Reinforcement Behavior in Laser Welding of A356/TiB2p MMCs

Haichao Cui, Fenggui Lu+, Xinhua Tang and Shun Yao

Shanghai key Laboratory of Laser Processing and Materials Modification, School of Materials Science and Engineering,Shanghai JiaoTong University, Shanghai 200240, P. R. China

TiB2p behavior was systematically studied in laser welding of A356/TiB2p metal matrix composites (MMCs). TiB2 particles encounteredthree different types of evolution under the direct irradiation of laser, including oxidization, escaping out of the bead and immigration in thewelding pool. TiB2 particles remained solid state in the welding pool. The clusters of solid TiB2 particles were pushed into smaller clusters sizeand dispersive TiB2 particles by the outflow of Al plasma. After solidification, the distribution of TiB2 in the bead got more homogeneous thanpre-welding. TiB2 and Al matrix bonded in strong semi-coherent relationship. [doi:10.2320/matertrans.M2011385]

(Received December 13, 2011; Accepted May 23, 2012; Published July 25, 2012)

Keywords: laser welding, oxidization, distribution, interface

1. Introduction

In situ TiB2 particulate reinforced aluminum metal matrixcomposites (MMCs) have attracted more and more attentionsfor its high damping capacity, high wear resistance and highstrength at lower and higher temperature.1­3) This compositewas recognized as one of the promising materials because ofits fine reinforcement, good distribution character and cleanparticulate/matrix interface, especially high wear resistanceand strength in comparison with other composites such asreinforced by SiC.4) The problem on the weldability of TiB2

reinforced A356 matrix has been analyzed and solved usinglaser beam technology,5) but there were also some problemsunclear. For instance, when welded with laser, aluminummatrix composite will melt after absorbing laser beam energy.Absorption capability of material to laser beam (¡) dependson resistivity of the material (μ) and wave length of laser (­),as indicated: ¡ ¼ ffiffiffi

μ

­

p, whereas resistivity of TiB2 as non-

metal reinforcement is larger than that of aluminum alloy.6)

So the ¡ value of TiB2 is larger than that of aluminum matrix.When laser shines on the composite, the temperature of TiB2

will go up quickly at first then transfer the heat flux tosurrounded aluminum matrix. During this process, TiB2

may encounter some variations because it directly absorbsthe laser energy and gets to the temperature of ten thousanddegrees or so. So in order to ascertain above questions,the further research is required to carry out. This paperresearched the TiB2 evolution during the laser weldingprocess and the subsequent bonding interface between TiB2

and Al matrix after solidification.

2. Experimental Material and Method

The A356 (Al­7Si­0.4Mg­0.4Zr) reinforced by 13mass%TiB2 composites were employed in the present study,which were fabricated with an exothermic reaction processvia K2TiF6 and KBF4 salts by the Institute of Ecology andEnvironmental Materials of Shanghai Jiao Tong University.15 kW CO2 laser welding system (TRUMPF TLF 15000T)was used to weld the A356/TiB2p composite. Due to thereflection of Al on laser, larger power density was necessary

on the surface in order to form the keyhole welding fordeeper penetration. So the laser beam was focused on the topsurface of workpiece, and the diameter of laser beam spotwas 0.8mm. Bead-on-plate welds were performed on theplate of 6mm thickness. Before welding the plates weremachined to get rid of oxidation film, and cleaned withacetone. Pure helium as shielding gas with a flow rate of28L/min was ejected by a forward nozzle inclining to thebeam axis 45°. In order to observe the oxidization of TiB2,the comparative experiments of A356 alloy and A356/TiB2

composite were performed using electron beam welding withthe power of 3 kW and the speed of 24mm/s. The vacuumlevel in the welding chamber was up to 5 Pa.

In order to recognize the phase formation of keyholeregion in the weld seam, the sample for X-ray diffraction(XRD) was sectioned along the middle bead in weldingdirection. The variation of TiB2 in distribution and morphol-ogy was characterized by scanning electron microscope(SEM) equipped by energy dispersive X-ray spectroscopy(EDS). High-resolution transmission electron microscope(HTEM) was employed to observe the interface betweenTiB2 and Al matrix.

3. Results and Discussion

3.1 TiB2 evolutionXRD results including the bead and base metal respec-

tively were shown in Fig. 1. It was seen that the peaks of thebead were almost the same with the base metal, especially forthe TiB2 peak. This indicated that most TiB2 particles withinkeyhole area were not varied after laser welding. But it wasknown that if the content of analyzed phase was low, XRDwas difficult to detect it. In order to further examine whetherTiB2 particle transformed, SEM combining EDS wasemployed to analyze the composition of particles locatingat the bead center. The compositions of particles wereshown in Fig. 2. The particles of TiO2 could be determinedaccording to Ti/O atom ratio. Except some TiO2 particlespresent, most remaining particles were TiB2 marked by thearrow in Fig. 2 which was distinguished by EDS. The phaseTiO2 was from the transformation of TiB2 because otherphases did not contain Ti element according to XRD result ofbase metal. The same results were found by Zhenlin Yang7)+Corresponding author, E-mail: lfg119@sjtu.edu.cn

Materials Transactions, Vol. 53, No. 9 (2012) pp. 1644 to 1647©2012 The Japan Institute of Metals

that TiB2 was changed into TiO2 when they were locating inthe circumstance of air at elevated temperature (1273K). Thelaser welding was also conducted in the air although theshielding gas was chosen in order to prevent the air into thewelding pool. However, it was impossible to protect the pooltotally well due to the presence of laser plume. So accordingto Zhenlin Yang7) suggestion, the following reactionoccurred:

TiB2 þ 5=2O2 ! TiO2ðsÞ þ B2O3ðlÞð�G0 ¼ �374:6 kcal at 873 kÞ

¦G value was negative indicating the reaction spontaneously.So the oxidization reaction was permitted according tothermodynamics rule. Because the oxidized particles locatedin the bead center, particles were close to the keyhole wallduring welding process where it was possible to expose to air.While at this location the temperature was higher than thetemperature of Al boiling point, so the B2O3 with low boilingpoint (2133 k) changed according to the followed reaction:

B2O3ðlÞ ! B2O3ðgÞB2O3 vaporized and escaped out of welding pool, while TiO2

remained in the pool. In A356 alloy there was Al and Mgelement (0.35mass%) present, but the oxidized products ofMgO and Al2O3 were not found in the bead center. Dickon8)

calculated the forming Gibbs free energy of MgO and Al2O3

products:

Mgþ 1=2O2 ! MgO ð�G0 ¼ �241:8 kcal at 873 kÞAlþ 3=4O2 ! 1=2Al2O3 ð�G0 ¼ �222:3 kcal at 873 kÞCompared to above reactions, the forming Gibbs free energyof TiB2 oxidization was lower and inclined to react at first.Hence, the other oxidized products like MgO or Al2O3 werenot found in the bead.

In order to further confirm whether TiB2 oxidizationoccurred, the presence of B2O3 phase needed to beconfirmed. After solidification of welding pool finished,vaporized B2O3 would drop onto the surface of bead due tocondensation. So the vacuum welding was required in orderto collect the oxidized product if the oxidized reaction reallyoccurred. Electron beam welding was employed whichpossessed similar welding mechanism with laser weldingand conducted under low vacuum level. Comparativeexperiments were prepared for the electron beam weldingA356 and A356/TiB2 composite respectively. The appear-ance of bead was shown in Fig. 3. It was found that thesurface of A356/TiB2 bead got black obviously and A356bead presented bright. The oxidization in electron beamwelding was inevitable because it was conducted undervacuum level of 5 Pa. This experiment proved that thetransformation of TiB2 particles took place. For the blackoxide it was required to further confirm whether it was B2O3

through electron probe micro-analyzer. According to aboveanalysis, TiB2 particles could be oxidized when they werein the circumstance of elevated temperature (1273K) with

20

87

6

5

3

2Theta(deg.)

bead

parent1 2

4

Inte

nsity 9

806040

Fig. 1 XRD pattern of the bead (P = 8.5 kW, v = 50mm/s).

TiB2

TiB2

Fig. 2 Particles profile and corresponding EDS result.

(a) (b)

Fig. 3 Examination for TiB2 oxidization using electron beam welding; (a) A356/TiB2 (b) A356.

Reinforcement Behavior in Laser Welding of A356/TiB2p MMCs 1645

oxygen present. Hence, it was inferred that oxidized TiB2

particles were from the region of keyhole where TiB2

particles met the oxidized condition. According to EDS andXRD results, few TiB2 particles in the bead center wereoxidized. It is know that the diameter of keyhole is nearlyequal to the diameter of focused laser spot (0.8mm), whereasthe width of bead is about 5mm. So if all the TiB2 particlesoriginally locating in the keyhole region are oxidized, theamount of TiO2 is about 16% which can be found by XRD.In fact the amount of TiO2 is quite slight.

In order to further explore the reason why other TiB2 didnot transform under the direct laser irradiation, detailedwelding process was observed by the high speed camera. Itwas found that much welding spatter occurred as shown inFig. 4. While in laser welding of A356 alloy, little spatter waspresent. The reason of spatter in laser welding was ascribedto the mixture of solid and gaseous state. At the beginning ofinteraction between laser and composite, the absorptionenergy of TiB2 to laser was not much larger than the Almatrix due to the lower interaction time (1/v = 0.02 s/mm)although TiB2 had the higher absorption rate to laser than Almatrix. So TiB2 remained solid state when Al alloy began toboil due to its higher melting temperature (3500K) whichwas higher than the boiling temperature of Al alloys(2700K). The solid TiB2 particles inhibited the overflow ofaluminum plume and then were blown out of the weldingpool that led to the occurrence of spatter.

With the increase of interaction time between laser andcomposite, partial TiB2 particles were blown out of the bead,but other TiB2 particles that did not inhibit the outflow of Alplasma from the pool were pushed into the liquid region ofwelding pool. In this interaction process, larger TiB2 clusterswere separated by the Al plasma, then pushed into the liquidregion of pool and subsequently distributed sparsely in theliquid pool. The variation of TiB2 clusters from larger size(Fig. 5(a)) of pre-welding to smaller size (Fig. 5(b)) afterwelding could prove the push of plasma on TiB2 clusters.Hence the distribution of TiB2 after welding was much morehomogeneous than pre-welding.

TiB2 particles in the liquid pool fully touched with Alliquid. Whether the reactions between TiB2 and Al matrixoccurred was required to study. Liang et al.9) have evaluatedthe forming Gibbs energy about each compound composed ofAl, Ti and B elements in the melt as was listed in Table 1. Itwas found that forming Gibbs energy of TiB2 was lower thanthat of any other product, especially much lower at theboiling point (2700K) of Al, so the reaction in the weldingpool was difficult to take place. TiB2 particle was stablest inall the constituting compounds of Al, Ti and B as wasreported by Yue et al.10) Figure 5 shows the TiB2 distributionin the bead center and base metal respectively. It was shownthat TiB2 particles distributed more homogeneously than pre-welding due to the push from vaporized aluminum plasma.The cluster size of TiB2 got smaller than pre-welding.

Finally, the above research indicated that TiB2 particles didnot melt after absorb laser energy directly but remained solidstate in welding pool except the oxidized particles. Thereasons for TiB2 without melting in laser welding processwere summarized as follows. First, TiB2 possessed highermelting temperature that was higher than the boilingtemperature of A356 matrix. A356 matrix would vaporizethen decrease the fluid temperature because of latent heat.

spatter

Fig. 4 Spatter profile during laser welding of A356/TiB2.

(b)(a)

TiB2 cluster

TiB2 cluster

Fig. 5 TiB2 distribution and morphology; (a) In base metal (b) In bead (P = 8 kW, v = 50mm/s).

Table 1 ¦G values of the formation to TiB2, TiB, Al3Ti and AlB2.

ReactionExpressions of ¦G

(J/mol)¦G values (J/mol)

T = 2700K

[Ti] + Al¼ Al3Ti ¹335920 + 175.3T 137390

[Ti] + 2[B]¼ TiB2 ¹4615411 + 92.3T ¹4366201

Ti + B ¼ TiB ¹302990 + 82.8T ¹79430

2[B] + Al¼ AlB2 ¹237812 + 182.5T 254938

H. Cui, F. Lu, X. Tang and S. Yao1646

Second, the size of in situ TiB2 particles was within the200 nm. The smaller size made TiB2 easily transfer the heatflux to A356 matrix and decreased its temperature quickly.Third, A356 alloy with high thermal conductivity and couldquickly transfer the heat flux from welding pool to workpiecearound. Fourth, under the high welding speed, the interactiontime between laser and TiB2 was short.

3.2 TiB2 bonding interface with Al matrixFigure 6 shows the HTEM bright field image of TiB2

particles. The sample for HTEM was taken from the center ofbead. TiB2 particles presented hexagonal and rectangularmorphology. The hexagonal morphology was because the{0001} face was parallel to the electron beam. Similarly,the particles with rectangular morphology were the resultsof f�1100g or f10�10g faces parallel to the electron beam.

Figure 6(b) shows the interface between TiB2 and Al matrix.The interface was very clean and no impurities were detected.The semi-coherent relationship was determined according tocombined character of lattice structure. Moreover, there wasdistortion of lattice around the TiB2 particles which wasmarked in Fig. 6(b). This supplied the evidence that thedifference of thermal expansion coefficient between the TiB2

(8.1 © 10¹6/k) and Al matrix (2.4 © 10¹5/k) produces thestress concentration during the solidification.

It is very interesting that TiB2 is still semi-coherent with Alafter laser welding. Z. Y. Chen11) has reported that semi-coherent relation between TiB2 with Al was present in as-castspecimens, but for the welding joint, semi-coherent relationwas also gained under the fast solidification rate. The goodinterface enables the joint to bear more loads and improvesthe strength of joint.

4. Conclusions

(1) In laser welding of A356/TiB2 composite, TiB2

particles were oxidized: TiB2 + 5/2O2 ¼ TiO2(s) +B2O3(l). The oxidized product of TiO2 was present inthe bead and B2O3 was blown out of the bead.

(2) Due to the rapid welding speed, TiB2 was not heated upto the melting temperature firstly although it had highabsorption rate to laser compared to aluminum matrix.When the A356 alloy got to the boiling temperature,TiB2 remained solid state in the pool. The solid TiB2

was pushed to other places of welding pool by theplume that led to the more homogeneous of TiB2

compared to pre-welding.(3) TiB2 after welding bonded in strong semi-coherent

relationship with A356 matrix.

Acknowledgements

This work was supported by grant No. 11ZR1417500 fromShanghai Natural Science Funding. The experimentalmaterials were supplied by the Institute of Ecology andEnvironmental Materials of Shanghai Jiao Tong University.The authors are grateful.

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5605­5608.

(b)

Al

(100)

0.254nm

(001)

0.330nm0.238nm

(111)

0.123nm (311)

TiB2

(a)

Fig. 6 High revolution TEM observation for TiB2 interface; (a) TiB2

particles and its diffraction pattern (b) The interface between TiB2 and Al.

Reinforcement Behavior in Laser Welding of A356/TiB2p MMCs 1647