Basic Steps For Successful Brazing

1. Joint Design 2. Pre Cleaning
3. Fluxing The Parts 4. Assembly For Brazing
5. Heating The Joint And Applying The Filler Alloy 6. Cleaning The Brazed Joints

1. Joint Design: {Drawing} A Comparison Of The Different Joint Designs Used In Welding And Brazing Is Shown Below: The Most Common Type Of Joint Used In Brazing Is The Lap Joint In The Case Of Tubular Components. To Desing A Good Lap Joint, Two Criteria Should Be Considered: A. The Joint Gap B. The Degree Of Overlap. It Is This Two Parameters Determine The Ultimate Joint Strength, And Not The Properties Of The Filler Metal. The Joint Clearence Between Parts Should Not Be Too Tight Nor Shouls Be Too Loose. An Optimum Clearence Is About 0.4 Mm.

2. Pre Cleaning: All Grease, Rust Or Plain Dirt Must Be Throughly Removed Chemically Or Mechanically. Mecanical Removal Is Preferable Because The Surface Is Roughened, And Excellent Bonding Is Obtained. Oil And Grease Removal Is Best Carried Out Using A Solvent Degreasing Agent.

3. Fluxing The Parts: Apply Paste Flux With A Brush On Joint Surface And Filler Alloy Before Heating. This Will Prevent Oxidation Of Parts During Heating Resulting In Free Flow Of Brazing Filler Metal. A Flux Powder Should Be Mixed To A Creamy Consistency With Water And Few Dropes Of Detergent.

4. Assembly For Brazing: Parts Should Be Securely Held In Position (Proper Jig And Fixture) During Brazing.

5. Heating The Joint And Applying The Filler Alloy: When Heating A Joint For Brazing It Is Essential That It Is Slowly And Evenly Heated To The Brazing Temprature. Apply Brazing Alloy When The Flux Is Molten. Continue Heating Until The Molten Filler Alloy Smoothly Flows Around Joint Surface.

6. Cleaning The Brazed Joint: After Brazing Clean Flux Residues From Brazed Joint By Soaking And Then Brushing Under Hot Water. When Alloy Is Solidified The Joint Can Be Quenched In Water To Help Remove Flux Residues. Quenching Should Only Be Carried Out When It Will Not Damage The Properties Of The Parent Metal Or Cause Cracking Because Of Stresses Caused By The Thermal Shock.

Flux Properties Technical Considerations

  • Be capable of dissolving the oxides of the base metals on which it is used.
  • Retain low viscosity to permit its ready displacement from capillary gaps by brazing material.
  • Melt at lower temperature than the melting point of brazing material to be used with it.
  • Mix readily with water to form a smooth paste free from coarse crystals.
  • Retain its paste form for a reasonable time and be capable of being remixed if it dries out.
  • Wet the work to which it is applied as an aqueous paste.
  • Cover the work adequately while in the stage heating.
  • Remain on vertical surface when fused.
  • Have a reasonably sufficient life without tiring when brazing operation are prolonged.
  • Have a residues which are easy to remove.

Brazing Alloys And Suggested Fluxes Chart

Silver Brazing Alloys Bearing Cadmium

NOMINAL COMPOSITION % MELTING RANGE SUGGESTED FLUXES
Ag Cu Zn Cd - - - Solidus ⁰C Liquidus ⁰C  
50 15 16 19 - - - 620 640 Lomelt 50
45 15 16 24 - - - 607 618
43 16 20 21 - - - 615 620 Lomelt A
40 19 21 20 - - - 595 630
38 20 22 20 - - - 605 650 Lomelt 58
35 26 21 18 - - - 605 700
30 28 21 21 - - - 600 690
25 30 27.5 17.5 - - - 607 682 Lomelt B
25 35 26.5 13.5 - - - 605 745 Lomelt 55
20 40 25 15 - - - 605 765 Lomelt 58
17 41 26 16 - - - 620 760 Lomelt 59
12 50 31 7 - - - 620 825 Lomelt 60

Silver Brazing Alloys Cadmium Free With Tin

NOMINAL COMPOSITION % MELTING RANGE SUGGESTED FLUXES
Ag Cu Zn Sn - - - Solidus ⁰C Liquidus ⁰C  
56 22 17 5 - - - 618 652 Lomelt B
55 21 22 2 - - - 630 660
45 27 25 3 - - - 640 680 Lomelt 55
40 30 28 2 - - - 650 710 Lomelt 58
38 32 28 2 - - - 650 720 Lomelt A
34 36 27 3 - - - 630 730
30 36 32 2 - - - 665 755 Lomelt 59
25 40 33 2 - - - 680 760 Lomelt 60
18 50 30 2 - - - 720 790

Silver Brazing Alloys Cadmium Free

NOMINAL COMPOSITION % MELTING RANGE SUGGESTED FLUXES
Ag Cu Zn Sn - - - Solidus ⁰C Liquidus ⁰C  
50 34 16 - - - - 688 744 Lomelt B
45 30 25 - - - - 670 740 Lomelt 58
40 30 30 - - - - 675 725 Lomelt 55
35 32 33 - - - - 680 750
30 38 32 - - - - 680 765 Lomelt 59
25 41 34 - - - - 700 800 Lomelt 60
20 44 35.9 0.1 - - - 690 810
12 48 40 - - - - 800 830 Oxiflux 630

Silver Brazing Alloys For Tungsten Carbide Tipped Tools

NOMINAL COMPOSITION % MELTING RANGE SUGGESTED FLUXES
Ag Cu Zn Ni Mn Cd - Solidus ⁰C Liquidus ⁰C  
50 15.5 15.5 3 - 16 - 630 690 Oxiflux
50 20 28 2 - - - 660 750 Oxiflux 600
49 27.5 20.5 0.5 2.5 - - 670 690 Oxiflux 630
49 16 23 4.5 7.5 - - 625 705
40 30 28 2 - - - 670 779 Oxiflux 650
27 38 20 5.5 9.5 - - 680 830 Oxiflux 630
25 38 33 2 2 - - 707 801

Silver - Copper - Phosphorous Brazing Alloys

NOMINAL COMPOSITION % MELTING RANGE SUGGESTED FLUXES
Ag Cu p - - - - Solidus ⁰C Liquidus ⁰C  
18 75 7 - - - - 645 650 Lomelt A
15 80 5 - - - - 650 800
6 87 7 - - - - 643 718 Lomelt 58
5 89 6 - - - - 650 810
2 91 7 - - - - 643 788 Lomelt B
2 91.4 6.6 - - - - 643 824
1 92.5 6.5 - - - - 650 810 Lomelt 57
- 92.7 7.3 - - - - 710 820
- 93.8 6.2 - - - - 710 880  

Brass Brazing / Bronze Brazing Alloys

NOMINAL COMPOSITION % MELTING RANGE SUGGESTED FLUXES
Cu Zn Si Mn Sn Ni others Solidus ⁰C Liquidus ⁰C  
60 39.7 0.3 - - - - 890 900 Bronze Flux
60 39.6 0.2 0.2 - - - 900 915 Brass Flux
59 39.7 0.2 - 1 - - 888 899 Braze Flux
58 39.4 0.1 0.3 0.9 0.6 0.7fe 866 882
50 39.7 0.3 - - 10 - 890 920  
50 39.7 - 0.3 - 9 1 Ag 890 920 Braze Flux
57 39 - 2 - - 2co 890 930 Ht Flux
55 35 - 4 - 6 - 880 920

Flux Selection

While selec a flux for any particular application the following points need to be considered

Selected heating method Heating cycle time
Base metals Acceptable working temperature
Selected filler alloy Production on quantities
Clearance and joint design Flux removal

The most important technical property of fluxes is there ac ve range. This property is indicated by two temperatures : the lower one is the temperature at which flux starts to be ac ve, the upper one is the is the temperature at which the flux is exhausted and will not perform it's deoxidant and protective function.

It is good rule to choose flux in a such a way so that the lower temperature is at least 50°c lower than the solidus temperature (melt) of the brazing alloy to be used, and the upper temperature is at least 50°c higher than the liquidus temperature (flow) of the alloy. Many different type of fluxes are available, each with the different chemical composi on, ac vity temperature range and proper es, that may be used with different alloys, different range products and for different applica ons. for example when reference is made to EN 1045 FH10 or AWS TYPE FB3-A, many fluxes are given the same classifica on, yet they have different properties and characteristics.

The fact that fluxes are proprietary formula on o en causes problems if one wants to change from one Manufacturers' product to another's. Operators will say that the flux does not work as well. This could be the case, but in many cases what the operator is saying is that it works differently, or perhaps more likely that it reacts differently when heated. This is too expected, as each formula on will result in a flux with different characteris cs. What must be assessed is whether the difference is good, bad, or indifferent and whether the joints produced are of acceptable quality.

Rather than just selec ng one flux from JO FLUX® range, it could be well be worth tes ng two or three that appears to be suitable, to see which provides the best 'on-the-job 'performance.