the microstructure and strength of copper alloy brazing...

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Introduction

In automobiles, the air-cooled heat ex-changer (radiator) has an important rolein engine efficiency and weight. Generally,aluminum radiators are used in passengercars because of their good thermal con-ductivity, lightweight, and easy joints.However, the brazing joint of aluminumalloys loses its strength at temperatureshigher than 150°C. At present, the higherservice temperature of the engine is drivenby the requirements of high power outputas well as low energy consumption. Thus,copper and brass alloys become good al-ternative materials for automobile radia-tors because of their high thermal conduc-tivity, strength, and service temperature.

It is known that there are basic re-quirements to achieve sound brazing

joints, i.e., cleaning of the parts and fillermetal, fluxing the parts, assembling theparts, fixing the assembly, brazing the as-sembly, and postcleaning the brazed joint.Since brazing Cu alloys involves multidis-ciplinary sciences, e.g., material sciences,metallurgy, and processing, practicalguidelines for Cu-alloy brazing are re-quired for practitioners. Unfortunately,there was little research on joining Cu al-loys in last decade. For Al alloys, the braz-ing guidelines have been established to en-sure proper wetting of filler metal onsubstrate, proper geometry design andalignment, optimum brazing process, andcost. However, these guidelines may notbe adequate for Cu-alloy brazed joints.

Shabtay et al. (Ref. 1) found that brass-tube and copper-fin radiators could with-stand high operating temperatures (0° to300°C) without substantial loss instrength, i.e., the tensile strength of brass-tube alloys decreased from 400 to 260MPa, and the tensile strength for copper-fin alloys decreased from 350 to 260 MPa.Li et al. (Ref. 2) have evaluated the thick-ness losses of Cu foil in contact withmelted Cu-P and Cu-Ag binary alloys atdifferent temperatures. They found thatthe amount of dissolved copper in Cu-Pliquid alloys was larger than that in Cu-Agliquid alloys. Zhang et al. (Ref. 3) havebrazed copper to copper in a vacuum con-dition using phosphor-copper filler metal.They found that the joint structure con-sisted of hypoeutectic alloys. The primaryphase was copper solid solution (α-Cu)and silver solid solution (α-Ag) while theeutectic phase was copper solid solution(α-Cu), silver solid solution (α-Ag), andCu3P.

Elmer et al. (Ref. 4) brazed oxygen-free copper (UNS C10100) in a high-vacuum atmosphere using alloy shims(25–50 μm thickness) and sputter depositcoating (1–3 μm layer) of Ag, Au, and Au-Ni alloy as filler metals. Although someporosities in joints were found for bothfiller metal, the tensile strength of thebrazed joints was about 75% of that of thecopper substrate (230 MPa). Merlin et al.(Ref. 5) have studied induction and laserbeam brazing of Cu-Zn-Ni alloys usingCu50-AgZn50 filler metal in a reducinggas atmosphere. The flux has been used todissolve oxide during brazing; however,the oxide layer and porosity occurred inthe joint due to localized overheating.Karamis et al. (Ref. 6) have investigatedthe microstructures and nonconfor-mances of joined zones in brazed coppertubes widely used in solar collector manu-facturing. They found that zone cleaningto be brazed was an important factor inpreventing porosities. The tendency forporosity increased when the filler metalcontained elements such as Zn and Cd.

The clearance between substrates is

The Microstructure and Strength ofCopper Alloy Brazing Joints

Exploring the influence of contact pressure as well as the amount of filler metal on the microstructure and strength of Cu-35Zn-3Pb brazing joints

BY A. HASAP, N. NORAPHAIPHIPAKSA, AND C. KANCHANOMAI

KEYWORDS

BrazingCopper AlloyFiller MetalContact Pressure

A. HASAP, N. NORAPHAIPHIPAKSA, and C.KANCHANOMAI ([email protected]) arewith the Department of Mechanical Engineering,Faculty of Engineering, Thammasat University,Pathumthani, Thailand. HASAP is also with theMaterial Properties Analysis and DevelopmentCentre, Thailand Institute of Scientific and Tech-nological Research, Pathumthani, Thailand.

ABSTRACT

The influences of contact pressures (2 to 47 kPa) and amounts of filler metal (Cu-9Sn-7Ni-6P with 0.04 to 0.64 mm thickness) on the microstructure and strength ofCu-alloy (Cu-35Zn-3Pb) brazing joints were evaluated for joint characteristics, i.e.,joint thickness, microstructure, and strength. Insufficient contact pressure and fillermetal resulted in the formation of cavities within the joint. The causes of cavities wereimperfect wettability of melted filler metal on substrate surfaces as well as entrappedgas within joints. At low contact pressure and a small amount of filler metal, a sig-nificant amount of Cu3P was drawn into an opening and formed Cu3P phases at themiddle of the joint. At high contact pressure and a large amount of filler metal, theflux and excess melted Cu3P were pressured out of an opening; therefore, joint mi-crostructure was a uniform combination between Cu3P and Cu-Zn-Sn phases. Jointstrength was influenced by the formation of cavities and Cu3P phase. Cavities werethe sites of crack initiation due to their high stress concentration. Consequently,cracks propagated through brittle phases of Cu3P and cavities, coalesced to othercracks, and caused final fracture. After minimizing the formation of cavities and Cu3Pphases, the joint strength was 346 MPa, which was about 86% of the substratestrength.

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one of the major concerns in brazing to en-sure that the capillary force is high enoughto draw the melted filler metal into anopening. In practice, the opening clear-ance could hardly be maintained at hightemperature due to the dimension changefrom thermal expansion. Accordingly, it isbelieved that the contact pressure be-tween substrates should be controlled in-stead of an opening clearance during braz-

ing. Although there were research worksdevoted to metallurgical evaluation forbrazing joints of copper alloys, none ofthese works emphasized the influences ofcontact pressure, amount of filler metal,and shape of cavity on joint strength. It is,therefore, the objective of this work toevaluate the influences of amount of fillermetal and contact pressure on microstruc-ture, defect, and strength of Cu-alloy (Cu-35Zn-3Pb) brazed joint. Finite elementanalysis (FEA) was carried out to evaluatethe influence of defect on joint strengths.The microstructure and fracture surfaceof the brazed joint were studied, then thefracture mechanism was discussed.

Materials and Procedures

Materials

In the present work, the substrate wasCu-35Zn-3Pb with 900°C melting temper-ature. It was manufactured from a colddrawn process to obtain a rod shape with8 mm diameter. Substrate was annealed ina nitrogen protective atmosphere. Thefiller metal was Cu-9Sn-7Ni-6P foil with8.25-g/cm3 density (VZ2250, VitrobrazeCo.), which was produced by a rapid so-lidification technique. The solidus and liq-uidus temperatures of the filler metal are600° and 630°C, respectively. Thus, the640°–680°C brazing temperature is recom-mended by the manufacturer. The com-positions of substrate and filler metal areshown in Table 1.

Tensile tests of as-drawn and annealedsubstrates were performed in accordancewith ASTM Standard E8M (Ref. 7) whilethe Vickers hardness tests were performed

in accordance with ASTM Standard E92(Ref. 8). The tensile tests were repeatedthree times, while the Vickers hardnesstests with 0.3 kgf force were performed onfive locations. Both results were averaged,as shown in Table 2. The tensile strength,yield strength, and hardness of substratewere affected by annealing. The annealingat 600°C for 2 h resulted in complete re-crystallization and stable mechanical prop-erties. Accordingly, the substrate with thisanneal condition was used in brazing. Forfiller metal foil, the tensile test is not possi-ble. However, it was melted, solidified intoa plate with 1 mm thickness, and then theVickers hardness tests with 0.3 kgf forcewere performed on five locations. The av-erage hardness was 174 HV.

Microstructures of substrates beforeand after annealing are shown in Fig. 1.Due to the addition of 34.7% of Zn intocopper, the microstructure of substrate re-veals two-phase structures, i.e., α-Cu ma-trix with dispersed β phase. Since Pb is in-soluble in solid copper, there are discreteand fine Pb precipitates at grain bound-aries. The annealed substrate revealstwinned grain boundaries due to the cold-working process.

To evaluate the filler metal microstruc-ture, the filler metal was placed on a ce-ramic plate and melted using brazing rec-ommended by the manufacturer. Itsmicrostructure is shown in Fig. 2. Energy-dispersive X-ray spectroscopy (EDS)analysis was repeated three times, and theresults were averaged. The EDS showsfour distinct phases with approximatecompositions as follows: 1) Cu-15Sn-15.5Ni-5.5P, 2) Cu-14Sn-1Ni, 3) Cu-2Ni-12.5P, and 4) Cu-3Sn-6.5Ni-11P, respec-tively. It is noted that the EDS analysis of

Fig. 1 — Microstructures of substrates. A — Before annealing; B — after annealing. Fig. 2 — Filler metal microstructure.

Table 1 — Compositions (wt-%) of Substrate and Filler Metal

Materials Cu Zn Ni Fe Si Sn Pb Mn P Co Al

Substrate 61.8 34.7 0.07 0.18 <0.0005 0.24 3.03 <0.0003 — 0.01 0.003Filler metal 77.4 — 7 — — 9.3 — — 6.3 — —

Fig. 3 — Schematic drawing of brazing fixture. A— Substrate; B — filler metal; C — fixture; D —insert; and E — weight (dimension in mm).

A B

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elemental composition has a low accuracy;therefore, the EDS technique can be ap-plied for comparative analysis only.

Substrate and Filler Metal Preparations

Two pieces of substrate with identicalsize and length (24 mm length and 8 mmdiameter) were used for the brazed joint.The substrates were prepared by a lathemachine to have uniform and parallel mat-ing surfaces. The surface roughness was 5-m Ra. The filler metal foil with 40 μm

thickness was cut to a 10-mm-diameterdisc and stacked to required thickness.Since cleanliness of substrate and fillermetal influences the brazed joint quality,both substrate and filler metal werecleaned by acetone in an ultrasonic bath toremove oil and grease. Then, they werecleaned by 2% citric acid and deionizedwater in an ultrasonic bath to remove oxi-dation. Once the substrate and filler metalwere cleaned and dried, brazing startedwithin 1 h.

To protect the formation of oxidationand improve wettability, the flux paste(Sure-Flo of Lucas-Milhaupt) was used inthis work. This flux contains 5–20% boricacid, 15–30% potassium fluoride, and15–35% potassium pentaborate. The fluxwas applied on the substrate by dipping

and then driedwith blowingwarm air. Thebrazing fluxstarts to dis-solve oxides when the temperaturereaches 315°C and removes the oxide layeron the copper substrate within a600°–871°C temperature range. Althoughpreferential brazing flux in the amount of20–30 g/m2 is suggested by a U.S. patent(Ref. 9), a brazing experiment using thisflux amount produced a brazed joint withporosity and flux residue. Therefore, ap-propriate flux concentration was studied,and the flux amount of 13.3 g/m2 couldlower porosity and flux residue.

Brazing Process

To hold the substrates and filler metal,a special design fixture (Fig. 3) was usedduring brazing. This fixture was made bybrass to reduce uneven thermal expan-sion. Identical upper and lower substrateswere placed in the fixture with filler metalfoil in between. An insert guide was usedto align both substrates and facilitate thedisassembling of brazing specimen. Vari-ous weights were placed on the upper sub-strate to maintain contact load duringbrazing.

A silicon-carbide, heated muffle fur-nace was built and used for brazing. Thefurnace had a 5 L volume. It was speciallydesigned for leak tight and uniform N2 cir-culation. A schematic drawing of the braz-ing furnace is shown in Fig. 4. The brazingtemperature was controlled by a platinum-rhodium thermocouple type R while tem-perature distribution in the furnace wasseparately monitored by two thermocou-ples type K, i.e., one at the middle of thefurnace and another beside the brazingfixture. At steady state, the temperaturedifference was lower than 5°C.

It is known that the oxygen content hasa detrimental effect on the brazed joint,i.e., oxidation of filler metal and substratecan easily occur at an elevated tempera-ture. The oxide prevents molten fillermetal from the wetting substrate surface,which results in a poor brazed joint. In thisstudy, the oxygen content in the furnacewas kept minimum to avoid oxidation. Be-fore brazing, the furnace was purged byflowing 10 L/min ultrahigh-purity argon(99.995%) for 15 min. Consequently, thefurnace was purged by ultrahigh-purity N2

Table 2 — Mechanical Properties of Substrate

Materials Yield Strength Tensile Strength Elongation at 30-mm Grain Size Hardness(MPa) (MPa) Gauge Length (%) (µm) (HV)

As drawn 440 516 8.23 8 613

Annealed at 225 415 20.5 8 115450oC for 2 h



Fig. 4 — Schematic drawing of brazing furnace.

Fig. 5 — FEA mesh and boundary condition of the brazing joint model.



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(99.999%) through a preheat tube to min-imize the temperature difference betweencold inlet gas and argon inside the furnace.The flowing N2 was also kept at a constantrate of 10 L/min throughout brazing. Ac-cordingly, the pressure inside the furnacewas positive above normal atmosphericpressure, ensuring that no air leaked intothe furnace.

The fixtures with substrates and fillermetal were placed into the furnace andheated at a constant rate of 50°C/min to590°C. The temperature was held at 590°Cfor 10 min and then heated to a brazing tem-perature of 680°C at the same rate. Thebrazing temperature was held for 10 min,then the cooling process took place by con-trolling N2 flow. The furnace was cooleddown to 300°C, then the furnace wasopened and left for air cooling to room tem-perature. Finally, the brazing specimen wasremoved from the furnace and soaked inwarm water to remove flux residue.

Elmer et al. (Ref. 4) reported that aninsufficient interfacial pressure resulted inporosity within the brazed joint, increasedjoint thickness, and reduced jointstrength. They suggested that the contactpressure of 3.45 kPa or higher was suffi-cient to produce a thin joint with goodjoint strength. However, joint thicknesscan be controlled not only by varying con-tact pressure but also by varying theamount of filler metal. In the presentstudy, the joint thickness, joint mi-crostructure, and joint strength were eval-uated using various combinations betweencontact pressures (2, 7, 14, and 47 kPa)and filler metal thicknesses (0.04, 0.16,0.32, and 0.64 mm).

Evaluation of Brazed Joint

The brazed joint was evaluated in threecategories, i.e., joint thickness, joint mi-crostructure, and joint strength. To ob-serve joint thickness, the brazing specimenwas sectioned in longitudinal direction,polished by abrasive paper, and observedby an optical microscope. Joint thicknesswas measured by image analyzer softwareat 5 locations, and the average joint thick-ness was calculated. The brazed joint wasetched with etchant (5-g FeCl3, 50-mLHCl, and 100-mL water), then its mi-crostructure was observed under an opti-cal microscope. The chemical composi-tion of the brazed joint was analyzed usingthe energy-dispersive X-ray spectroscopytechnique (Hitachi S-3400N with EDS). Atensile test was used to evaluate the jointstrength. The remaining filler metalaround the joint was removed by a lathemachine to avoid a discontinuity, whichcould produce an erroneous tensile

strength. The universal tensile test ma-chine (Instron 5969 with 50-kN load celland 0.5 mm/min crosshead speed) wasused in the present work. The tensile testswere repeated three times, and the jointstrengths were averaged. The error barsshowing the minimum and maximum val-ues of repeated tests were added into theresults. The fractures of tensile specimenswere investigated using an optical micro-scope.

Finite Element Analysis

To understand joint strength, the dis-tribution of Von Mises equivalent stress inthe brazed joint was calculated using 2DFEA. As a commercial FEA software,ABAQUS (Ref. 10) was used in the pres-ent work. The geometry models were sim-ilar to the selected brazing joints. The lin-ear-elastic, plane-strain FEA model wasdivided into two regions (Fig. 5), i.e., a re-gion of fine-level mesh around the brazedjoint and its defects, and a region ofcoarse-level mesh of substrate. Thequadrilateral elements with 8 nodes wereused in both regions. Initially, arbitrarysizes of element were applied, and the VonMises equivalent stresses (σv) were nu-merically calculated,

where σx, σy, σz are the normal stress com-ponents in a rectangular Cartesian coordi-nate while τxy, τyz, τxz are the shear stresscomponents in a rectangular Cartesian co-ordinate. The sizes of element were varied

σ

σ σ σ σ

σ σ τ τv

x y x z

y z xy xz=

− + −

+ − +

( ) ( )

( ) 6 ( +

2 2

2 2 2 ++ )

2(1)

2τyz

Fig. 6 — Micrographs of brazing joints. A — 2 kPacontact pressure and 0.04–mm filler metal thickness;B — 47-kPa contact pressure and 0.04–mm fillermetal thickness; C — 2–kPa contact pressure and0.64-mm filler metal thickness; and D — 47–kPacontact pressure and 0.64–mm filler metal thickness.

Fig. 7 — Relationships between joint thickness and contact pressure at various filler metalthicknesses.

A

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C

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until the invariance of the numerical re-sults was satisfied. Then, the independ-ency of numerical results on mesh densitywas confirmed. The size of fine elementswas 5 μm while that of coarse elementswas 100 μm. The total amount of elementwas approximately 70,000 elements.

The far-field stress (σ) of 50 MPa wasapplied to simulate elastic deformation ofthe brazed joint. The elastic modulus (E)of 97 GPa and Poisson’s ratio (v) of 0.3were used for both substrate and brazingalloys. As a ratio between localized VonMises equivalent stress and far-fieldstress, a stress concentration factor (SCF)was calculated, i.e. SCF = σv/σ. The con-tour plots of SCF were compared betweenvarious brazed joints.

Results and Discussion

Joint Thickness

The filler metal melts and flows withinan opening between substrates when itstemperature reaches the brazing temper-ature. The melted filler metal is expectedto wet the surfaces of substrate withoutresidue flux and cavities. After the fillermetal’s solidification, the brazed joint isformed. Examples of brazed joints areshown in Fig. 6, i.e., joint A: 2-kPa con-tact pressure and 0.04-mm filler metalthickness; joint B: 47-kPa contact pres-sure and 0.04-mm filler metal thickness;joint C: 2 kPa contact pressure and 0.64-mm filler metal thickness; joint D: 47-kPacontact pressure and 0.64-mm filler metalthickness.

Relationships between joint thicknessand contact pressure at various filler metalthicknesses are shown in Fig. 7. For jointswith 0.04-mm filler metal thickness (jointsA and B), the 0.04-mm filler metal thick-ness causes a small opening, which createsa capillary force and draws melted filler

metal from the out-side perimeter of thejoint into an opening.Accordingly, thewidths of both jointsbecome larger thantheir filler metalthicknesses. Sincethe melted fillermetal is drawn intoan opening, the oxideand gas may be en-trapped within thebrazed joint. Whencomparing between joints A and B, it isfound that the higher contact pressure ofjoint B (47 kPa) results in smaller jointthickness than that of joint A (2 kPa). Forjoints with 0.64-mm filler metal thickness(joints C and D), the flux and excessmelted filler metal are pressured out ofopenings; therefore, the joint thicknessesare smaller than their filler metal thick-nesses.

As a discontinuity within the brazedjoint, cavities are understood to reducejoint strength. Due to the imperfect wet-tability of melted filler metal on substratesurfaces as well as entrapped gas withinthe joint, cavities are possible to occurthere. These cavities are influenced byboth contact pressure and the amount offiller metal (Fig. 6). The cavities withinjoint A (2-kPa contact pressure and 0.04mm filler metal thickness) are a combina-tion between flat-shaped cavities at thejoint interface and spherical-shaped cavi-ties within the joint. It is known that thewettability of melted filler metal on a sub-strate depends on the amount of oxide ona substrate. The flux is expected to dis-solve the oxide and prepare a clean sur-face for perfect wettability of the meltedfiller metal on a substrate. Unfortunately,the 2-kPa contact pressure is insufficientto pressure the flux with the dissolved

oxide as well as entrapped gas within themelted filler metal out of joint A. The im-perfect wettability of the melted fillermetal on a substrate causes flat-shapedcavities at the joint interface, whereas theentrapped gas causes spherical-shapedcavities within the joint. With increasingcontact pressure, the entrapped flux andgas within melted filler metal become less.Therefore, the wettability of melted fillermetal on a substrate is improved, and onlycavities within the joint are observed forjoint B (47-kPa contact pressure and 0.04mm filler metal thickness).

With sufficient contact pressure and anamount of filler metal, the flux and excessmelted filler metal are pressured out of anopening, i.e., joint thickness is smaller thanfiller metal thickness. It is known that oxidecan dissolve in flux as well as filler metalwith a phosphorous composition (Ref. 11);the flow of excess melted filler metal out ofjoint may decrease the amount of oxide andenhance wettability. Accordingly, no cavi-ties at the joint interface (imperfect wetta-bility) are found; only spherical-shaped cav-ities (entrapped gas) are found within jointsC and D (0.64-mm filler metal thickness).Since higher contact pressure can pressuremore entrapped gas out of melted fillermetal, the cavities within joint D (47-kPacontact pressure) are smaller in size and less

Fig. 8 — Micrograph and EDS analysis of brazing joint A (2-kPa contact pres-sure and 0.04–mm filler metal thickness).

Fig. 9 — Micrograph and EDS elemental mapping of brazing joint D (47-kPa contact pressure and 0.64-mm filler metal thickness).



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in number than those within joint C (2-kPacontact pressure).

Joint Microstructure

The microstructure of brazed joint A(2-kPa contact pressure and 0.04-mm fillermetal thickness) is shown in Fig. 8. Al-though the EDS analysis of elementalcomposition has a low accuracy, it is pos-sible to apply EDS technique for compar-ative analysis. In each area of joint A (Fig.8), the EDS analysis was repeated threetimes, and the element wt-% was aver-aged. The amounts of major elements in

various areas were compared, then thecharacteristic of joint microstructure wasdiscussed.

Diffusions of elements between thejoint and substrate are observed, e.g., Zndiffuses from the substrate (area 1) intothe joint (areas 2 and 3). Since phospho-rous has good solubility in Cu, the grayphase of Cu3P (area 4) forms in the mid-dle of the joint. Diffusion of Zn from thesubstrate into the joint causes the forma-tion of Cu-Zn-Sn compound, i.e., light-gray phase (area 2). The combination be-tween Cu3P and Cu-Zn-Sn phases isobserved in area 3. The average hardnessof the Cu3P phase is 286 HV while that ofthe Cu-Zn-Sn phase is 133 HV.

The element distribution of joint D (47kPa contact pressure and 0.64-mm fillermetal thickness) was evaluated by EDS el-emental mapping, as shown in Fig. 9. Thespherical-shaped cavities are mostly foundat the P-rich phase.

Since the operated temperature rangeof flux is 600°–871°C, it is expected that theflux is in liquid state at brazing tempera-ture (680°C). The formation of voids dueto flux volatiles is therefore unlikely. Therandom distribution of spherical-shapedcavities should be observed if the spheri-cal-shaped cavities are the flux volatilesand/or entrapped N2 (brazing environ-ment). However, the spherical-shapedcavities are mostly found at the P-richphase of the present joints. According toLide (Ref. 12), the boiling point of phos-phorous is 280°C, and its heat of vaporiza-tion (12.1 kJ/mol) is 9.5 times lower thanthat of zinc (115.3 kJ/mol). Thus, it is be-lieved that the porosities at Cu3P phaseare related to vaporization of phospho-rous during brazing.

Microstructures of joints A, B, C, andD are compared in Fig. 10. For joints Aand B (0.04-mm filler metal thickness),the Cu-Zn-Sn phase forms near substrateswhile the Cu3P phase forms at the middleof the joint. Since the phosphorus contentwithin the filler metal can improve flowcharacteristic (Ref. 13), it is believed that

the flow of melted Cu3P within an theopening is possible. The lower contactpressure of joint A causes a small opening,creates a capillary force, and draws meltedCu3P from outside into an opening. Sincethe high contact pressure of joint B (47-kPa) can reduce the amount of meltedCu3P that flows into the opening, theamount of Cu3P phase within joint B (47-kPa contact pressure) is less than that ofjoint A (2-kPa contact pressure), as shownin Fig. 10.

On the other hand, a more uniformcombination between Cu3P and Cu-Zn-Sn phases is observed for joints C and D(0.64-mm filler metal thickness). Withhigh contact pressure and amount offiller metal, the flux and excess meltedCu3P are pressured out of the opening;therefore, the formation of a pure Cu3Pphase at the middle of joints C and D isunlikely. The higher contact pressure ofjoint D can pressure out more amounts ofCu3P and entrapped gas out of the open-ing; therefore, the cavities and Cu3Pphases of joint D are less than those ofjoint C (Figs. 6 and 10).

Joint Strength

The shape, size, and number of cavitiescan result in premature cracking and re-duced strength of the brazed joint. To un-derstand the effect of cavities on jointstrength, the SCF of various brazed jointswere numerically analyzed and compared.Geometry models were similar to the se-lected brazing joints, as indicated in Fig. 6.Contour plots of SCF are shown in Fig. 11.Joints A, B, and C show high SCF acrossthe joint interface (dashed line in Fig. 11)due to their large cavities. The cavitiesfrom imperfect wettability (joint A) showthe highest SCF. Due to high SCF of adja-cent cavities, these cavities can link up andare likely the location of the final failureof brazed joint. Based on FEA, thestrength of joint A should be the lowest.Since the cavities from entrapped gas ofjoint D are spherical and small, joint D has

Fig. 10 — Microstructures of brazed joints. A — 2-kPa contact pressure and 0.04-mm filler metal thickness; B — 47-kPa contact pressure and 0.04-mm fillermetal thickness; C — 2-kPa contact pressure and 0.64-mm filler metal thickness; and D — 47-kPa contact pressure and 0.64-mm filler metal thickness.

Fig. 11 — Contour plots of SCF for brazed joints. A— 2-kPa contact pressure and 0.04-mm filler metalthickness; B — 47-kPa contact pressure and 0.04-mm filler metal thickness; C — 2-kPa contact pres-sure and 0.64-mm filler metal thickness; and D —47-kPa contact pressure and 0.64-mm filler metalthickness.

A B C D

A

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C

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the smallest SCF. Therefore, it is expectedthat the strength of joint D is the highest,while the magnitudes of SCF for joints Band C are higher than that of joint D butlower than that of joint A. The strength ofjoints B and C should be in between thestrengths of joints A and D.

To verify the influence of cavities onjoint strength, the joint strength at variouscontact pressures and filler metal thick-nesses are shown in Fig. 12. Joint D has thehighest average strength of 346 MPa,which is about 86% of substrate strength.While the average strengths of joints A, B,and C are significantly low and marginallydifferent, i.e. 125 to 140 MPa, it is notedthat the joint strength is not only influ-enced by cavities but also joint mi-crostructure. For the brazed joints at con-tact pressures of 7 and 14 kPa, thevariation of filler metal thickness (0.04 to0.64 mm) gives the average joint strengthin the range of 80–180 MPa, which are sig-nificantly lower than the strength of jointD. This finding corresponds to the obser-vations of many cavities within these braz-ing joints.

Failure Analysis

Cross-sectional micrographs of frac-tured joints are shown in Fig. 13. Fracturesof joints A, B, and C occur in the middleof the brazing joints. At high magnifica-tion, evidence of cracks propagatingthrough the Cu3P phase is clearly seen injoints A, B, and C. Cavities are likely thesites of crack initiation due to their highstress concentration (Fig. 11) togetherwith the fact that the Cu3P phase (286HV) is harder and more brittle than theCu-Zn- Sn phase (133 HV). Therefore,cracks propagate through the Cu3P phaseand cavities, and coalesce to others. Con-sequently, final fracture occurs at the

brazing joint.Since the Cu3P phase forms at the mid-

dle of joints A and B (Fig. 10), the crackscan easily propagate through the Cu3Pphase (Fig. 13). Thus, strengths of joints Aand B are similar (Fig. 12), and influencedby both cavities and Cu3P phase. Althoughthe combination between Cu-Zn-Sn andCu3P phases is observed for joint C (Fig.10), the large size and high amount ofspherical cavities (Fig. 6) can reduce theload-bearing area of a joint. Therefore,the strength of joint C is dominated by theinfluence of cavities, which results in lowjoint strength (Fig. 12).

A different crack path is observed forjoint D (47 kPa contact pressure and 0.64mm filler metal thickness), i.e., crackpropagates through both the joint-sub-strate interface and joint. Since the cavi-ties and Cu3P phase within joint D are sig-nificantly less than other joints, the strainmismatch at joint-substrate interface islikely the main driving force for crack ini-tiation. A number of cracks initiate andpropagate at the joint-substrate interface,then they coalesce to others. The coales-cence of these cracks occurs across thejoint (Fig. 13). At high magnification, it isobserved that these cracks propagatethrough the Cu3P phase. Since the sizeand amount of spherical cavity and Cu3Pphase are significantly smaller than thoseof other joints, the highest averagestrength is obtained for joint D, i.e., 346MPa or about 86% of substrate strength.

Based on the present findings, thehigh-strength, Cu-alloy brazing joint couldbe achieved by employing the appropriatebrazing, plus proper combination betweencontact pressure and amount of fillermetal. The influences of contact pressureand amount of filler metal on joint defects,joint microstructure, and joint strengthare understood. These understandings

also provide some guidelines for practi-tioners to obtain successful Cu-alloy braz-ing, e.g. 1) a selection of contact pressureand amount of filler metal to avoid defectsand weak phase microstructure, and 2) acompromise between joint strength andamount of filler metal (optimize cost offiller metal). It is recommended that thepresent brazing should be performed with47 kPa contact pressure and 0.64 mm fillermetal thickness.

Conclusion

The influence of contact pressure (2 to47 kPa) and amount of filler metal (0.04 to0.64 mm thickness) on the microstructureand strength of the Cu-alloy brazed jointhas been evaluated. The findings can besummarized as follows:

1. Furnace brazing of the Cu-Zn alloywith a Cu-Sn-Ni-P filler metal is sensitiveto oxidation, i.e., the effectiveness andconsistency of the cleaning process as wellas a controlled environment are necessary.Successful brazing can be achieved byusing a flux and a nitrogen atmosphere.

2. Both contact pressure and theamount of filler metal influence joint char-acteristics, i.e., joint thickness, joint mi-crostructure, and joint strength. Insuffi-cient contact pressure and filler metalresult in the formation of cavities withinthe joint; the causes of cavities are imper-fect wettability of the melted filler metalon substrate surfaces as well as entrappedgas within the joint.

3. At low contact pressure and a smallamount of filler metal, a significantamount of Cu3P is drawn into the opening,and the Cu3P phases form at the middle ofthe joint. At high contact pressure and alarge amount of filler metal, the flux andexcess melted Cu3P are pressured out ofopening; therefore, joint microstructure is

Fig. 12 — Relationships between joint strength and contact pres-sure at various filler metal thicknesses.

Fig. 13 — Failures of brazing joints. A — 2-kPa contact pressure and 0.04-mm filler metalthickness; B — 47-kPa contact pressure and 0.04-mm filler metal thickness; C — 2-kPacontact pressure and 0.64-mm filler metal thickness; and D — 47-kPa contact pressureand 0.64-mm filler metal thickness.

A B C D


a more uniform combination betweenCu3P and Cu-Zn-Sn phases.

4. Joint strength is influenced by theformations of cavities and Cu3P phase.Cavities are the sites of crack initiationdue to their high stress concentration.Consequently, cracks propagate throughbrittle phases of Cu3P and cavities, coa-lesce to other cracks, and cause final frac-ture. After minimizing the formations ofcavities and Cu3P phases, the jointstrength is 346 MPa, which is about 86%of the substrate strength.

Acknowledgment

The authors would like to acknowledgesupport from the C. H. Wattanayont Co.,Ltd., the Thailand Research Fund(TRF–IUG5280003), Thailand Commis-sion on Higher Education of Thailand (Na-tional Research University Project), Na-tional Research Council of Thailand(NRCT), and National Metal and MaterialsTechnology Center (MTEC), Thailand.

References

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2. Li, Y. N., Peng, Z. L., and Yan, J. C. 2012.The dissolution mechanism of copper weldbrazing with Cu-based brazing alloys. MaterialsScience Forum 697–698: 394–398.

3. Zhang, P. L., Yao, S., Ding, M., Lu, F. G.,and Lou, S. N. 2010. Microstructural analysis inthe vacuum brazing of copper to copper using aphosphor-copper brazing filler metal. Interna-tional Journal of Materials Research 101(11):1436–1440.

4. Elmer, J. W., Klingmann, J., and Van Bib-ber, K. 2001. Diffusion bonding and brazing ofhigh purity copper for linear collider accelera-tor structures. Physical Review Special Topics –Accelerators and Beams 4(5): 12–27.

5. Merlin, M., Crepaldi, I., Garagnani, G. L.,and Trebbi, L. 2009. Influence of the brazingprocess parameters on the microstructuralcharacteristics of copper alloy joints. WeldingInternational 23(8): 606–615.

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F. 2003. Microstructural analysis and disconti-nuities in the brazed zone of copper tubes. Jour-nal of Materials Processing Technology 141(3):302–312.

7. ASTM E8M, Standard Test Method forTension Testing of Metallic Materials. Volume3.01, Annual Book of ASTM Standards. 2004.

8. ASTM E92, Standard Test Method forVickers Hardness of Metallic Materials. Volume3.01, Annual Book of ASTM Standards. 2003.

9. United States Patent Number 5190596,Method of brazing metal surfaces, 1993.

10. ABAQUS User’s Manual. ABAQUS Inc.,2006.

11. Şerban, V. A., Codrean, C., Uţu, D., andOpriş, C. 2009. Amorphous alloys for brazingcopper based alloys. The 13th InternationalConference on Rapidly Quenched andMetastable Materials. Dresden, Germany: IOPPublishing.

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13. Sim, R. F., and Willingham, J. A. 1987.Copper phosphorus based (self-fluxing) braz-ing alloys used for joining copper and its alloys.FWP Journal 27(7): 33–39.


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