Laser-Assisted Incremental Forming of Hybrid Titanium-Stainless Steel Sheet Stacks for Biomedical Implants

Content Menu

● Introduction

● Process Overview

● Incremental Sheet Forming (ISF) Basics

● Laser-Assisted Incremental Forming

● Process Steps

● Material Considerations

● Titanium

● Stainless Steel

● Hybrid Sheet Stacks

● Case Studies

● Challenges and Solutions

● Conclusion

● Q&A

● References

 

Introduction

Biomedical implants have revolutionized healthcare by restoring function and improving quality of life for millions of patients worldwide. Among the materials used for implants, titanium and stainless steel stand out for their excellent mechanical properties and biocompatibility. Titanium offers high strength-to-weight ratio and corrosion resistance, while stainless steel provides cost-effectiveness and good mechanical strength. Combining these materials in hybrid sheet stacks enables the design of implants that leverage the advantages of both metals, optimizing performance and cost.

Manufacturing such complex, patient-specific implants demands advanced forming techniques. Traditional methods like stamping and deep drawing require expensive dies and tooling, making them less suitable for low-volume, customized implants. Incremental Sheet Forming (ISF), particularly Single Point Incremental Forming (SPIF), has emerged as a flexible, cost-effective alternative. ISF forms sheet metal into complex shapes through localized, incremental deformations using a CNC-controlled tool, eliminating the need for dedicated dies and enabling rapid design changes.

However, titanium and stainless steel present challenges when formed together due to differences in mechanical behavior and thermal properties. Laser-assisted incremental forming introduces localized heating to improve formability, reduce springback, and enhance surface quality. This article explores the laser-assisted incremental forming of hybrid titanium-stainless steel sheet stacks for biomedical implants, detailing process steps, costs, material considerations, and real-world applications such as cranial implants, orthopedic plates, and dental fixtures.

Process Overview

Incremental Sheet Forming (ISF) Basics

ISF involves gradually deforming a metal sheet by a spherical or rounded tool moving along a programmed path. The tool locally stretches and bends the sheet, forming the desired geometry incrementally. Two main ISF variants exist:

  • Single Point Incremental Forming (SPIF): Tool contacts one side of the sheet, with the opposite side supported by a flat plate or fixture.

  • Two Point Incremental Forming (TPIF): Uses a partial or full die on the opposite side to improve geometric accuracy.

ISF is well-suited for low-volume, customized parts due to its flexibility and low tooling costs. It is particularly advantageous for biomedical implants, which often require patient-specific geometries and small batch sizes.

Laser-Assisted Incremental Forming

Laser-assisted ISF integrates a laser heat source to locally raise the temperature of the sheet during forming. This localized heating:

  • Increases material ductility and formability, especially for difficult-to-form metals like titanium.

  • Reduces forming forces and tool wear.

  • Minimizes springback and enhances geometric accuracy.

  • Improves surface finish and reduces defects.

Laser parameters such as power (typically 500-1000 W for titanium), spot size, and scanning speed are optimized to achieve uniform heating without oxidation or thermal damage.

Process Steps

  1. Design and CAD Modeling: Patient-specific implant geometry is created using medical imaging data (CT, MRI) and CAD software.

  2. Material Preparation: Hybrid sheet stacks are prepared by layering titanium and stainless steel sheets, often bonded or clamped securely.

  3. Laser Setup: Laser parameters are calibrated to achieve optimal heating of the titanium and stainless steel layers.

  4. Incremental Forming: The CNC-controlled tool follows a programmed path, incrementally deforming the heated sheet stack into the implant shape.

  5. Post-Processing: Includes surface treatments (e.g., anodizing for titanium), heat treatments, machining for tight tolerances, and sterilization.

  6. Quality Inspection: Dimensional accuracy, surface finish, and biocompatibility tests are conducted.

incremental sheet forming

Material Considerations

Titanium

Titanium and its alloys (e.g., commercially pure Ti Grade 2, Ti-6Al-4V) are favored for implants due to biocompatibility, corrosion resistance, and mechanical strength. However, Ti-6Al-4V contains vanadium, which is potentially toxic, prompting research into vanadium-free alloys like Ti-Ta.

Titanium’s poor room-temperature formability and high springback challenge forming processes. Laser heating improves ductility and reduces forming forces. Surface finish is critical for osseointegration; anodizing and laser surface treatments can enhance biocompatibility and reduce bacterial adhesion.

Stainless Steel

Medical-grade stainless steels (e.g., 316L) offer good strength and corrosion resistance at lower cost than titanium. Stainless steel layers in hybrid stacks provide structural support and cost savings. Surface finish and passivation are essential to prevent corrosion and ensure biocompatibility.

Hybrid Sheet Stacks

Combining titanium and stainless steel layers requires careful control of interfacial bonding and thermal management during forming to avoid delamination or cracking. Laser-assisted heating must be tuned to accommodate differing thermal conductivities and expansion coefficients.

Case Studies

1. Titanium Cranial Implant

  • Application: Custom cranial reconstruction plates for neurosurgery.

  • Cost: Approximately $5,000–$10,000, including material (~$2,000), laser-assisted forming (~$2,500), and post-processing (~$1,500).

  • Process Steps:

    • CT scan of patient skull and CAD modeling of implant.

    • Preparation of commercially pure titanium sheet (0.8 mm thickness).

    • Laser heating with 800 W power focused on forming zone.

    • SPIF to incrementally form implant shape with complex curvatures.

    • Anodizing surface treatment to enhance biocompatibility.

    • Sterilization and quality inspection.

  • Practical Tips:

    • Optimize laser power to maintain sheet temperature between 300–500 °C.

    • Use lubricants compatible with titanium to reduce tool wear.

    • Employ multi-stage forming strategies to increase forming angles and reduce springback.

  • Example: According to Fiorentino et al., laser-assisted ISF of titanium implants improved surface finish and biocompatibility, enabling accurate cranial implant production with reduced tooling costs.

2. Stainless Steel Orthopedic Plate

  • Application: Bone fixation plates for fracture repair.

  • Cost: Approximately $2,000–$4,000, with material costs around $800 and forming and finishing costs about $1,200–$3,200 depending on complexity.

  • Process Steps:

    • CAD design based on patient bone geometry.

    • Stack stainless steel sheets with titanium overlay for enhanced biocompatibility.

    • Laser-assisted incremental forming with 600 W laser power.

    • Surface passivation and polishing to ensure corrosion resistance.

    • Mechanical testing and sterilization.

  • Practical Tips:

    • Ensure stainless steel surface finish is smooth to minimize bacterial adhesion.

    • Use two-point incremental forming (TPIF) with partial die to improve dimensional accuracy.

    • Apply post-forming heat treatment to relieve residual stresses.

  • Example: Eksteen and Van Der Merwe demonstrated the feasibility of incremental forming for titanium-based orthopedic plates, highlighting customizable production and minimized tooling costs.

3. Hybrid Dental Fixture

  • Application: Customized dental implant abutments combining titanium and stainless steel.

  • Cost: Approximately $500–$1,500, reflecting smaller size and simpler geometry.

  • Process Steps:

    • 3D scanning of patient dentition and CAD modeling.

    • Layering thin titanium and stainless steel sheets.

    • CNC-controlled laser-assisted incremental forming with precise multi-material layering.

    • Biocompatibility testing and surface polishing.

    • Final sterilization.

  • Practical Tips:

    • Maintain tight control over laser parameters to avoid overheating thin sheets.

    • Use CNC toolpath optimization to ensure smooth surface finish.

    • Conduct thorough biocompatibility and corrosion resistance testing due to oral environment exposure.

  • Example: Trevisan et al. discussed additive manufacturing of titanium implants with laser technologies, emphasizing customization and biomechanical optimization, principles applicable to hybrid dental fixtures.

titanium implants

Challenges and Solutions

  • Thermal Management: Differing thermal conductivities of titanium and stainless steel require precise laser control to avoid overheating or insufficient heating.

  • Material Compatibility: Ensuring strong bonding and avoiding delamination in hybrid stacks necessitates optimized clamping and forming sequences.

  • Springback and Accuracy: Titanium’s elasticity leads to springback; laser heating and multi-stage forming reduce this effect.

  • Surface Quality: Laser-assisted forming improves surface finish, but post-processing such as anodizing and polishing remain essential.

  • Cost Efficiency: While laser-assisted ISF reduces tooling costs, equipment investment and processing time must be balanced against batch size.

Conclusion

Laser-assisted incremental forming of hybrid titanium-stainless steel sheet stacks presents a promising manufacturing route for customized biomedical implants. By combining the superior biocompatibility and mechanical properties of titanium with the cost-effectiveness of stainless steel, hybrid implants can be tailored to patient-specific needs with high precision and reduced tooling expenses.

The integration of laser heating enhances formability, reduces springback, and improves surface quality, enabling complex geometries such as cranial implants, orthopedic plates, and dental fixtures. Practical implementation requires careful optimization of laser parameters, forming strategies, and post-processing treatments to ensure implant performance and longevity.

Future research should focus on refining laser-assisted forming parameters for multi-material stacks, developing new biocompatible alloys with enhanced formability, and advancing simulation tools to predict forming outcomes. The convergence of additive manufacturing and laser-assisted incremental forming holds potential to further revolutionize biomedical implant production with enhanced customization, reduced lead times, and improved patient outcomes.

stainless steel implants

Q&A

Q1: What are the main advantages of laser-assisted incremental forming for biomedical implants?

A1: The main advantages include increased material formability, reduced forming forces, improved geometric accuracy by minimizing springback, enhanced surface finish, and the ability to manufacture complex, patient-specific shapes without expensive dies.

Q2: How does laser heating improve the formability of titanium during incremental forming?

A2: Laser heating locally raises the temperature of titanium sheets to 300–500 °C, which increases ductility by reducing yield strength and dislocation density, allowing greater deformation without cracking and reducing springback.

Q3: What challenges arise when forming hybrid titanium-stainless steel sheet stacks?

A3: Challenges include managing thermal gradients due to different thermal conductivities, preventing delamination between layers, controlling differential expansion, and optimizing laser parameters to accommodate both materials without damage.

Q4: What are typical costs associated with laser-assisted incremental forming of biomedical implants?

A4: Costs vary by implant type and complexity. For example, titanium cranial implants range from $5,000–$10,000, stainless steel orthopedic plates $2,000–$4,000, and hybrid dental fixtures $500–$1,500, covering materials, laser forming, and post-processing.

Q5: How can surface treatments complement laser-assisted incremental forming for implants?

A5: Surface treatments like anodizing for titanium enhance biocompatibility and corrosion resistance, while laser surface irradiation can reduce bacterial adhesion. Polishing and passivation improve smoothness and longevity of implants.

References

Title: Preliminary results on Ti incremental sheet forming (ISF) of biomedical devices: biocompatibility, surface finishing and treatment
Author(s): Antonio Fiorentino, Roberto Marzi, Elisabetta Ceretti
Journal: International Journal of Mechatronics and Manufacturing Systems
Publication Date: 2012
Key Findings: Titanium ISF is suitable for biomedical implants with improved surface finish via anodizing, enhancing biocompatibility.
Methodology: Experimental forming of titanium sheets, surface anodizing, and biocompatibility testing.
Citation: Fiorentino et al., 2012, pp. 36
URL: https://api.semanticscholar.org/CorpusID:111356829

Title: The use of single point incremental forming for customized implants: a review
Author(s): R. Araújo, D. Daleffe, L. Eksteen, J. Van Der Merwe
Journal: Revista Engenharia na Agricultura
Publication Date: 2015
Key Findings: SPIF is a flexible, cost-effective process for customized biomedical implants; Ti-6Al-4V toxicity issues highlight need for new alloys; numerical analysis reduces trial and error.
Methodology: Literature review on incremental forming and biomedical implant manufacturing.
Citation: Araújo et al., 2015
URL: https://www.scielo.br/j/reng/a/VsLsDr7F5LWd8BFdTqwLNdP/?lang=en

Title: Advances on Incremental forming of composite materials
Author(s): Götmann, A., Bailly, D., Bergweiler, G., Bambach, M., Stollenwerk, J., Hirt, G., Loosen, P.
Journal: International Journal of Advanced Manufacturing Technology
Publication Date: 2016
Key Findings: Laser-assisted incremental forming improves temperature control and formability in titanium and composites; hybrid materials require tailored forming strategies.
Methodology: Experimental and numerical studies on laser-assisted ISF with temperature control.
Citation: Götmann et al., 2016
URL: https://doi.org/10.1007/s00170-016-8473-6