Flux-free brazing processes including direct electric resistance and induction heating processes that use protective-gas and local heating methods are being chosen over furnace brazing in many applications. Examples include brazing steel ring-piece pipe joints using resistance or induction heating, brazing Cr-Ni alloy steel fuel lines using resistance heating and brazing steel servo-steering parts in automated production processes. Advantages of flux-free brazing over traditional flame and protective-gas furnace processes include:
- Lower energy costs
- Brazing without using flux
- Easy integration of heating equipment into automated production processes
- Meeting tight tolerances in the brazing of bent pipes
Process description
An oxide layer present on a metal surface to be brazed must be removed to allow wetting of the surface by a brazing alloy. This can be accomplished by heating the metal, which causes the oxide layer to break up due to the different thermal expansions of the material and oxide. This process is material and temperature dependent; materials have a certain minimum temperature at which wetting will occur. The metal will not reoxidize if oxygen partial pressure is kept low enough. This level depends on the proportion of oxygen in the protective-gas used, on the quantity of isolated oxygen pockets around the components to be brazed, on preexisting oxides on the surface of the equipment (furnace and protective chamber) and on the brazing time. The lower the chamber pressure, the less oxide there is on the metal surface. The shorter the brazing time, the lower the oxygen content in the brazing-atmosphere and, thus, the lower the oxygen partial pressure. Local resistance and induction heating meets these conditions. This allows steel and Cr-Ni alloy steel to be well wetted without the need for flux using a variety of brazing alloys including copper, copper-silver, copper-silver-zinc, copper-manganese-nickel, copper-nickel, silver-manganese, nickel-chrome and iron-nickel-chrome.
Brazing steel pipes/tubes using steel ring-pieces
Ring-pieces made of DIN 7642 are used as connection elements for pipes/tubes (e.g., oil and fuel lines) used in automobiles and mechanical equipment. The ring-piece typically is joined using high-temperature brazing in a protective-gas, continuous type furnace. This requires fixing the joint position via spot welding and fluxing before brazing. Disadvantages of the process include total or partial loss of brazing paste and the inability to be sure whether enough brazing paste is available to completely fill the brazing gap. Thus, when brazed parts must meet higher safety standards, expensive alloy rings often are substituted for brazing paste.
Furnace brazing is most effective when brazing several locations on smaller assemblies. Protective-gas, local-heating methods are more effective when brazing fewer pieces and thin-walled parts. Thus, brazing of ring-piece pipe joints using resistance or induction heating is more economical.
In resistance heating, the brazing joint is mounted in the secondary circuit of a transformer, and a sufficiently large current is applied, which warms the joint. The heat source lies within the ring-piece pipe-joint, thus the process heats from within. This warming process is suitable for brazing of small, compact parts. A ring-piece joined to a steel pipe using a Cu-Sn alloy ring and resistance-heating is shown in Fig. 1. Figure 2 shows a resistance-brazing apparatus used to join ring-piece pipe joints for a 8 to 12 mm (0.3 to 0.5 in.) diameter pipe.
In induction heating, the inductor and component part are coupled without contact. The necessary working temperature for brazing is obtained in the outer layer of the component part through induced eddy currents. Induction heating is superior for brazing large, thin parts. Protective gases for both processes include argon, forming gas (90% nitrogen/10% hydrogen) and technical nitrogen.
The brazing device (Fig. 3) essentially consists of a protective-gas chamber, two part electrodes (one stationary and one mobile) and one in the height-adjustable pipe electrode. A height-adjustable device between the part electrodes picks up the ring-piece. The ring-piece is placed in the fixture together with a pipe previously fitted with an alloy-ring. To start the automatic brazing process, a part electrode is driven against the ring-piece, which closes the secondary circuit. At the proper temperature, the brazing alloy liquifies and flows through capillary action into the braze gap. One braze joint is produced every 41 seconds, and 28 seconds are required to bend the pipe and check the seal. After a 10-second hold under the protective-gas, the chamber is opened and the brazed piece is removed. An induction-brazing apparatus is shown in Fig. 4.
The ring piece is placed in the part fixture of the open brazing device (Fig. 5), and the brazed joint is completed for a pipe that is previously fitted with a Cu-Sn alloy ring. Closing the protective-gas chamber moves the inductor into brazing position and heating begins.
Using two brazing systems, a finishing-time of 20 seconds is achieved. The pause-time of the brazing personnel amounts to 15 seconds, in which the soundness of the brazed joint is tested, and the mounting of the alloy-ring is executed.
Brazing Cr-Ni steel fuel lines
A fuel-line assembly (Fig. 6), previously brazed by hand using a flame torch, consists of a part to be brazed and a bent pipe measuring 4 mm x 1.2 mm (0.15 x 0.05 in.). Flame brazing is difficult because there must be no blockage inside of the pipe (it must maintain the same flow capability) following brazing, which makes it necessary to use highly trained personnel.
Automatic flux-free brazing is initiated by closing the protective-gas chamber. Brazing requires 14 seconds, and the piece is removed after cooling for 10 seconds under the protective gas. Checking the assembly for braze quality and proper wall thickness requires 24 seconds.
Automatic brazing steel servo-steering parts
Servo-steering parts traditionally have been brazed in protective-gas, continuous-type furnaces with a low failure rate of brazed parts. Failures that occur generally are due to a partial loss of brazing alloy while in the oven.
Because the required annual quantity totals over 1-million pieces, the brazing production process must be automated. Mounting the alloy automatically and monitoring at the brazing temperature can reduce the failure rate. Both local heating processes are suitable for this application. In automatic resistance-heating brazing, the pipes and the ring-pieces are isolated, automatically mounted and brazed under a technical nitrogen gas. The accuracy of applying brazing wire is automatically examined by video inspection. Short and long pieces are alternately brazed with ring pieces after they have been automatically upset and collected opposite the brazing station.
An automatic wire-feed device feeds the brazing alloy. The wire feed is warmed and follows the natural action of the brazing alloy even before reaching the working temperature of the brazed joint. The alloy is fed out after reaching this temperature. As a result, the heated alloy melts immediately upon the second dispensing. This differs from traditional methods of applying cold wire, where the wire often is pushed past the brazing joint.
Following brazing, assembled components are transported by conveyor belt to the testing station, where a camera system checks the concave fillet and heated pipe area for flaws. Heating of the pipe area and the quality of the concave fillet are automatically tested. Parts detected having defects in the concave fillet exceeding 0.122 mm (0.005 in.) are removed from the process using a special procedure. The brazed assembly is turned four times. Flaws are displayed as black picture points, and the sum of these is used as a threshhold value. When the camera registers fewer than 60 black points (0.015 mm2) in a region, the program approves the detail picture. If the program counts more than 50 black points, a failure report is given. A part that passes inspection continues to the bending station.
Brazing in three brazing stations enables a joining-time of 15 seconds. During this time, the brazed assembly is upset, brazed, tested and bent.
For more information: For more information: Peter Salzberg is manager, Brazing, Wolf & Partner GmbH, Rudower Chaussee 29, 12489 Berlin, Germany. Contact Uwe Manzke, sales manager; tel: +49 30 6392 6110; fax: +49 30 6392 6350; e-mail: vertrieb@wp-berlin.de
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