Wetting and Joint Filling
4.3.4 Wide and Narrow Gap Brazing
Under normal circumstances, a brazed joint will naturally tend to be a few tens of microns (of the order of a mil) thick. Recommended joint clearances for brazing are generally in the region of 50 lm (2 mils) for reduced atmosphere braz-ing and closer to 100 lm (4 mils) for molten flux brazing (see Table 4.10). Sometimes it is necessary to create joints that are either signifi-cantly thinner or thicker. Thin joints benefit from capillary flow and may possess superi or me-chanical, physical, and aesthetic characteristics, although they are more vulnerable to brittle fail-ure when stressed, as pointed out in section 4.3.3.1 of this chapter. Thick joints tend to be encountered where:
● The mechanical tolerance of the components does not allow for joints to be consistently made narrower.
● Machining tolerances are not maintained within tight limits, usually in order to reduce production costs.
● Mating parts have surfa ces of different and possibly nonuniform curvature.
● The srcinal joint surfaces have been dressed by the physical removal of material, as in repair work.
Although wide gap joints can be filled with pre-forms or paste of the filler metal at room tem-perature, if the joint gap is too wide, then the molten filler will simply run out of the joint. The term “wide gap” brazing normally refers to
joints with a clearance of between 500 lm (20 mils) and 4 mm (160 mils), and “narrow gap”
brazing where the clearance is less than 30 lm (1 mil).
4.3
4.3.4. .4.1 1 Nar Narrow row Gap Gap Bra Brazin zingg
It is practicable to make brazed joints that are as thin as 1 to 2 lm (40–80 lin.), even in volume manufacturing. Joints of this thinness require that the braze is preapplied to one of the joint surfaces as a high-quality film. As pointed out in section 4.3.3.1 of this chapter, it is generally not possible to make thin brazed joints by simply using a narrow joint gap and hoping that the braze from a reservoir will be drawn in by cap-illary action and fill it. Copper brazing of mild steel is one of the few exception s. In thin brazed joints, molten fluxes interfere with wetting and spreading because the brazing fluxes tend to be fairly viscous. Similar ly, volatile speci es that are evolved during the heating cycle have trouble escaping from a narrow gap. Narrow joints are therefore best made using a self-fluxing braze, a gaseous flux, or fluxless (see Chapter 3, sections 3.3, 3.1 and 3.4, respectively).
Another method of making a thin joint is to carry out a conventional brazing operation with a standard amount of braze, either drawn in from one or more edges of the joint or inserted as a preform. Once the joint surfaces have been wet-ted by the molten filler, sufficient compressive stress is applied to overcome the hydrostatic pressure of the braze, and the surplus material is simply extruded from the joint gap. Physical stops can be used to control the final joint gap.
If the lower component has a larger area parallel to the joint than the upper one, lands can be pro-vided to catch the overspill in a controlled man-ner. The stress required to reliably force a molten
182 / Principles of Brazing 182 / Principles of Brazing
T
Table 4.11 able 4.11 RelatRelationshiionship between clep between clean room class designan room class design ation and airboation and airborne partirne particle sizecle size distribution
distribution
Federal standard 209F airborne particulate cleanliness classes
Class limits complexity of the jigging to apply the compres-sive stress and the possibility of damaging more delicate components.
A frequently overlooked consideration when attempting to make thin joints is the cleanliness of the components and particularly the environ-ment in which the jiggi ng prior to joinin g is con-ducted. When the desired joint gap is just a few microns (sub-mil) wide, there is no point jigging the components in a room where the airborne particles are larger! Therefore, the brazing pro-cess must be undertaken in a semiconductor-grade clean room, and great attention must be paid to the particulate content of all proces s gases, cleaning chemicals, and tools. Table 4.11 shows the correlation between the various classes of clean room and their particle size dis-tributions. Clearly, if the requirement is for a joint gap less than 5 lm (200 lin.), then a class M4 (class 100) or better clean room is required.
4.3
4.3.4. .4.2 2 Wi Wide de Gap Gap Bra Brazin zingg
When endeavoring to make particularly thick brazed joints (500 lm, or 20 mils), the prob-lem encountered is how to retain the braze in the joint gap. The traditional method of solving this problem has been practiced for generations in the jewelery and plumbing industries and for the manufacture of musical instruments. The ap-proach is to select a braze that has a wide melt-ing range and to conduct the joinmelt-ing operation below the liquidus temperature when the filler
alloy is in a pasty state. This process is carried out with commensurate skill by craftsmen in these trades. The presen ce of the solid phase drastically modifies the viscosity of the alloy and prevents it from flowing out of a wide joint gap.
However, brazes with wide melting ranges are generally effective only for joint clearances less than 1 mm (40 mils), althou gh applications such as lugless joining of tubular frames usually lie within this constraint.A number of strategies have been developed to address wide gap joining. Most of these com-prise mixtures of a conventional brazing alloy and a high-melting-point metal that provides
“body” for bridging a gap. The high-melting-point material may simply be powder of the par-ent material. Joints of up to several millimeters in width in Inconel components can be bridged by this means using nickel-phosphorus as the braze, and in titanium aluminides using copper as the filler metal [Gale et al. 2002]. An advan-tage of using the parent material in the joint gap is that it helps speed removal of low-melting-point components by diffusion (see Chapter 6 on diffusion brazing).
Some methods of wide joint gap brazing in-volve inserting into the joint gap, at the time of jigging the assembly, a solid structure such as a
plate, mesh, or honeycomb, which acts as a spacer that the filler alloy can wet. One of the more interesting types of such structures that is now available for this purpose is metal foam.
Nickel foams, in particular, are now commer-cially available with a wide variety of pore di-mensions and packing densities [Liu and Liang 2000]. Inserting thin parallel shims of copper,
Chapter 4: The Role of Materials in Defining Process Constraints / 183 Chapter 4: The Role of Materials in Defining Process Constraints / 183
for example, into the joint effectively partitions the joint gap into a series of much thinner joints and enables conventional joining methods to be employed. This approach is also used to reduce the effects of thermal expansion mismatch be-tween abutting components (see section 4.2.2 in this chapter).
One of the more common approaches to wide joint gap brazing is to use a composite filler
metal, comprising a regular brazing alloy, such as a copper- or nickel-base braze and spherical powder of a higher-melting-point component, usually called the “gap filler,” or “additive,” that does not substantially melt at the process tem-perature. This constituent can be appropriately selected to improve the mechanical properties of the braze [Chekunov 1996]. One example of the latter is provided by powder of Ni-20Cr alloys used in conjunction with nickel-base brazes. The two alloy fractions are introduced in the joint as a compact of mixed powders, held in a poly-meric binder [Yu and Lai 1995; Lim, Lee, and Lai 1995; Tung and Lim 1994]. Provided the high-melting-point metal is largely insoluble in the braze, the apparent viscosity of the filler metal can be altered independently of tempera-ture by varying the ratio of braze to gap filler material. Thus, by judicious selection of the gap filler propor tion and size distribution of the pow-der, it is possible to fill joints over 4 mm (160 mils) in width [Radsijewski 1992]. Other com-binations reported include iron powder in copper braze and iron-nicke l powder in copper-zi nc and copper-manganese brazes. Because the powder remains solid at the brazing temperature, the particle morphology, size, and size distribution all need to be controlled to get the desired bal-ance of wetting and spreading characteristics for the composite filler. The optimum proportion of the high-melting-point fraction is typically 30 to 40 vol% of the mixture. Investigations have shown that this balance achieves sufficient stiff-ness in the mixture to bridge the gap, while en-suring that there is sufficient fluidity to fill in-terstices within the joint.
Besides incomplete infiltration of the braze into the “gap filler” when carrying out joining operations using powder mixtures, sponges, or foams, defects such as porosity arise from shrinkage, and this problem grows as the joint width, depth, and volume fraction of the gap filler material increases. For satisfactory results, pressures of 5 to 10 MPa (700–1,400 psi) need to be applied to assemblies during the joining operation, which must be carried out in a vacuum or in a reducing atmosphere because
brazing flux residues only exacerbate the for-mation of porosity and other defects in the joint.
Shrinkage can be reduced by hot isostatic press-ing (HIPPpress-ing) the braze/“gap filler” compact into a dense preform prior to the brazing opera-tion. This approach, combined with the appli-cation of 1 MPa (145 psi) pressure during the brazing cycle, has proved effective in achieving fully filled joints when using a mixture of nickel-base filler metal (AMS 4777 AWS BNi-2) and up to 30 vol% nickel [Wu, et al. 2001]. A fast HIPPing cycle was used (10 min holding time). Also, the temperature in this pressing operation was kept below the solidus tempera-ture of the filler 988 C (1810 F), in order to prevent the melting depressants in the alloy, namely boron and silicon, from diffusing into the nickel “gap filler,” and so lose the melting point differential between the two constituents.
Infiltration of the molten braze into interstices is aided by high process temperatures provided that the fluidity per degree change in tempera-ture increases faster than does consumption of the braze by reaction with the “gap filler” ma-terial. Obtaining void-free joints when a “gap filler” is present becomes more difficult the wider is the joint since empty spaces (regions of lower packing density) are always the last to fill, owing to the reduced capillarity in these air- and vapor-filled regions [Tung and Lim 1995]. Often metalloids such as boron and silicon are in-cluded in the braze formulation as both melting point depressants and wetting promoters, as in Nicrobraz LC (Wall Colmonoy, UK) (Ni-14Cr-4.5Fe-4.5Si-3B). Where these metalloids are used, a relatively high superheat is required to disperse them and prevent formation of brittle intermetallic inclusions. The process is then akin to diffusion brazing (see Chapter 6). This re-stricts the use of wide gap brazes based on met-alloids to applications where high temperatures (950C, or 1740F) and extended cycle times can be tolerated, which considerably limits their applicability.
The reason that much of the work on wide gap joining processes pertains to nickel-base al-loys owes to the requirement to repair cracks that develop in aeroengine components. This has been satisfactorily addressed by the dual powder approach as a method for repairing defects that developed in service. The dressed crack will be of the order of 0.5 mm (20 mils) wide, which is too wide to be bridged by most conventional brazing alloys. Welding as a repair solution is also not usually a practical option because the nickel-base superalloys used in aeroengine hot
Chapter 4: The Role of Materials in Defining Process Constraints / 184 Chapter 4: The Role of Materials in Defining Process Constraints / 184
T
Table 4.13 able 4.13 CalcuCalculated crilated critical angltical angle for a e for a liquid to spontaliquid to spontaneouslneously infiltray infiltrate the interte the intersticestices in selecteds in selected close-packed structures, and the minimum packing density necessary to achieve filling even with close-packed structures, and the minimum packing density necessary to achieve filling even with perfect wetting
perfect wetting
Above the minimum packing density, spontaneous infiltration is relatively easy to achieve, even when the wetting is relatively poor.
Reinforcement type Reinforcement type
Critical wetting angle for Critical wetting angle for spontaneo
spontaneous us infiltrationinfiltration, , degreesdegrees
Minimum volume fraction
Minimum volume fraction of of reinforeinforcementrcement for
for spontaneouspontaneous s infiltrationinfiltration at 0
at 0contact angle, %contact angle, %
Unidirectional fibers 45 40
Body-centered cubic packed mono-sized spheres 65 20
Face-centered cubic/hexagonal close-packed mono-sized spheres 50 40
T
Table 4.12 able 4.12 Braze quBraze quality conality control map delitrol map delineatineating regiong regions of joint qualitns of joint qualit y as a function of the “gapy as a function of the “gap filler” content, brazing temperature, and gap width
filler” content, brazing temperature, and gap width
For In-625 nickel-base superalloy brazed with Nicrobraz LC (74Ni-14Cr-4.5Fe-4.5Si-3B) braze and 80Ni-20Cr “gap filler” powder Process
(a) Unsound joint containing microvoids
section components are difficult to weld, and their complex geometries are not compatible with this method.
The methodology that is usually followed is to pack the dressed crack with a mixture of “gap filler” and braze powder, with an additional sup-ply of the braze deposited outside of the joint gap. The gap filler in this case takes the form of spherical particulate with a mean diameter in the region of 75 lm (3 mils). Aeroengine parts are generally (fluxless) vacuum-brazed. The process requires some degree of skill to successfully im-plement, with minimum shrinkage voids and other joint defects, which would be highly del-eterious to the fatigue life of the repaired com-ponent. The process window with regard to gap filler fraction and joining temperature can be represented in a braze quality map, an example of which is given in Table 4.12 [Tung and Lim 1995, Lim, Lee and Lai 1995].
When contemplating using fiber or particu-late-reinforced brazes, one of the key targets is to obtain a void-free joint; otherwise, poor joint filling mitigates the strengthening effect. This end is greatly assisted when the infiltration of braze into the gap filler is promoted not only by metallurgical wetting of the braze, but surface tension forces are exploited to achieve sponta-neous infiltration into the interstices. This
situ-ation has been studied from a theoretical stand-point, albeit simplified and some of the key results are presented in Table 4.13. In summary, provided the wetting angle of the lower-melting-point braze to the reinforcement material (or metallization applied to it) is below 45 , then spontaneous infiltration should take place irre-spective of the aspect ratio of the reinforcement.
If the reinforcement medium (gap filler) is not closely packed, then the critical wetting angle decreases accordingly. The corollary is that un-less the minimum conditions given in Table 4.13 are achieved, the resulting joint will contain voids, unless external pressure is applied to force the molten metal into the interstices of the reinforcement material [Yang and Xi 1995].
The composite filler approach for wide-gap brazing has been successfully combined with diffusion brazing (otherwise referred to as tran-sient liquid-phase bonding) to produce well-filled and high-strength joints. This topic is dis-cussed in Chapter 6, section 6.4.