Analysis of Diffusional Solidification in a Wide-Gap Brazing Powder Mixture Using Differential Scanning Calorimetry
- PDF / 5,382,195 Bytes
- 14 Pages / 593.972 x 792 pts Page_size
- 59 Downloads / 213 Views
TRANSIENT liquid-phase bonding or brazing (TLPB) is a technique commonly used to join Ni-based superalloys for use in aero engine applications.[1–4] During TLPB, a filler metal containing a melting point depressant (MPD) is used to join two base metal components. The most common MPD elements are boron and silicon. During brazing, the MPD diffuses into the base metal such that the joint undergoes diffusional solidification (DS). DS is used as a general definition of a reduction in the liquid fraction at the joint, due to the diffusion of the MPD from the liquid into the base metal. It would cover the phenomenon both during heating (i.e., non-isothermal conditions) and during an isothermal hold at the braze temperature. Isothermal solidification (IS) is defined as a subset of DS where the diffusion process takes place isothermally. Ideally, the DS process creates a solid solution c-Ni microstructure in the joint gap, producing properties similar to the base metal. If the
STEPHEN F. CORBIN, P.Eng, NSERC/Pratt and Whitney Canada Industrial Research Chair, is with the Department of Mechanical Engineering, Dalhousie University, 1360 Barrington Street, Box 15000, Halifax, NS, B3H 4R2 Canada. Contact e-mail: [email protected] D. CLARK MURRAY, formerly Research Associate with the Department of Civil and Resource Engineering, Dalhousie University, is now Materials Laboratory Manager with STELIA North America, Lunenberg, NS, Canada. ALAIN BOUTHILLIER, Manager, Fellow Special Processes, is with the Manufacturing Technology Development & Capital, Pratt and Whitney Canada, 1000 Marie-Victorin Blvd., Longueuil, QC, J4G 1A1 Canada. Manuscript submitted February 15, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
DS process is incomplete upon cooling from the braze temperature, the remaining liquid phase solidifies through binary or ternary eutectic processes which produce a range of brittle Ni or Cr intermetallic boride compounds. When borides form this way during solidification, they are present in a high volume fraction and distributed continuously through the joint. This resultant microstructure can have a significant negative impact on the mechanical properties of the joint.[5] The time required to achieve complete isothermal solidification scales with the square of the initial joint gap thickness or clearance Wo. Therefore, complete DS can be achieved through the use of a narrow joint gap. In practice, TLPB joint gaps are below 250 lm, with a value of 50 lm being the most common. For cases where joint gaps must exceed 250 lm, an alternative process known as wide-gap brazing (WGB) has been developed.[5] In this process, the joint gap is filled with a combination of filler metal and base metal additive, both usually in the form of powders. The primary functions of the base metal additive powder is to increase the volume of material within the joint’s wide gap without adding excessive filler metal material and to act as a diffusive ‘‘sink’’ for boron and silicon thus increasing the rate of DS and provide capillary pressure to draw molten
Data Loading...
