Abstract:
A compound arrangement comprising a first component of metal being brazed to a second component of metal. The first component has an external cylindrical surface touching an cylindrical internal surface of the second component. The second component clasps the first component tightly, so that the second component exerts compressive stress on said external surface of the first component.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/994,559, filed on Nov. 27, 2001, now publication no. US 2002/0033056 A1, which is a continuation of application Ser. No. 09/618,068, filed on Jul. 17, 2000, now U.S. Pat. No. 6,352,196; which is a continuation of application Ser. No. 09/110,606, filed on Jul. 6, 1998, now U.S. Pat. No. 6,168,069, which is a nonprovisional of provisional application No. 60/056,285, filed on Sep. 3, 1997. 
    
    
     FIELD OF THE INVENTION 
     This invention deals with a novel use of silver-copper-palladium brazing alloys. 
     BACKGROUND OF THE INVENTION 
     Such brazing alloys are commercially available, cf. “Welding Journal, October 1990, pages 31 to 34, which describes, among many other brazing alloys whose ability to wet 316L steel is investigated, a 68Ag-27Cu-5Pd brazing alloy designated as “Palcusil 5”, a 58Ag-32Cu-10Pd brazing alloy designated as “Palcusil 10”, a 65Ag-20Cu-15Pd brazing alloy designated as “Palcusil 15”, and a 54Ag-21Cu-25Pd brazing alloy designated as “Palcusil 25”. 
     Since these silver-copper-palladium brazing alloys properly wet stainless steel, they can be used for brazing components made of this material. It is also possible, however, to braze components of titanium with these silver-copper-palladium brazing alloys. 
     SUMMARY OF THE INVENTION 
     When examining how to braze a component of titanium to a component of stainless steel, i.e. without first having to apply an intermediate layer of another metal to the steel, for instance nickel to 304L steel, cf. “Welding Journal, May 1991, page 112, the inventor first noted only that, if flat surfaces of the two components are brazed, the joint is brittle after having cooled down. 
     This is due to the rather different coefficients of thermal expansion of these two materials; the expansion coefficient of steel is quite a bit greater than that of titanium. 
     Surprisingly, however, silver-copper-palladium brazing alloys, which have hitherto been offered only for the brazing of components of the same material, are also very well suited for brazing titanium to stainless steel if, according to one feature of the invention, the second component, i.e., the component of stainless steel, clasps the first component, i.e., the component of titanium, tightly, so that the cold joint is under constant compressive stress. 
     Accordingly, a first variant of the invention consists in the use of silver-copper-palladium brazing alloys for brazing a first component of titanium to a second component of stainless steel which clasps the first component tightly. 
     A second variant of the invention provides a method for forming a compound arrangement by brazing a first component of titanium to a second component of stainless steel which clasps the first component tightly, using silver-copper-palladium brazing alloys, wherein
         the first component of titanium is provided with a cylindrical first end
           which has a smaller outside diameter than an adjacent main portion   whose external surface is, at least in part, a first surface to be brazed;   
           the second component is a cylindrical steel sleeve
           whose inside diameter is equal to the outside diameter of the main portion of the first component and   whose internal surface is, at least in part, a second surface to be brazed;   
           a silver-copper-palladium brazing alloy is placed around the first end of the first component;   the steel sleeve is slipped over the main portion of the first component; and   the first and second components and the silver-copper-palladium brazing alloy are heated in a vacuum or an inert gas until the silver-copper-palladium brazing alloy melts and wets the surfaces to be brazed, and are then allowed to cool down;
           whereby the compound arrangement is formed.   
               

     A first development of the second variant of the invention provides a method wherein
         the steel sleeve has an end projecting beyond the first end of the first component of titanium;   the first component has a tapped blind hole at the first end;   a tube of stainless steel which has an outside diameter equal to the inside diameter of the steel sleeve is provided at a first end with an external thread fitting the thread of the tapped blind hole; and   the projecting end of the steel sleeve is brazed to the tube.       

     A second development of the second variant of the invention, which can also be used together with the first development, provides a method wherein the main portion of the first component of titanium is provided with a collar remote from the first end, said collar being covered by and serving as a stop for the steel sleeve. 
     A third development of the second variant of the invention, which can also be used with the first development and/or the second development, provides a method wherein
         the first component of titanium is provided with an axial bore whose diameter is equal to the inside diameter of the tube of stainless steel;   a titanium tube whose outside diameter is virtually equal to the inside diameter of the tube is inserted into the tube and into the axial bore; and   the titanium tube is electrically welded to the first component in an inert-gas atmosphere.       

     In a preferred embodiment of the first or second variant of the invention, which can also be used with the above developments, a composition of 86.5 wt. % silver, 26.5 wt. % copper, and 5 wt. % palladium is used which is as free of residues as possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be explained in more detail with reference to the accompanying drawings, in which embodiments are shown schematically in the form of longitudinal sections, and in which like reference characters have been used to designate like parts. In a figure following a figure in which a reference character appeared for the first time, this reference character is not shown again. 
         FIG. 1  shows a compound arrangement formed according to the second variant of the invention; 
         FIG. 2  shows a compound arrangement formed according to the above first development; 
         FIG. 3  shows compound arrangement formed according to the above third development; 
         FIG. 4  shows compound arrangement formed according to the above second and third developments; and 
         FIG. 5  shows the use of the second variant of the invention in a single-tube Coriolis mass flow sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a compound arrangement  1  of a first component  11  of titanium and a second component of stainless steel in a sectional view. According to the second variant of the invention, compound arrangement  1  was formed by brazing with a silver-copper-palladium brazing alloy. 
     For this purpose, component  11  was provided with a cylindrical first end  111  which has a smaller outside diameter than an adjoining main portion  112 . The external surface  113  of the latter is, at least in part, a first surface to be brazed; in  FIG. 1  this is the entire external surface  113 . The main portion is followed, via a constriction  114 , by an integral flange  115 . 
     At its end  111 , component  11  is provided with a tapped blind hole  116  which extends into main portion  112 . From end  111 , component  11  was provided with an axial bore  117 ; its function and the functions of flange  115  and tapped blind hole  116  are explained below. 
     The second component is a cylindrical steel sleeve  12  whose inside diameter is equal to the outside diameter of main portion  112  of component  11 , and whose internal surface  123  is, at least in part, a second surface to be brazed; in  FIG. 1 , this is the surface touching external surface  113  of component  11 . 
     A first end  121  of steel sleeve  12  terminates at the beginning of constriction  114 , while a second end  122  projects beyond the end of component  11 . This is by no means mandatory: Steel sleeve  12  may also be flush with or recede from end  111 . 
     To form the compound arrangement, steel sleeve  12  is slipped over main portion  112  of component  11 , i.e., the outside diameter of the main portion is slightly less than the inside diameter of the steel sleeve, so that the latter can be easily slipped on. Thus, in this condition, steel sleeve  12  encloses component  11  without clasping it tightly for the time being. 
     After steel sleeve  12  has been slipped on, a silver-copper-palladium brazing alloy  13  is placed around the first end  111  of component  11 , as indicated by broken lines. The amount of brazing alloy  13  is chosen to be sufficient for brazing the two surfaces  113 ,  123 . Brazing alloy  13  may take the form of a prefabricated silver-copper-palladium wire, a corresponding ribbon, or a corresponding paste. 
     A silver-copper-palladium brazing alloy which has proved especially suitable is a composition of 68.5 wt. % silver, 26.5 wt. % copper, and 5 wt. % palladium which is as free of residues as possible. 
     The arrangement consisting of component  11 , steel sleeve  12 , and silver-copper-palladium brazing alloy  13  is then heated in a vacuum or an inert gas, since titanium oxidizes quickly when heated, until the brazing alloy melts and penetrates into the gap between the surfaces to be brazed and wets these surfaces as completely as possible. Then the arrangement is allowed to cool down, so that steel sleeve  12  clasps component  11  tightly. The formation of compound arrangement  11  is thus completed. 
       FIG. 2  shows a sectional view of a compound arrangement  1 ′ formed according to a development of the method explained with reference to  FIG. 1. A  tube  14  of stainless steel which was provided at a first end  141  with an external thread  142  fitting the thread  116  of the tapped blind hole was screwed into the blind hole. Tube  14  has an outside diameter equal to the inside diameter of steel sleeve  12 . The projecting end  122  of steel sleeve  12  was welded to tube  14 , as illustrated by a weld  143 . 
       FIG. 3  shows a sectional view of a compound arrangement  1 ″ formed according to another development of the method explained with reference to  FIGS. 1 and 2 . A titanium tube  15  whose outside diameter is virtually equal to the inside diameter of tube  13  was inserted into axial bore  117 . A first end  151  of titanium tube  15  was electrically welded at  153  to component  11  in an inert-gas atmosphere. 
       FIG. 4  shows a cross-sectional view of a compound arrangement  1 * formed according to still another development of the method explained with reference to  FIGS. 1  to  3 . Main portion  112  of component  11  of titanium was provided with a collar  118  remote from first end  111 . Collar  118  is covered by steel sleeve  12  and serves as a stop for the latter. To this end, steel sleeve  12  was provided with a recess  128  which fits collar  118 . 
       FIG. 5  shows a cross-sectional view of a single-tube Coriolis mass flow sensor  10  in which the second variant of the invention, shown in  FIGS. 1  to  4 , was used to advantage twice. Tube  14  of compound arrangement  1 * expands into a funnel-like end portion  144  having a greater diameter than tube  14 . 
     A compound arrangement  1 # which is symmetrical with respect to compound arrangement  1 * has a funnel-like end portion  144 ′. End portions  144 ,  144 ′ are permanently connected with one another by a support tube  16 , for example by being welded to the support tube all around. For this purpose, end portions  144 ,  144 ′ are so designed that support tube  16  can be slip-fitted to them and that the external surfaces of end portions  144 ,  144 ′ are flush with the external surface of support tube  16 . 
     The diameter of end portion  144 , which is greater than the diameter of tube  14 , is chosen so that the resulting hollow space can serve to mount an exciter assembly and sensors etc. on titantium tube  15 . These, as is well known, are necessary for a Coriolis mass flow sensor but have been omitted in  FIG. 5  for clarity. 
     By using the invention with a single-tube Coriolis massflow sensor, which, as is usual and as shown in  FIG. 5 , is provided with titanium tube  15  as a vibrating measuring tube, very good joints can be produced between support tube  16  of stainless steel and flange  115  of titanium and between titanium tube  15  and flange  115 . 
     These joints between titanium and titanium and between titantium and steel are necessary since both the junction between titanium tube  15  and (titanium) flange  115  and the junction between steel tube  14  and (titanium) flange  115  must remain tight under all operating conditions, particularly in case of changes in temperature. This is guaranteed, since the maximum permissible operating temperature of Coriolis mass flow sensor  10  is far below the temperature of the above-explained brazing. 
     By means of flange  115  and the corresponding flange  115 ′ at compound arrangement  1 #, the single-tube Coriolis massflow sensor  10  can be installed in a pipe conducting the fluid to be measured fluid-tight. 
     The invention can be used to particular advantage in a single-tube Coriolis mass flow sensor with a cantilever mass as is described in the prior U.S. Provisional Applications Ser. No. 60/032,906 filed Dec. 16, 1996, and Ser. No. 60/036,192 filed Jan. 21, 1997 as well as the corresponding U.S. Non-Provisional Application Ser. No. 08/940,644 filed Sep. 30, 1997 which are incorporated herein by reference.