Resistance Welding a Niobium Strip to a Nickel Block Dan Sorensen 4 December, 2013

Background and Motivation

• The medical device industry has requirements not seen in other industries: – Small part and component sizes – Relatively short list of biocompatible materials (Ti and its

alloys, Ta, Nb, Hf, NiTi, Pt and Pt alloys, Co-Ni-Cr-Mo, Au, austenitic stainless steel, bulk metallic glasses?)

– High reliability requirements (at least 6 sigma) – MRI compatibility (no ferromagnetic materials) – Welding dissimilar metals is a common need or

requirement. – Very often limited data exists in these exotic systems.

Welding Techniques Common to the Medical Device Industry

• Laser welding – Excellent for welding similar metals (or compatible

dissimilar metals) to create strong hermetic joints. – Welding reactive materials requires welding be

performed in an inert atmosphere (Ar or He) – Expensive equipment, tight joint design tolerances,

complete melting and mixing of work pieces.

• Ultrasonic welding – Excellent for solid state welding of thin foils and sheet. – Common in battery manufacturing.

• Percussive Arc Welding – Allows fine wires to be butt welded together. – Difficult to fixture

• Resistance welding (Parallel Gap) – Solid state joining process that uses

plastic deformation and contact resistance to make a metallurgical bond.

– Often used when only top of work pieces are accessible to the welding electrodes.

– Process results in two fusion zones. – The work pieces may not melt and a

solid state bond is often formed.

Welding Techniques Common to the Medical Device Industry

www.microjoining.com

Application of Computational Tools

• The small size of the system we are joining and the uncommon dissimilar material pair (Ni-Nb) requires modeling to understand the forces, temperature profile, and phase transformation that occur when resistance welding.

• The parts were first modeled using Pro-Engineer (CAD program) and imported into Abaqus.

• Abaqus will be used to model the deformation and temperature profile that occurs during the resistance welding process.

• Following the thermo-mechanical analysis Thermo-Calc will be used to model the phases present in the Ni-Nb fusion zone and their approximate phase fractions.

Modeled Geometry and Materials • Ni 200 Block

– 1.3mm X 1.3mm X 0.89mm – ¾ Hard condition – Mechanical properties taken from mechanical tests on bulk samples, converted to true

stress – true strain. – Thermophysical properties obtained from Handbook of Thermophysical Properties of

Metals at High Temperatures p364. • Nb Grade 1

– 2.63mm X 0.25mm X 0.2 mm – Fully Annealed – Mechanical properties from bulk samples, converted to true stress – true strain. – Thermophysical properties obtained from Handbook of Thermophysical Properties of

Metals at High Temperatures p276. • W - Cu Electrodes

– All properties taken from www.brouwermetaal.com. – W – Cu was taken to be elastic in the displacement range of the electrodes so no

plasticity data was used.

Literature Review Part 1: The Ni/Nb System • Very little literature exists regarding the Ni-Nb system.

– Most papers focus on novel Ni-Nb metallic glasses [1] or Nb as a small alloying addition to Ni based super alloys [2].

• Only one paper could be found that investigated the Ni-Nb system and focused mostly on high temperature sulfidation of the alloy [3]. – Microstructures of the cast alloys were reported however.

As-cast microstructure of Ni-40Nb [3]

[1] Lee PY and Koch CC. Journal of Non-Crystalline Solids. 94:1 (1987) 88-100

[2]Sundararaman M, Mukhopadhyay P, Banerjee S. Metallurgical Transactions A. 19:3, (1988) 453-465

[3] Chen MF, Douglass DL, Gesmundo F. Oxidation of Metals, 31:3/4 (1989) 237-263

Literature Review Part 2: Resistance Weld Modeling • The earliest work on modeling resistance welds was

performed by Nied [1] who modeled opposed electrode resistance spot welds on 321 stainless steel sheets. His results showed reasonable agreement with experiments related to nugget size.

• Further work was performed using a coupled thermal-electrical-mechanical approach by Khan, Xu, and Chao [2]

• More recently Yang et al. created resistance weld models predicting residual stresses and resulting mechanical properties of the weld zone. [3]

[1] H. A. Nied: Weld. J., 1984, 63, 123s–132s.

[2] J. Khan, L. Xu and Y. J. Chao: Sci. Technol. Weld. Join., 1999, 4, 201–207.

[3] Yang YP, Babu SS, Orth F, Peterson W. Sci. Technol. Weld. Join., 2008, 13, 232-239

Literature Review Part 3: Micro Resistance Welding Dissimilar Metals

• Although micro resistance welding is a commonly utilized technique in the micro electronics industry, very little peer reviewed literature could be found.

• Examples include micro resistance spot welding Kovar, steel, and Nickel [1], fine Nickel wires [2], and electrode sticking of thin steel sheets [3].

• These papers, while interesting, are of limited use for the uncommon dissimilar metal system found in the current analysis.

• No papers on modeling micro resistance welding could be found, most of the data was emperical.

[1] Ely KJ, Zhou Y. Science and Technology of Welding and Joining 2001 Vol. 6 No. 2

[2] Fukumoto S, Zhou Y. Metall Mater Trans. 35A, October 2004 pg 3165

[3] Dong SJ, Kelkar GP, Zhou Y. IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 25, NO. 4, OCTOBER 2002

Physics Involved in Resistance Welding – Application of a Mechanical Load

The first step involves bringing the electrodes and work pieces into contact. • In the case of this weld instead of a load of 27 N (6 lbf) is used. • The displacement of the electrodes has been experimentally measured to

be 50 μm in the z direction. • This sustained load helps to bring the work pieces together by breaking

the native oxide layers and compressing asperities down. • Displacement was added as a boundary condition at the end of the

electrodes. • The weld time was approximated as a 1.6 ms duration pulse. No time

ramp was used. • The conductivity (both electrical and thermal) as a function of distance was

unknown. I set the interactions as follows: – Thermal conductance (conductance, clearance) (0.08, 0), (0,0.001) – Electrical conductance (conductance, clearance) (8000, 0), (0, 0.001)

Physics Involved in Resistance Welding – Electrothermal Processes

• The second step involves a current being passed through one of the electrodes into the work pieces.

• Resistance between the work pieces and electrodes when a current is passed over a period of time causes heating at the interfaces.

𝑄 = 𝐼2𝑅𝑅 – Where Q is the heat generated, I is the current, R is the

contact resistance, and t is the time current is applied.

• The heating coupled with plastic deformation may result in a solid state weld.

• This is likely the case when high melting temperature materials are joined such as Ni and Nb

Assumptions Made for this Analysis • Mechanical

– The mechanical properties stay constant throughout the welding cycle. Only room temperature elastic and mechanical properties where used for inputs.

– The electrode materials are purely elastic. This is reasonable considering their high elastic modulus and low strains they experience.

• Electrothermal – The contact resistance between the electrodes, Ni block,

and Nb ribbon stays constant upon loading and heating.

Boundary Conditions, Interactions, and Loads Used for this Analysis

• The area of the top of the electrode was measured and a current density was calculated and used.

• In this case the current used for this welding process is 500A and the area of the electrode is 3.175 mm2 (50 A/mm2)

• A second boundary condition on the top of the second electrode was 0 electric flux.

Preliminary Results – Mechanical Loading Step Von Mises Stress

The initial model shows stresses on the order of 200 MPa under the electrodes. This seems reasonable based on the contact area and Nb ribbon strength.

Preliminary Results – Mechanical Loading Step PEEQ

The equivalent plastic strain results are also reasonable. Cross sections of these resistance welds show similar Nb ribbon deformation geometries

Preliminary Results – Temperature Profile

While the temperature profile appears reasonable, the magnitude of the temperature is off by a factor of 3-5. This is likely an artifact of the interface conductivity inputs.

Preliminary Results – Temperature Profile

While the temperature is not correct there is elevated temperature where the weld pools are formed, a positive sign.

Preliminary Results: The Nickel – Niobium System

Nb

Ni

Fusion Zone

• FIB/SEM, EBSD, and EDS show the fusion zone is not BCC Nb or FCC Ni.

• No recrystallization or grain growth was observed experimentally.

• Nanoindentation shows the fusion zone to be locally harder than the Ni and Nb base materials

Experimental Validation

• The small size of the work pieces makes experimental validation of the model difficult.

• It may be possible to fixture thermocouples in the Ni block. This will be expensive, time consuming, and difficult to validate.

• Cross sections can be used to measure the actual geometry of the deformed Nb ribbon and compare it with the model.

• The cross sections can also be used to indirectly verify the temperature profile in the weld. – TEM can be used to identify phases present. – EBSD can be used to inspect the microstructure for grain size

variation and strain relief in the fusion and heat affected zone.

Further Work and Refinements to the Model • The main drawback of the current model is the

assumption that the contact resistance is constant over all temperatures. This is not correct. – Professor Victor Li has proposed an improved method for

modeling contact resistance as a function of temperature [1].

• A layer of elements on the surface of the Ni block needs to be added. This will add an extra layer of complexity to the models. – Thermo-Calc does not have a large database for the Ni-Nb-Au

system and all thermochemical calculations would need to be experimentally validated using TEM.

• A large validation effort will be required before this model can be used in the heavily regulated medical device industry.

[1] M. V. Li, P. Dong, and M. Kimchi, "A Contact Resistance Model for Resistance Spot Welding Processes: Theoretical Formulation and Numerical Implementation," Proc. Int. Conf. Advances in Welding Technology, September 17-19, 1997, Columbus, OH, EWI/AWS/NJC/SME/TWI, pp. 357-369.

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