Surface Reactins and Wetting Mechanisms f Titanium and AluminumCntaining NickelBase and IrnBase Allys During Brazing under Vacuum. _i DC (/) CC Different thermal expansins at brazing temperature cause xide, nitride and carbide surfaces n base metals t crack and allw filler metals t penetrate and wet the underlying base metals 0. i If) Intrductin T btain quality brazed jints, the perfect wetting f the parts being brazed is a necessity. Even relatively thin surface layers n base metals, particularly xides, can cause insufficient wetting. A decisive factr in the separatin f surface layers during brazing in vacuum is the stability f the surface structure; this is a functin f the cmpsitin f the base metal. n stainless steel, base metal surface wetting is attained by reducing the chrmium xide layer with carbn in the steel t prduce carbn mnxide under vacuum f 10"' 4 t 10 5 mbar at temperatures abve 900 C, i.e., 1652 F (Ref. 1). This paper discusses the temperaturedependent surface reactins f titaniumand aluminumcntaining base metals under vacuum heat treatment. These elements develp extremely stable xide, nitride, and carbide surface structures; the wetting behavir f hightemperature brazing filler metals n these Based n a paper presented at the 13th Internatinal A WS WRC Brazing and Sldering Cnference held in Kansas City, Missuri, during April 2729, 1982. Au E. tugscheider is Prfessr, Material Science Divisin, Technical University f Aachen; M. MAIER is with the Institute f Semicnductr Electrnics, Technical University f Aachen, Federal Republic f Germany; and H. ZHUANG is Prfessr, Beijing Institute f Aernautics and Astrnautics, Beijing, China. BY E. GSCHEIDER, H. ZHUANG AND M. MAIER materials during brazing under vacuum cnditins is reprted. Test Prcedure pies plished and cleaned ultrasnically " / trr = 1.33 mbar Table 1Cmpsitin f Base Metals and Brazing Filler Metals, Wt% C Cr Al Ti M C B Si P Fe Ni BTR: (a) Base metals Nially 0.10 14.9 1.8 1.8 2.9 0.5 Feally 0.02 1.6 4.0 12.0 18.0 The fllwing base metals were investigated: a nickelbase ally similar t Incnel 700 and a martensitic irnbase ally. Bth base metals were brazed with three hightemperature brazing filler metalsbni2, BNi7, and BAu4 (Table 1). Temperaturedependent base metal surface reactins were tested during vacwere annealed in a vacuum f 2 10~ 5 mbar (1.5 10~ 5 trr)* at temperatures up t 1150 C (2100 F). The wetting tests tk place in a vacuum f 2 10 5 mbar at the recmmended brazing temperatures. The fllwing methds f investigatin were applied: Auger electrn spectrscpy (AES) with sputter etching. Secndary in mass spectrmetry (SIMS). BNi2 0.06 6.5 3.0 4.5 2.7, 1010 1175 Filler metals BNi7 0.10 13.0 0.1 10.0 0.2 925 1065 BAu4 82.0 18.0 950 1005 (a) BTR = brazing temperature range in C with F equivalents as fllws: 925 C = 1697 F; 950 C = 1742 F; 1005 C = 1841 F; 1010 C = 1850 F; 1065 C = 1949 F; 1175 C = 2147 F; 0. x t 0 a. (A WELDING RESEARCH SUPPLEMENT 1295s
1 ik~^r; l.5 Nix3 jmr^^j^^ x0.6 e Cr x 0.6 jr^ww, 0.3 i i i i 0 10 20 nm 30 distance frm the surface Fig. 7AES c/eptfi prfile f nickelally base metal In purified assupplied cnditin 1 3 1 10 20 distance frm the surface nm 30 Fig. 2 AES depth prfile f nickelally base metal after vacuum annealing at 2 10~ s mbar and 1000 C (1832 F) fr 10 min. /.,.. fm*hi'**l*hi*l* r Scanning electrn micrscpy (SEM). Electrn (EMA). micrprbe analysis Thermgravimetry (TC) AKx) 6 8 10 12 14 energy, e\m0 2 Fig. 3 Surface auger spectrum f nickelally base metal after vacuum annealing at 2 10 5 mbar and 1000 C (1832 F) fr 10 min CD CD NickelAlly Base Metal The AES surface analysis f the nickelally base metal befre heat treatment is shwn in Fig. 1. T determine the depth prfile f nickel, bth highenergy (848 ev) and lwer energy (61 ev) auger lines were applied. The latter cincides clsely t that f aluminum. Mlybdenum and irn were nt determined. The "asreceived" ally was evidently carbncntaminated. The surface layer had a thickness f apprximately 2.5 nm (25 A"), and cnsisted nt nly f carbn, but als chrmium, nickel, aluminum, titanium, and xygen. After vacuum annealing fr 10 minutes (min) at 1000 C (1832 F) the nickelally base metal had an almst metallic shine. The AES (depth prfile f the sample) shwed an increase t apprximately 20 nm (200 A") in the thickness f the base metal's surface layer Fig. 2. The exterir layer area was free f nickel and chrmium; hwever, carbn cntaminatin as well as high aluminum and xygen cntents were recrded. An increase in titanium and carbn cncentratin was bserved in a lwer zne. The augerspectrum f the surface als shws a layer f aluminum xide at 1378 evfig. 3. After vacuum annealing fr 10 min at 1150 C (2100 F), the nickelally base metal sample had a clear metallic shine; and the AES depth prfile (Fig. 4) shwed 10 20 distance frm the surface nm 30 Fig 4AES depth prfile f nickelally base metal after vacuum annealing at 2 10~ s mbar and 1150''C (2100 F) fr 10 min 2%s CTBER 1983
a reductin f the base metal surface layer thickness t abut 5 nm. The layer nw cntaining nickel and chrmium shwed an increased titanium and carbn cntent. Cnspicuus is the distinct reductin f the xygen and aluminum cncentratin, evidenced by the lack f surface Al 2 0 3 after annealing at 1000 C (1832 F). Thermgravimetrical measurement (Fig. 5) f the nickelally base metal shwed that a distinct reductin in weight is the cnsequence f annealing in a vacuum f 2 10" 5 mbar at 1130 C (2066 F). The evapratin f AI23 at these temperatures and pressures is nt pssible. Therefre, the questin that must be cnsidered is this: Can the aluminumxide in the surface be reduced by the carbn f the base material, as is the case in Cr 2 C3 catings n stainless steel? Theretically, the reductin f AI23 by carbn crrespnding t the reactin 3 C + Al 2 0 3 = 2 Al 4 3 C is pssible. The free enthalpy A Gr f the chemical reactin is 12758 calries (cal.) fr a temperature f 1150 C (2100 F) and a pressure f 10 7 atm. A reductin t vlatile aluminum subxide by the reactin, 2 C 4 AI23 = Al 2 0 + 2 C is mre likely due t the lw carbn cntent. The A GT value fr this reactin at a temperature f 1150 F (2100 F) and a pressure f 10~ 7 atm. is 21465 cal. Even with a carbn activity f 0.001, the A Gj is still 8300 cal. T experimentally test this hypthesis, a mixture f aluminum xide (AI23) and carbn pwder was heated at 1150 C (2100 F) under a vacuum f 2 10" 5 mbar fr 1 hur (h) in a steel sheet cvered melting pt. The steel lid vaprized. The vaprizatin prduct culd be clearly identified as an aluminumxygen prduct with the aid f AES analysis. The absence f an AI23 surface layer CU cn a 20 40 60 time 1150 C 2NiCMTi 1812 C nickel base ally 80 mm 100 J 0,1 mg/cm 2 0.05 mg/cnr Fig. 5 Temperaturedependent weight alteratin f the surfaces f nickel and irnally base metals under annealing treatments in a 2 10~ 5 mbar vacuum n the nickelally base metal annealed in a vacuum at 1150 C, i.e., 2100 F (Fig. 4) can, therefre, be attributed t aluminumsubxide vlatilizatin due t carbn reductin. IrnAlly Base Metal The surface layer thickness f the irnally base metal was apprximately 2.5 nm when supplied in a purified cnditin. The AES depth prfile in Fig. 6 shws heavy carbn cntaminatin as well as nickel, irn, titanium, and xygen. The surface layer increased by abut 90 nm after heat treatment under vacuum at 1000 C (1832 F) fr 10 min. The AES depth prfile shwed a distinct increase in xygen, carbn, and titanium cncentratin in the exterir layers Fig. 7. The absence f irn and nickel in the, base metal surface layer was als demnstrated clearly in the augerspectrum Fig. 8. A SIMS analysis shwed that, after 10 min vacuum annealing at 1000 C (1832 F), the surface layer f the irnally base metal was cmpsed f titanium carbide, nitride, and xide Fig. 9. These were als nted by the light yellw clring f the sample. The high aluminum peak shwn in Fig. 9 was attributed t residual Al 2 0 3 plishing cmpund n the surface as well as the sensitivity f the SIMS analysis t this element. Vacuum annealing at 1150 C (2100 F) caused bvius grwth Fe x 1.3 Ni x 0.4 10 20 distance frm the surface Fig. 6 AES depth prfile f irnally base metal in purified cnditin nm 30 nm 150 distance frm the surface assupplied Fig. 7AES depth prfile f irnally base metal after vacuum annealing at 2 ICr 5 mbar and 1000 C (1832 F) fr 10 mm WELDING RESEARCH SUPPLEMENT I 297s
in the titanium cmpund surface layer f the irnbase ally Figure 10. This did nt ur n the nickelally base metal. In additin, heat treatment at 1150 C (2100 F) led t distinct diffusin f irn and nickel in the irnally base metal surface layer. The cnstant increase in weight f the irnally base metal with increased temperature, mainly due t the titanium reactin with residual gas f the vacuum, is demnstrated by the vacuum thermgravimetry results Fig. 5. The surface f the sample shws a dark yellw clring at 1150 C (2100 F). Wetting Tests Tests were cnducted t determine the wetting behavir f the nickel and irnally base metals fr high temperature brazing filler metals. T this end, filler metal catings were brazed n base materials with filler metals BAu4 and BNi7 at 1000 C (1832 F) and with BNi2 at 1040 C (1904 F). The brazing time was 15 min, and the pressure was belw 2 10~ 5 mbar. Satisfactry wetting f the materials with small wetting angles was bserved Fig. 11. Hwever, under these brazing cnditins, the nickelally base metal was cated with an aluminum xide layer, and the irnally base metal with a titanium nitride, titanium xide, titanium carbide layer. The base metal surface layers cracked at high temperatures, apparently because f the substantial difference in thermal expansin f the surface layer and the base metal. The filler metals culd thus energy, ev10 Fig. 8 Auger spectrum f the irnally base metal surface after vacuum annealing at 2 10~ s mbar and 1000 C (1832 F) fr 10 min penetrate int the cracks and wet the underlying base metals. The AES analysis shwed that the xide, nitride, and carbide layers then flated up thrugh the liquid filler metals. Almst the same auger spectrum was registered n the surface f the BAu4 filler metal after applicatin t the nickelally base metal (Fig. 11B) as was fund n the base metal after vacuum anneal fr 10 min at 1000 C, i.e., 1832 F (Fig. 3). It was mainly aluminum xide. After the wetting f the irnally base metal, the auger spectrum f the surface f the BAu4 filler metal (Fig. 11E) shwed the line pattern typical f apprpriately heattreated base materials Figs. 8 and 12. Titanium cmpunds prevailed, and gld frm the filler metal was absent. Metallgraphic investigatin f base metal surface layers that flated upward in liquid filler metal and their presence n the filler metal surface was carried ut by brazing bth the nickel and irnally base metals with a cating f BNi7 filler metal. T gain thicker surface layers, the base metals were systematically prexidized befre brazing under vacuum at 2 10 mbar and 1000 C (1832 F). This was dne by annealing the nickelally base metal fr 1 h at 1000 C (1832 F) in a vacuum f I 2 mbar and the irnally base metal fr 10 min at 1150 C (2100 F) in a vacuum f 10~ 4 mbar. Micrgraphs, in Fig. 13, f the BNi7 filler metal brazed n bth base metals shwed distinct upward flating f base metal surface layers and reduced wetting f the filler metal. The micrprbe analysis f the elements at the BNi7/nickelally base metal and BNi7/irnally base metal interfaces (Fig. 14) shwed distinct increases in the aluminum and xygen cncentratin (Fig. 14A) and in la J a "a. E a i ) cn 10 30 50 70 mss Fig. 9SIMS analysis f the irnally base metal surface after vacuum annealing at 2 1CT 5 and 1000 C (1832 F) fr 10 min 50 100 nm 150 distance frm the surface Fig. 10 AES depth prfile f the irnally base metal after vacuum annealing at 2 10 5 mbar and 1150 C (2100 F) fr 10 mm 298S CTBER 1983
5 0 _l I Crt C a. _j Fig. 11 Micrgraphs f varius filler metals after being brazed under vacuum fr 10 min at 2 10~ 5 mbar nt the surfaces f nickel and irnally base metals: A BNi7 n Nially base metal, 1000 C;B BAu4 n Nially base metal, 1000 C;C BNi2 n Nially base metal, 1040 C;D BNi7 n Feally base metal, 1000 C;EBAu4 n Feally base metal, 1000 C; FBNi2 n Feallybase metal, 1040 C(nte: land 1040 C equivalent t 1832 and 1904 F, respectively). 50 cr Crt cn (JN(E) de [ ^^ict c ' N fi Ti Ni. Crt 1 1 1 1 1 1 1 1 1 1 4 6 10 energy, ev 10 2 Fig. 12 Auger spectrum f BAu4 filler metal surface after being brazed under Irnally base metal at 2 ICr 5 mbar and 1000 C (1832 F) fr 10 min vacuum n _i r,/.. Fig. 13 BNi7 filler metal after being brazed under vacuum nt the surfaces f nickel and irnally base metals at 2 ICr 5 mbar and 1000 C (1832 F): A Feally base metal, 10 min, 200; B Feally base metal, 10 min, 500; CFeally base metal, 15 min, 200; DNially base metal, 10 min, 200; ENially base metal, 10 min, 500; FNially base metal, 15 min, 200 (reduced 68% n reprductin) _l LJ tr V) CC WELDING RESEARCH SUPPLEMENT 299s
BNi7 Nickel bse ally Irn base ally BNi7 Cr (500 cps) Fe M Ni (500cps) Ni (T cps) M (70 cps) Al (300 cps) M (200 cps) Fe (500 cps) Cr (300 cps) ^^ywsarwv Ti (100 cps) jv^v^v^^v^v 0 (300cps) Ti Cr Ti(300cps) Fig. 14 Micrprbe analysis n the ally element distributin fllwing the brazing under vacuum f BNi7 filler metal n tw base metals at 2 ICr mbar and 1000 C (1832 F) fr 10 mm: A Nially base metal; BFeally base metal the titanium cncentratin (Fig. 14B). The penetratin f BAu4 high temperature filler metals int the cracks f the surface layers f the nickel and irnally base metals during brazing, with subsequent wetting f the base metal, can als be bserved in the scanning electrn micrscpe graphs. Figure 15 shws a fissured surface structure, almst the same in bth filler metal and base metal, at the transitin zne between the braze cating and base metal. Summary During brazing in vacuum (2 10 5 mbar) f nickel allys cntaining titanium and aluminum, the frmatin f base metal surface layers is extremely temperaturedependent. A cnstant grwth in the thickness f the predminantly alumi Fig. 15 (right) SEM micrgraph f the transitin zne f BAu4 filler metal/base metal fllwing brazing under vacuum at 2 10~ 5 mbar and 1000 C (1832 F) fr 10 min: A Nially base metal, 500; BFeally base metal, 430 (A and ti reduced 53%, n reprductin) num xide surface layer was bserved at temperatures up t apprximately 1130 C (2066 F). Abve apprximately 1150 C (2100 F), the aluminum xide was reduced t aluminum subxide by the base metal carbn; the remaining layer cnsisted mainly f titanium cmpunds and was abut as thick as was the riginal layer befre heat treatment. A titaniumcntaining irnally base metal heattreated under the same vacuum cnditins shwed a cnstant increase in the thickness f its surface layer with increasing temperatures. The surface layer was predminantly cmpsed f titanium xide, nitrides, and carbides. Brazing tests in a 2 10 5 mbar vacuum were cnducted with BNi2, BNi7 and BAu4 filler metals at temperatures f 1000 and 1040 C (1832 and 1904 F). Results shwed that wetting f the base metal takes place despite surface layer frmatin. Due t the differing thermal expansins at brazing temperature, the xide, nitride and carbide surface layers n the base metal crack and allw the filler metal t penetrate and wet the base metal. The surface layers then start t flat up thrugh and n the liquid filler metals. Nevertheless, difficulties in filling small brazing clearances and nnmetallic inclusins in the brazed jint d reduce jint strength and shuld be taken int cnsideratin. Reference 1. Lugscheider, E., and Zhuang, H. Schweissen und Schneiden 34:490. 1982. 300s CTBER 1983