Metallurgical Investigation of Battery Frame Components: A Deep Dive into Sample Preparation & Analysis

Dr. Evans Mogire

As the push for electrification accelerates, the structural integrity and reliability of battery frames in electric vehicles (EVs) become a focal point of engineering excellence. These frames, comprising various metallic components such as battery enclosures, busbars, module lids, and cell lattices, must withstand mechanical stresses, corrosion, and thermal cycling. Ensuring their robustness demands meticulous metallurgical investigation.

This article explores the rigorous sample preparation and metallurgical analysis processes involved in examining battery frame components. From sectioning to etching, every step plays a crucial role in detecting weld quality, voids, and defects that could compromise performance and safety.

Sample Preparation: The Foundation of Accurate Metallurgical Analysis

To perform precise metallurgical evaluation, representative samples should be extracted from different battery frame components. The number of sections per component varies, considering the complexity of manufacturing, and how critical they are to the performance and integrity of the frames:

• Battery Electric Vehicle (BEV) or Plug-in Hybrid Electric Vehicle (PHEV) Frame; can have multiple welded components that should be analysed. Depending on the frame type, a considerable number of weld sections need to be extracted for metallurgical analysis. BEV frames due to their size, and preference of welded aluminium profiles for flexibility in design, have more sections for analysis compared to a PHEV battery frame. PHEV frames can also be designed from cast aluminium due to their smaller size in general.

• Busbars: These connect modules to form the overall electrical network in the pack and are typically made of copper or aluminium coated with protective layers. These present different challenges attributed to joint reliability, and ability to resist fatigue. They do require metallurgical evaluation.
• Cell Lattice: A key design area of a battery frame responsible for enhancing frame stiffness and ensures structural integrity and are responsible for crash energy absorption. They are key to load path distribution within the frames, and as such, require metallurgical inspection.

For steady-state production monitoring, at least five of each component categories should be analysed on a weekly basis to ensure ongoing quality assurance. To achieve this, a sequence of metallographic preparation steps should be put in place with the correct equipment and related methodologies to ensure samples are true and representative of the component microstructures.

Sectioning – extract specimens with minimal damage

Challenges in Sectioning

• Battery frames (typically aluminium or steel) require low-deformation cutting to avoid sectioning damage or cause heat-affected zones on areas of interest.
• Battery busbars (copper or aluminium) are prone to smearing and edge rounding if improper cutting techniques are used, such as excessive feed rates on automatic sectioning equipment or heavy handedness during manual sectioning.
• Multi-material interfaces (e.g., Al-Cu, Al-Ni joints) require careful sectioning to preserve joint integrity without distortion or damage.

Recommended Cutting Methods

a. Band Saw Sectioning:

• For large battery frames, a bandsaw can be used to re-size the larger frame to a medium sized component that can fit into large sectioning equipment. A full frame cannot be fitted into an abrasive sectioning machine. Initial sectioning plane should have minimal impact on the critical areas of interest for frame analysis.

b. Abrasive Sectioning:

• Use diamond or alumina-based blades for sectioning battery frames, the component size after band-saw sectioning, will have a bearing on the sectioning machine (AbrasiMet XL pro or AbrasiMet L Pro) and associated blades to be used. AbrasiMet M is ideal for quick manual sectioning of smaller components.
• During sectioning, ensure machine “floods” enough coolant to the cutting point to minimise thermal damage that can occur during sectioning. This is highly localised, however, if sectioning is done aggressively or with low coolant flow, heat build-up causes microstructural changes.
• As a general guide, abrasive sectioning is recommended for large sectioning of components such as a battery frames.

c. Precision Sectioning:

• This is more suited for delicate joints/weld section as seen in busbars (Al-Cu, Al-Ni).
• A key benefit of using precision sectioning equipment is little to no thermal damage, less materials loss or mechanical deformation of components. Use of precision cutters does prevent sectioning induced-strain due to controlled feed rates and less sectioning forces being applied to components or test pieces.

Mounting – protecting your sample

Encapsulating sectioned components protects small or delicate test pieces, such as, busbars and tab weld on cell casings. This ensures that the sample is protected by preventing edge rounding, filling pores/cracks with resin providing additional strength. Mounting samples does results in uniform specimen sizes for ease of semi-automatic preparation in a grinder polisher but also ensuring consistent ease of handling during microscopic analysis.

Recommended Mounting Methods

a. Cold Mounting (Epoxy Resins):

• Ideal for battery busbars and delicate interconnects.
• Minimizes shrinkage, preventing gaps at metal interfaces.
• For specimens with voids/cracks, and intricate geometries, using casting resins ensures proper impregnation. This is aided using vacuum/pressure system (SimpliVac).

b. Hot Compression Mounting (PhenoCure, TransOptic):

• Best for battery frames that require additional edge protection, and where high compression forces in the mounting press (SimpliMet 4000) will not affect specimen integrity.
• The technique is fast curing but can introduce minor thermal stresses as the resins used in compression require elevated temperature (150-180°C) and pressure to form mounts.

Grinding and Polishing – Achieving a pristine surface

Grinding is carried out using silicon carbide (SiC) papers for several reasons; materials making up battery frames, busbars, cell interconnects consisting of various dissimilar materials that have different abrasions rates attributed to their hardness. SiC paper is preferred, as it causes less sub-surface damage to these materials. Additionally, depending on the sectioning method used, it is recommended to start with finest grit possible that also ensures good material removal.

Recommendations on grinding

• Uses silicon carbide (SiC) paper.
• Start with coarse grit (240-320) to remove sectioning damage – depends on sectioning method.
• Progressively fine grits (600-1200) to refine the surface for polishing stages.
• Lubricate (using water/water-based coolants) to prevent heat buildup.

Polishing stages are carried out using diamond suspension on a no napped surface (VerduTex, Trident, and ChemoMet). The diamond polishing stage grit size is governed by the last grinding step grit size, a factor of five is generally optimal.

• Diamond Suspension (1-3μm): Best for hard materials (steel, nickel-coated busbars).
• Colloidal Silica (0.05μm): Enhances final polish for soft materials (aluminium, copper).
• Vibratory Polishing: Reduces deformation, ideal for electroplated busbars.

Etching - Revealing Weld Profiles & Defects

Sample etched using various etchants (of varying compositions) to aid with contrast enhancement for microscopical analysis. Etching helps to reveal grain boundaries, phase distributions, and defects found in welded components. It also enhances welded and diffusion-bonded regions in battery components.

Common Etchants Used

• Aluminium Components: Kellers by swabbing or immersion, NaOH (10-20%) by immersion.
• Copper Busbars: Ferric chloride (FeCl3), Marbles, solution (5-10 sec).
• Steel Frames: Nital (2% or 3%) for ferritic microstructure.
• Nickel Coatings: Glyceregia, Marbles.

Figures below illustrate dissimilar material welds (busbars) revealing different weld profile and compounds formed within the welded regions.

Common defects

a. Weld & Bonding Defects:

• Porosity & Voids: from improper laser welding or ultrasonic bonding.
• IMC Formation: Excessive intermetallic compound growth in Al-Cu/ Al-Fe /Al-Ni joints.

b. Coating Failures:

• Cracking & Peeling: Due to thermal expansion mismatch.
• Oxide Layer Formation deleterious to electrical conductivity.

c. Fatigue and Stress Fractures:

• Microcracks in frames and busbars from cyclic loading.
• Delamination at plated interfaces in electroplated connectors

Microscopical techniques

Optical Microscopy (OM); ideal for grain size evaluation, microstructural changes, porosity checks, cracks, precipitates, and inclusions. Use of microscopical imaging modes applicable to optical microscopy, such as phase contrast,? brightfield (BF) and differential interference contrast (DIC), allows one to discern more details at the same magnification without loosing resolution.

For routine measurements on welds and related profiles, use of stereo microscope is recommended due to its larger field of view allowing full welds to be observed and analysed. Use of automated measurements allows one to increase analysis throughput as illustrated by OmniMet weld analysis module, and the various parameters you can measure.

Scanning Electron Microscopy (SEM) + Energy-Dispersive Spectroscopy (EDS); ideal for high resolution imaging of battery components, but it is the elemental distribution information around welds and associated IMC compounds that make it an endearing technique.

Hardness Testing (Vickers, Knoop)

Hardness testing is used to evaluate hardness variation across welded components to assess the integrity of the welding process. It also allows one to detect the heat affected zones on battery frame/ interconnects but also hardness variations attributed to IMC compounds within the welded regions, alloying element migration in dissimilar materials causing an increase or decrease in hardness. For example, on a laser weld between a 1050 Al grade and AlSi10Mg alloy, Si migration into 1050 Al results in microstructural changes causing higher hardness values. Additionally, welding processing parameters such heat input, traverse speed, among others can also result in hardness variations.

Conclusion: Advancing Metallographic Techniques

As electric vehicles continue to evolve, the demand for advanced metallurgical investigation grows. Automated sample preparation, AI-driven microstructural analysis, and real-time data analytics are paving the way for the next-generation quality control in battery frame components.

For companies engaged in EV manufacturing, establishing a robust metallurgical analysis workflow is crucial. Ensuring weld integrity, structural reliability, and defect-free manufacturing not only enhances performance but also safeguards passengers and maximizes battery efficiency.

By leveraging state-of-the-art preparation tools and analysis techniques, metallurgists and engineers are enabling a new era of durable and high-performance battery frames.

Have you worked on battery module or pack metallographic analysis? Share your experiences and insights. For further queries please contact us on [email protected] [email protected] or our website at www.buehler.com and for solutions applicable to automotive industry.

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