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Brazing - a guide to best practice Section 2. Base/parent material combinations Introduction Filler metal selection factors Table 1: guide to brazing Table 2: base metal/filler metal combinations Table 3: selection of fillers/base materials/brazing processes Introduction Almost every metal or alloy can be joined by brazing. In the past few decades, brazing has also been used to join ceramic materials. This guide does not cover ceramic materials, although an overview of the available braze filler metals is given in Section 3. A number of factors must be considered when matching base material to filler metal and joining process. These include the effect of the brazing cycle on the base metal and the final joint strength required, since the brazed joint may be required to withstand some later treatment, for example quenching, solution treatment, austenitising. Filler metal selection factors When selecting the filler metal, it must be compatible with the base metal, joint clearance and brazing procedures to be used. The properties of the filler metal in the joint after brazing and heat treating must also be taken into consideration. Some of the more important factors are: filler metal melting temperature strength at service temperature fracture toughness corrosion resistance fatigue properties electrical properties thermal conductivity Table 1 shows the suitability of brazing for a range of base materials and Table 2 shows the appropriate generic combinations of base and filler metals for brazing. For a more detailed approach to which braze filler materials should be used, dependent on both base metal and process, Table 3 should be used as reference. Table 1 Guide to brazing Parent Material Brazing Suitability Notes Metals     Al alloys 1/3 Aluminium fillers essential with appropriate flux. Cr 2 Chromium plated surfaces are usually damaged by brazing. Special fluxes or hydrogen atmosphere required. Cu 1 Flux may not be required. Copper-aluminium 2 Special flux essential. Copper-beryllium 1 Special flux essential above 2.2%Be. Copper-cadmium 1   Copper-chromium 1 Heat treatment poses problems. Copper-nickel 2 Initial stress relief may be necessary to avoid stress cracking. Copper-tin 1   Copper-zinc 1   Copper-zinc-nickel 1   Au 1   Fe - cast 2 Difficult to wet grey iron. Surfaces require special cleaning or use of special fillers. Fe - malleable 2 Fe - wrought 1   Mg 3   Ni 2 Inter-crystalline failure possible unless initial stress relief applied. Nickel-copper 2 Nickel-chromium 2 Nickel-chromium-iron 2 Pt 1   Ag and Ag alloys 1   Steel - mild 1   Steel - stainless 2 Special flux and silver alloy fillers may be required to avoid interfacial corrosion. Stabilised steel recommended. Ti and Ti alloys 2 Special filler alloys required. Inert atmospheres or vacuum brazing recommended. W 2 Special filler alloys and fluxes. Zn 3 Soft soldering only. Zr 2 Special fillers. Inert atmospheres or vacuum recommended. Non-metallic materials     Carbon (diamond) 2 Direct brazing possible only with active filler alloys (containing Ti, Zr, etc). Metallised surfaces can be brazed with Cu or Cu-Ag eutectic fillers. Ceramics 2 Tungsten carbide 2 Silver alloy fillers with Ni or Mn to improve wetting recommended. 1=Brazing satisfactory. 2=Special precautions for good results. 3=Brazing with silver-bearing, copper or noble metal alloys impracticable or not recommended. Table 2 Base metal-filler metal combinations   Al & Al alloys Mg & Mg alloys Cu & Cu alloys Carbon & low alloy steels Cast iron Stain less steel Ni & Ni alloys Ti & Ti alloys Be, Zr, V, & alloys Reac tive metals W, Mo, Ta, Co & alloys Refra ctory metals Tool steels Al & Al alloys AL 1 1 1 1 AL AL AL 2 1 1 Mg & Mg alloys 1 * 1 1 1 1 1 1 1 1 1 Cu & Cu alloys 1 1 AG, AU, CP, HTN, CZ AG, AU, CZ, HTN AG, AU, CZ, HTN AG, AU, HTN AG, AU, CZ AG* AG AG AG, AU, CZ, HTN Carbon & low alloy steels 1 1 AG, AU, CZ, HTN AG, AU, CU, CZ, HTN AG, CZ, HTN AG, AU, CU, CZ, HTN AG, AU, CU, CZ, HTN AG* AG AG, CU, HTN AG, AU, CU, CZ Cast iron 1 1 AG, AU, HTN, CZ AG, CZ, HTN AG, CZ, HTN AG, AU, CU, CZ, HTN AG, CU, CZ, HTN AG* AG AG, CU, HTN AG, AU, CZ, HTN Stainless steel AL 1 AG, AU, HTN AG, AU, CU, CZ, HTN AG, AU, CU, CZ, HTN AG, AU, CU, HTN AG, AU, CU, HTN AG* AG* AG*, AU, CU, HTN CU, AG, AU, HTN Ni & Ni alloys AL 1 AG, AU, HTN AG, AU, CU, CZ, HTN AG, CU, CZ, HTN AG, AU, CU, HTN AG, AU, CU, HTN AG* AG* AG, AU, CU, HTN AG, AU, CU, CZ, HTN Ti & Ti alloys AL 1 AG* AG* AG* AG* AG* AG, AL* 2 2 1 Be, Zr, V & alloys (Reactive metals) 2 1 AG AG AG AG* AG* 2 2 2 1 W, Mo, Ta, Co & alloys (Refractory metals) 1 1 AG AG, CU, HTN AG, CU, HTN AG*, AU, CU, HTN AG, AU, CU, HTN 2 2 2 1 Tool steels 1 1 AG, AU, CZ, HTN AG, ZU, CU, CZ AG, AU, CZ, HTN AG, AU, CU, HTN AG, AU, CU, CZ, HTN 1 1 1 AG, AU, CU, HTN, CZ AL = Aluminium based filler metals 1 - Not recommended; however, special techniques may be practicable for certain dissimilar metal combinations. AG = Silver based filler metals 2 - Generalisations on these combinations cannot be made. AU = Gold based filler metals * - Special brazing filler metals are available and are used successfully for specific metal combinations. CP = Copper-Phosphorus based filler metals CZ = Copper-Zinc based filler metals HTN = Nickel based filler metals CU = Copper based filler metals Table 3 Selection of fillers and base materials for torch (T), induction (I) and furnace (F) brazing Filler material Parent materials Cu and Cu base alloys Mild steel Alloy steels Stainless steels and irons Malleable and wrought irons Cemented carbides Ni base alloys Cu   IpF IpF IpF IpF IpF I F Ag T I F TpFp TpFp Fp Fp IpFp IpFp CZ TpIpFp T I F T I F T IpFp T I F T I Fp T IpFp CP, Ag-Cu-P T IpF             Ag-Cu T I F TpI F TpI F I F IpFp IpFp I F Ag-Cu-Zn T I F T I F T I F T IpFp TpI F TpI Fp T IpFp Ag-Cu-Sn TpIpF TpIpF IpFp TpIpF IpFp IpFp IpFp Ag-Cu-Zn-Cd-Ni T I F T I F T I F T IpFp T IpFp T I Fp T IpFp Ag-Cu-Zn-Cd T I F T I F T I F T IpFp T IpFp T IpFp T IpFp Co-Cr-B   TpIpFp TpIpFp TpIpF IpFp   TpIpF Ni-Cr(+B+Si)   T I F T I F T I F IpFp TpIpFp T I F Ni-P       IpF     IpF Ag-Pd-Mn   T I F T I F T I F TpI F TpIpFp T I F Ni-Pd-Mn   T I F T I F T I F TpIpFp TpIpFp T I F Cu-Pd-Ni-Mn   TpIpF TpIpFp T I F TpIpFp TpI F T I F Pd-Ni   TpIpF TpIpFp TpI F TpFp TpIpFp TpI F Cu-Ni   IpF IpF IpF IpF IpFp IpFp Cu-Pd   T I F T I F T I F T I F T I F T I F Ag-Pd   T I F TpIpFp TpIpF TpIpFp IpFp TpF Ag-Cu-Pd T I F T I F T I F T I F TpI F TpI F T I F Au-Ni TpI F TpIpF TpIpFp TpI F TpIpFp TpIpFp TpI F Au-Cu T I F TpIpF TpIpFp TpI F TpIpFp TpIpFp   Table 3 (cont'd) Selection of fillers and base materials for torch (T), induction (I) and furnace (F) brazing Filler material Parent materials Co base alloys W Mo Ti Zr Ta Be Nb Cu IpFp         Fp     Ag Fp     IpFp Fp   IpFp   CZ Tp TpIp TpIp           CP, Ag-Cu-P   TpIp TpIp           Ag-Cu IpFp Tp Tp IpFp IpFp Fp IpFp   Ag-Cu-Zn TpIpFp T I Fp T IpFp Ip Ip       Ag-Cu-Sn IpFp TpIpFp TpIpFp IpFp Fp IpFp     Ag-Cu-Zn-Cd-Ni TpIpFp T I Fp T I Ip         Ag-Cu-Zn-Cd TpIpFp T I Fp T I           Co-Cr-B TpIpF               Ni-Cr(+B+Si) TpIpFp TpI F TpI F   Fp Fp   Fp Ni-P         Ip       Ag-Pd-Mn TpIpFp TpIpF TpI F   IpFp IpFp IpFp IpFp Ni-Pd-Mn TpIpFp TpIpF TpIpF   IpFp IpFp   IpFp Cu-Pd-Ni-Mn TpIpFp TpIpF TpIpF   IpFp IpFp   IpFp Pd-Ni TpIpFp I F I F   IpFp I F   IpFp Cu-Ni TpIpFp Fp Fp Fp IpFp IpFp   Fp Cu-Pd TpIpFp IpFp I F Fp IpFp IpFp IpFp IpFp Ag-Pd TpIpFp TpIpFp TpIpFp   IpFp IpFp IpFp IpFp Ag-Cu-Pd TpFp TpI F TpI F I F IpFp IpFp IpF IpFp Au-Ni TpIpFp TpI F TpI F IpFp IpFp IpF IpFp IpFp Au-Cu TpIpFp TpIpFp TpIpFp IpFp IpFp IpFp IpFp IpFp AL, Al-Si-Cu       TpIpFp Fp       p = Possible but not standard industrial practice. Note: Al and Al alloys are only brazed with AL based filler materials.