare also produced with this alloy [3].

Ti-6Al-4 V is a two phase aþ b alloy, with aluminum as thealpha stabilizer and vanadium as the beta stabilizer. The nominalchemical composition [1] is listed in Table 1.

Strengthening is achieved through heat treatment or thermome-chanical processing, although the best combination of propertiesresults from solution heat treatment and consequent rapid quench-ing and aging [2]. As a consequence of welding thermal cycles, asignificant variation of microstructure is noticed in the weldingbead. A specific mechanic performance of the bead is expecteddepending on the microstructure [4].

Conventional welding methods for titanium alloys include TIGand plasma arc welding [5]. Nevertheless, the need for industrialsustainable processes and systems have been highlighted in the lit-erature [6]; within this frame, autogenous techniques with highlyconcentrated energy sources, such as laser beam [7] and electronbeam [8], are receiving increasing interest. In particular, with laserwelding, much more simple fixturing would be benefited in com-parison with electron beam welding where a vacuum is neededand X-rays are produced [9].

Past research in the field of Ti-6Al-4 V laser welding hasfocused on CO2 lasers [10]; attempts have been made withNd:YAG source [11,12] and fiber lasers [13]; few works havedealt with disk-laser sources [14,15], which are expected toprovide better beam quality and efficiency in comparison withtraditional laser systems [16]. It has been proven that gas shielding

is crucial for bead protection in order to obtain sound joints. Withrespect to the assisting gas, which is required for the purpose ofmetal plume removal, previous works with CO2 sources hadshown that deeper penetration is achieved when considering he-lium instead of argon, due to the higher ionization potential of theformer [10]. Moreover, smaller grains in the fused zone, a nar-rower heat affected zone (HAZ) and harder welds result from laserprocessing in comparison to either TIG or plasma arc welding [7];nevertheless, a significant reduction in ductility may occur as aconsequence of the formation of aluminum oxides and micropores [14].

This paper aims to improve previous studies on laser weldingof Ti-6Al-4 V alloy. In particular, an Yb:YAG diode-pumped thindisk-laser source is employed: advantages arise indeed, as a con-sequence of only weak thermal lensing effects; better beam qual-ity is benefited because the divergence and the diameter variationalong the propagation axis decrease [16]. Furthermore, a properdevice is intended to be developed and tested for bead protection,as titanium is prone to oxidation when in fused state.

A number of regression models to predict certain geometricalfeatures of the beads, as a function of the main governing parame-ters, are developed and considered to discuss the main effects ofthe governing parameters. An optimization is carried out to find aproper set-up to for welding 3 mm thick plates in square buttwelding configuration.

Experimental Procedure

A proper choice of the governing parameters is based on the lit-erature and past experience [14,15]. Laser power and weldingspeed are primary factors as they determine the rate of energyinput to the work-piece [17], so they definitely must be taken intoconsideration in the study. In addition, successful laser weldingrequires optimization of other parameters, such as the size and thelocation of the focal spot [18]. Therefore, a change in the focusposition is worth investigating.

The range of the power was decided so that the specific thresh-old irradiance for conduction to key-hole transition (104 W/mm2)would be overcome even in defocused conditions. Sensible valuesfor welding speed were found via bead on plate tests aiming to

Manuscript received March 28, 2013; final manuscript received September 25,2013; published online November 18, 2013. Assoc. Editor: Yung Shin.

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produce full penetration with no significant drop-through on thelower surface [15].

Negative defocusing, i.e., with the beam focal point beneath themetal surface, was considered because it has been associated witha reduction of the grain size in a previous work [19]; the growthin grain size would otherwise produce a decrease in the tensilestrength of the welded structures [11].

Factorial experiments were planned. A three-level (� 1, 0, þ1)experimental plan with power (P), speed (s), and defocusing (f) asgoverning factors was arranged; factor levels for each parameterare listed in Table 2.

A fractional design was preferred, aiming to reduce the amountof welds: A central composite design was planned being it moreappropriate to create response surface [20]; a face-centeredscheme was chosen in order to explore the areas within the rangespreviously found in preliminary trials. Tests to be performed areplaced on a cubic lattice according to Fig. 1.

Three runs were planned for each condition in order to checkthe statistical significance of measurements [21]. A random testprocedure was arranged both to allocate the plates and to producethe specimens, so that the observations are independent randomvariables, aiming to reduce systematic experimental variation.

Two groups of geometric responses (Fig. 2) have been chosento be investigated, the first one including responses concerning thebead shape; the second one including possible imperfections assuggested by the referred specification [22]. Concerning theshape, the responses are the bead crown width (CW), the rootwidth (RW), the width of the heat affected zone on the uppersurface (HAZup), the width of the heat affected zone on the lowersurface (HAZlow), and the area of the FZ.

Concerning the bead imperfections, the evaluation alsoincluded right and left undercut (UC), reinforcement (R), and ex-cessive penetration (EP) at the key-hole root. The grain size was

additionally considered, given its influence on the resultingstrength.

Welding was performed in continuous wave emission; weldingsystem technical data are listed in Table 3. A preliminary proce-dure to check the focus position with respect to the surface of theplates to be welded was conducted [23]. Power and speed weremodified via the corresponding controllers in laser and robot,respectively; defocusing was modified manually through thesleeve on the welding head.

Based on literature references and past works on the same alloywith different types of laser sources [10,15], helium was preferredas assisting gas for plume removal via a leading nozzle with aflow rate of 20 l/min.

The special device (Fig. 3) developed for gas supply consists ofa side diffuser and a grooved box for top-side and back-sideshielding, respectively. The system has consequently beenpatented [24]. Argon was considered for both top-side and back-side shielding; air cross-jet prevented metal drops to reach theoptics. The gas flow rate, the welding direction, the nozzle angle,and the diffuser positioning were chosen based on trialexperiments.

Table 2 Factor levels for input parameters

Levels

�1 0 1P (W) 1500 1750 2000s (mm/s) 15 20 25f (mm) �3 �1.5 0

Fig. 1 Face-centered central composite design schemeFig. 2 Bead characterization: geometric features (above) andimperfections (below)

Table 3 Welding system technical data

Maximum output power (kW) 2.0Laser light wavelength (nm) 1030Beam parameter product (mm�mrad) 8.0Focal length (mm) 200Maximum power density (kW/mm2) 28.3Focus diameter (mm) 0.300Rayleigh range (mm) 2.81

Table 1 Nominal chemical composition [1] of Ti-6Al-4 V (wt. %)

Al V Fe O2 H2 C N2 Ti

5.5�6.8 3.5�4.5 0.4 0.2 0.015 0.08 0.05 Balanced

061012-2 / Vol. 135, DECEMBER 2013 Transactions of the ASME


Butt samples were cross-cut perpendicularly to the weldingdirection and then polished to a mirror finish with SiC paper andgrinding diamond paste on polishing cloths. Three cross-cuts,namely, at 25%, 50%, and 75% of the total weld length, wereexamined for each specimen. Chemical etching was performedusing a solution of hydrofluoric acid (48%, 10 ml), nitric acid(65%, 15 ml), and water (75 ml) at room temperature in order tohighlight the bead boundaries and microstructures in the cross-section [1]. The grain size was measured with lineal analysis viaspecific imaging software; namely, three reference lines in threedifferent sites in the same cross-section were considered for eachwelding condition.

Results and Discussion

Visual inspections were conducted just after welding to checkthe shielding effectiveness. As an example, the top-side and back-side aspects of the specimen processed in the condition of the cen-ter point of the experimental plan (P¼ 1750 W, s¼ 20 mm/s,f¼�1.5 mm) are shown in Fig. 4.

Discoloration of the bead would suggest that oxidation tookplace during the process [2]; in particular, no shielding at allwould result in grey discoloration, while blue or yellow spotswould reveal improper flow rate or device positioning. Based onthe fact that uniform, smooth, and shiny beads were produced, thedevice for bead protection was assumed to be appropriate andeffective. Moreover, since no spatters were observed both on thetop-side and on the back-side, the bead aspect is deemed to beimproved with this device with respect to other results in theliterature [13] where spattering is clearly noticed.

An overview of the transverse cross-section of the specimen inthe center condition of the plan (P¼ 1750 W, s¼ 20 mm/s,f¼�1.5 mm) is shown in Fig. 5. Pore formation due to trappedgas within the solidifying welding pool has been reported in the

literature [17]; pores are not found within the investigated rangeof the plan.

The base metal has been found to be composed of a darkb phase in a bright a matrix, which is the typical annealed struc-ture of the base alloy. Namely, the body-centered cubic structuredb phase distributes along the boundaries of the hexagonal close-packed structured a phase [8]. The corresponding micrograph, asobserved for a specimen of base metal, is shown in Fig. 6.

Laser beam welding promotes a diffusionless transformation ofthe b phase into a martensitic a0 microstructure due to high self-quench rates. A significant variation of the microstructure occurswhen moving from the base metal towards the weld center. Infact, the heat affected zone is a mixture of a0 and primary aphases. This corresponds to a structure which is quenched belowthe b-transus ranging from 720 to 985 �C [2]; the micrograph isshown in Fig. 7 with respect to midheight in the specimen corre-sponding to the center point of the plan. The fused zone, instead,mainly consists of acicular a0 martensite; the micrograph is shownin Fig. 8 and has been obtained in the core of the cross-section ofthe specimen corresponding to the center point of the plan(P¼ 1750 W, s¼ 20 mm/s, f¼�1.5 mm).

A similar structure is obtained when quenching the alloy fromthe b phase region above the b-transus, which is 985 �C approxi-mately. It has to be assumed that a value of 410 �C/s, required toattain a completely martensitic microstructure for Ti-6Al-4 Valloy, is overcome. Namely, during the bonding process, b grains

Fig. 3 System set-up

Fig. 4 Bead aspect of the specimen corresponding to thecenter point of the experimental plan

Fig. 5 Cross-section micrograph of the specimen correspond-ing to the center point of the plan

Fig. 6 Micrograph of the base metal

Journal of Manufacturing Science and Engineering DECEMBER 2013, Vol. 135 / 061012-3


grow opposite to the heat flow direction; the martensitic transfor-mation is experienced upon cooling [2,25].

The average values of the responses for each tested conditionare given in Table 4; missing data for RW, HAZlow, and EP aredue to the occurrence of incomplete penetration in the correspond-ing condition.

The imperfections in the other welding conditions matched therequirements for class A, as requested in the referred specificationfor fusion welding of aerospace applications [22], where accep-tance criteria to be satisfied are defined in terms of maximumallowed value for the imperfections; namely, 210 lm for UC and760 lm for R and EP, when 3 mm thick plates for class A welds.Referring to the same specification, excess of underfill and poros-ity were reported instead [12] when welding the same alloy with a4 kW Nd:YAG source, despite the fact that heat inputs and beadirradiance were provided in the same order of magnitude.

Given that the grain size is a statistic value due to the methodof lineal analysis, its variation is worth reporting: a standarddeviation in the range of 8.1–18.4 lm resulted.

Analyses of the Responses

Aiming to conduct the optimization of the process, the effec-tiveness of a proper synthetic parameter to describe the generalbehavior of the weld in terms of geometry must be first discussed,so that a proper constraint can be defined. Intuitively, FZ can beconsidered. As shown in Fig. 9, defocusing has no effect on thetrend of this response whose values are similar for a given thermalinput (Q), i.e., the power to speed ratio, irrespective of the focusposition. Nevertheless, when the focus position is neglected andFZ is studied as a function of power and speed as separate inputs,the expected basic relationships are confirmed, as shown in Figs.10 and 11. In particular, any increase in the laser power yields aproportional increase in FZ due to higher thermal input andenhanced fusion; conversely, any increase in the welding speedresults in a corresponding decrease in FZ.

Fig. 7 Micrograph of the heat affected zone for the specimencorresponding to the center point of the plan

Fig. 8 Micrograph of the fused zone (center point of the plan)

Table 4 Average values of geometric features and imperfections for the different welding conditions

P (W)s

(mm/s)f

(mm)CW(lm)

RW(lm)

HAZup

(lm)HAZlow

(lm)EP

(lm)R

(lm)UC right

(lm)UC left(lm)

FZ(mm2)

Grain size(lm)

1500 15 �3 3190 985 4076 2260 61 140 33 25 10.27 1682000 15 �3 3826 3419 4810 4477 167 123 59 52 15.07 1771500 25 �3 2414 — 3014 — — 25 66 70 5.63 1402000 25 �3 3077 1790 3643 2508 257 �8 100 147 6.13 1541750 20 �3 3122 1730 3758 2503 210 61 71 50 6.28 2101750 20 0 3017 2401 3296 3076 60 117 107 96 7.51 3851500 15 0 3389 2869 3799 3579 96 134 76 79 9.00 1452000 15 0 4185 4078 4760 4712 38 158 100 79 12.66 2401500 25 0 2525 493 2852 1660 �31 76 55 55 4.48 1522000 25 0 3048 2611 3386 3055 179 107 155 165 7.87 1861500 20 �1.5 2990 1682 3615 2399 237 75 86 99 5.93 2342000 20 �1.5 3737 2963 4343 3801 199 137 124 118 9.50 1471750 15 �1.5 4161 3615 4785 4420 150 145 98 79 11.30 1611750 25 �1.5 2963 1728 3514 2359 210 125 138 146 5.80 2101750 20 �1.5 3281 2503 3954 3227 229 64 123 105 8.29 187

Fig. 9 Fused zone as a function of thermal input



One may hence assume that defocusing has no effect at all onthe overall geometry, so a choice limited to the area of the fusedzone is not deemed to be satisfactory to effectively sum up thebehavior of the weld.

Actually, despite the fact that similar values of the fused zoneextent are produced for a given thermal input, the bead profile isclearly influenced by the focus position, since an effect on thecrown and the root width is noticed. As an example, for a giventhermal input of 88 J/mm, as obtained with a power of 1750 Wand a speed of 20 mm/s, focused and defocused conditions arecompared. The macrographs are shown in Fig. 12.

The obvious suggestion is that a shape factor involving CW andRW must be considered to describe the shape. The ratio of RW toCW is hence considered to be modeled in conjunction with FZ soto develop a satisfactory and effective description of the beadshape.

Polynomials have been fitted to the experimental data to obtainthe regression equation of the responses Y. A general form for theregression model is

Y ¼ f x1; x2;…; xkð Þ þ e (1)

where xi values of k factors are combined to give the output,which is affected by a residual e between the actual value and theprediction. The general quadratic model is hence

Y ¼ b0 þXk

i¼1

bixi þXk

i¼1

biix2ii þ

Xk

i¼1

Xk

j¼1j6¼i

bijxixj þ e (2)

where linear terms, interactions, and square terms are involved,since the response is capable of being influenced by single param-eter, resulting from the input parameters acting alone, or interac-tion of parameters as well, resulting when combinations of inputparameters are important.

The significant terms, and therefore the best regression modelfor each response, were selected via sequential statistical tests in astep-wise regression approach. Reinforcement, undercut, and ex-cessive penetration did not show significance, so these modelswere neglected; it is inferred that a random influence on theseresponses arises due to both the upper and lower gas fluxes, whichdirectly interact with the fused metal. Furthermore, low signifi-cance resulted for the width of the HAZ, both at the upper and atthe lower surface. Nevertheless, this is not thought to hamper thestudy, since it is assumed that a proper constraint of minimizationon the extent of the fused zone would also act in reducing theHAZ. The regression models for the responses which showed sta-tistical significance in the analysis of variance (ANOVA) aregiven in the following equations:

CW ¼ 5669:9þ 1:3P� 386:8s� 228:7fþ� 7:9sf þ 7:0s2 � 141:0f 2

(3)

RW ¼� 8281:2þ 13:5P� 178:8sþ 618:3fþ� 0:5Pf � 3:0� 10�3P2 � 197:7f 2

(4)

FZ ¼ 27:4� 3:9� 10�3P� 1:7s� 4:6� 10�4Psþþ 5:5� 10�6P2 þ 0:05s2

(5)

Fig. 10 Fused zone as a function of power

Fig. 11 Fused zone as a function of welding speed

Fig. 12 Bead profile with a focused (left) and defocused beam (right), all other pa-rameters being equal



RW=CW ¼� 1:7þ 3:9� 10�3P� 0:1sþ 0:3fþþ 6:9� 10�5Ps� 1:8� 10�4Pf � 1:3� 10�6P2þ� 1:3� 10�3s2 � 0:04f 2

(6)

Grain size ¼þ 63:4þ 0:4P� 31:2s� 64:6fþ� 7:5� 10�3Psþ 0:08Pf � 2:7sf þ 0:8s2

(7)

The responses for a given set of input values to estimate theeffect of a certain welding condition are provided. For eachresponse, the corresponding p-values and R-squared factors toassess the reliability of the model are listed in Table 5.

Since the overall p-value is a measure of probability of noiseoccurrence, models with p-values lower than 0.05 are intended tobe significant; additionally, the adjusted R-squared is expected tobe close to 1, being it a measure of the model capability inaccounting the variation of the dependent variable.

A deeper analysis has been conducted on the responses to beinvolved in the optimization process. Concerning the shape factor(RW/CW), two examples of corresponding surfaces resultingfrom the regression model are shown in Figs. 13(a) and 13(b), fora given speed of 20 mm/s, and a given negative focus position of3 mm, respectively.

For a given welding speed, increasing power results in increas-ing shape factor since the root width approaches crown width as aconsequence of higher thermal input. Negative defocusing insteadresults in larger spots on the upper surface, since the plan hasbeen set to overcome the threshold irradiance even when defocus-ing; a wider crown is then produced with consequent reducedshape factor. For a given focus position, it can be alternativelynoticed that an increase in laser power or a decrease in weldingspeed yields a corresponding increase in the shape factor as a con-sequence of increased delivered thermal input.

Concerning the grain size, two examples of correspondingsurfaces resulting from the regression model are shown inFigs. 14(a) and 14(b), for a given power of 2000 W and a givenspeed of 20 mm/s, respectively.

As a consequence of higher power or focused beam, higher irra-diances are provided, hence lower cooling rates are inferred to bein place; increased grain size results. As expected when designingthe experimental plan, a decrease in the mean grain size wasnoticed when defocusing; the effect of focus position on the grainsize is clear when considering the steepness of the responsesurface related to the highest power of the experimental plan.

Optimization

The optimization was carried out referring to the area of thefused zone, the mean grain size, and the shape factor. Minimiza-tion was required for both the extent of the fused zone and themean grain size. Aiming to produce a key-hole with a high aspectratio to be balanced with a proper constraint on the root width, inorder to ease the expulsion of vapor which could result in porosityformation otherwise, an optimal range between 0.5 and 0.7 waschosen for the shape factor [24]. Under the above-mentioned con-straint criteria, the optimization of the process was carried out.

The goals are combined into an overall desirability function[20]. This approach is one of the most widely used in industry forthe optimization of multiple response processes and it is based onthe idea that a product or a process with multiple quality charac-teristics is completely unacceptable if even only one of themdrops outside of some desired limits.

For each response Yi (x), the desirability function di (Yi) assignsnumbers ranging between 0 and 1 to the possible values of Yi,with di(Yi)¼ 0 representing a completely undesirable value of Yi

and di(Yi)¼ 1 representing a completely desirable or idealresponse value. The individual desirability values are thencombined using a geometric mean, which gives the overalldesirability D

D ¼ ½d1ðY1Þ � d2ðY2Þ � ::: � dhðYhÞ�1=h(8)

with h denoting the number of responses. When numerically solv-ing an optimization, a proper importance can be awarded to eachconstraint [26,27]; in particular, absolute importance was awardedin this study to the constraint involving the grain size, aiming toproduce a finer structure. Predictably, the suggested optimumwelding set-up condition would move towards a condition of neg-ative defocusing. In particular, a solution with a power of 1820 W,a speed of 23 mm/s with 3 mm of negative defocusing, and anoverall desirability of 92% was found.

Table 5 P-values and R-squared factors for the ANOVA-significant responses

Response variable p-value Adjusted R-squared

CW <0.0001 0.95RW <0.0001 0.92FZ <0.0001 0.93RW/CW <0.0001 0.83Grain size <0.0001 0.90

Fig. 13 (a) Shape factor for a given speed of 20 mm/s and (b)shape factor for a given negative focus position of 3 mm



Assessment of the Optimum

The suggested optimal condition was considered to producenew welding beads to be checked via X-rays. A number of weldswere cross-cut so to evaluate the welding quality level referring tointernational standards [22], as well as to compare the actualresponses to the predicted values of the models and to measurethe Vickers micro hardness along the welding bead. Additionalspecimens were produced with specific size and shape in accord-ance with the referred specification [28] for tensile testing.

The requirements for stringent quality were met for imperfec-tions in the cross-section. With respect to the issue of inclusionsor sub porosity, all of the beads complied with the specification interms of possible indications in the fused zone; as an example, anX-ray transmitted image of the welding bead is provided inFig. 15.

With respect to the predicted values of the responses as sug-gested from the corresponding regression models, the following

percentage errors were measured for the responses which hadbeen considered at the optimization stage: 2% for the extent of thefused zone, 10% for the shape factor, and 12% for the mean grainsize.

Vickers micro hardness tests were carried out to assess the fea-tures of the welding bead; indentations were made at midthicknessof the work-piece [29]. An indenting load of 0.300 kg was usedfor a dwell period of 10 s. As expected, maximum hardness wasfound at the center of the bead since laser beam welding providesa lower heat input and a more rapid solidification rate [11] whencompared with conventional techniques.

An example of the resulting trend of micro hardness along thewelding bead is provided in Fig. 16.

An average value of 322 HV0.3 resulted for the base metal. As aconsequence of the formation of acicular a0 martensite, anincrease to 344 HV0.3 is noticed on average in the HAZ, while anincrease to 383 HV0.3 is noticed in the fused zone, to a maximumvalue of 399 HV0.3. Similar findings have been discussed in theliterature [2,30] with respect to laser welding; also, the same trendhas been noticed when considering electron beam welding [8]. Inparticular, the increase in hardness in the fused zone, as a conse-quence of the formation of a0 martensite, is deemed to be reasonof improved strength in the welding bead in comparison with con-ventional welding. Even higher hardness is noticed in the fused

Fig. 14 (a) Grain size for a given power of 2000 W and (b) grainsize for a given speed of 20 mm/s

Fig. 15 X-ray transmitted image of the bead as obtained in thesuggested optimal welding condition

Fig. 16 Vickers micro hardness trend in the cross-section

Table 6 Mechanical properties of tested specimens

SpecimenYoung’s

modulus (GPa)Ultimate tensilestrength (MPa)

Elongation(%)

1 118.2 1014.2 1.262 111.5 1012.3 1.263 116.7 1036.2 1.39

Fig. 17 Fracture surfaces from tensile tests, top-side (left),back-side (right)



zone when considering pulse welding [11] with peak power rang-ing from 1.12 to 3.06 kW, due to different cooling rates.

Tensile tests were eventually conducted. The values of mechan-ical properties at room temperature for three tested specimens aregiven in Table 6. Fracture is experienced in the heat affected zone(Fig. 17).

An average decrease in 13% of the ultimate tensile strengthresulted in comparison with an expected value of 1170 MPa of thebase metal; the elongation is also degraded. The issue may beovercome with post weld heat treatment of the beads. Neverthe-less, if referred to actual application, the resulted strength isdeemed to comply with customer specification as efficiency of thebead is intended in terms of tensile strength and a value of 87% atleast is required.

Conclusions

Welding of Ti-6Al-4 V titanium alloy was investigated. Thestudy was carried out on 3 mm thick plates in square butt weldingconfiguration. The process was performed in continuous waveemission using a thin disk-laser source thus offering an element ofnovelty in comparison with previous research on the same subject.

A proper device for bead protection was developed and testedto shield the bead and prevent oxidation; given that uniform,smooth, and shiny beads with no spatters were produced, the sys-tem is assumed to be appropriate and effective; it has been conse-quently patented. No pores are found in the cross-section.

The fused zone of the bead has been found to consist of aciculara0 martensite, because the welding thermal cycles resulted inquenching the alloy from the b phase region above the b-transus.The class A requirements, as requested in the referred specifica-tion for fusion welding of aerospace applications, were met forimperfections such as undercut, reinforcement, and excessivepenetration.

Defocusing had no effect on the trend of the extent of the fusedzone; nevertheless, a clear effect of defocusing on the crown andthe root width was noticed, therefore, the bead profile is clearlyinfluenced by the focus position. Furthermore, considering theregression models, a decrease in the mean grain size was observedwhen defocusing.

The optimization was eventually carried out with minimizationrequired for both the extent of the fused zone and the mean grainsize. An optimal range for the root to crown width ratio has beendeemed to be crucial. A solution with a power of 1820 W, a speedof 23 mm/s with 3 mm of negative defocusing, and an overalldesirability of 92% was found and eventually tested. Convincingresults were achieved since no indications were found via X-rayinspections; an increase in hardness in the fused zone was noticedas a consequence of the formation of a0 martensite. A decrease inthe ultimate tensile strength resulted in comparison with the basemetal; nevertheless, the outcome is deemed to comply with cus-tomer specification as efficiency of the bead is intended in termsof tensile strength for aerospace application and a value of 87% atleast is required.

Acknowledgment

Fabrizia Caiazzo, Vincenzo Sergi, and Francesco Cardaropoliacknowledge financial support from PON Ricerca e Competitivit�a2007–2013 under grant agreement PON NAFASSY,PONa3_00007.Nafassy.

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