# Diffusion bonding

Diffusion bonding is a solid-state joining process in which two clean, close-fitting surfaces are united by heat and pressure so that atoms diffuse across the interface without melting. The American Welding Society's preferred term for the process is [diffusion welding](https://www.edgepedia.org/diffusion-welding), though diffusion bonding is more common in industry.<sup>[1](https://www.asminternational.org/results/-/journal_content/56/ASMHBA0001350/BOOK-ARTICLE/)</sup> Joining occurs below the melting point of the materials, usually above half of the absolute melting temperature, with loads below those that would cause macroscopic deformation of the parts.<sup>[1](https://www.asminternational.org/results/-/journal_content/56/ASMHBA0001350/BOOK-ARTICLE/)</sup> The International Institute of Welding adopted a definition of a monolithic joint formed by atomic-level bonding through local plastic deformation at elevated temperature, which aids interdiffusion in the surface layers.<sup>[2](https://www.phase-trans.msm.cam.ac.uk/2005/Amir/bond.html)</sup> Because the joint is monolithic, it can carry the parent metal's own properties; the process is used for aerospace structures, nuclear components, and compact heat exchangers,<sup>[3](https://www.ihi.co.jp/en/technology/techinfo/contents_no/__icsFiles/afieldfile/2023/09/14/Vol56No2_12.pdf)</sup><sup> • </sup><sup>[4](https://www.osti.gov/servlets/purl/2368568)</sup> although it remains expensive and is widespread mainly in aerospace engineering.<sup>[5](https://www.mdpi.com/2075-4701/10/5/613)</sup>

| Key fact | Detail |
|---|---|
| Bonding temperature | About 50–90% of the absolute melting point; other handbooks give 50–80%<sup>[2](https://www.phase-trans.msm.cam.ac.uk/2005/Amir/bond.html)</sup><sup> • </sup><sup>[3](https://www.ihi.co.jp/en/technology/techinfo/contents_no/__icsFiles/afieldfile/2023/09/14/Vol56No2_12.pdf)</sup> |
| Bonding time | A few minutes to a few hours<sup>[2](https://www.phase-trans.msm.cam.ac.uk/2005/Amir/bond.html)</sup> |
| Atmosphere | Vacuum (typically 10⁻³ mbar or better) or inert gas, to prevent oxidation<sup>[6](https://oro.open.ac.uk/99929/1/Issue%209%20Shirzadi%20pages%2013-15.pdf)</sup> |
| Bonding stages | Intimate contact, metallic bond formation, interdiffusion, and recrystallization and/or grain growth<sup>[7](https://cdn.ymaws.com/titanium.org/resource/resmgr/ZZ-WCTP1973-VOL1/1973_Vol.1-5-Pressure_Requir.pdf)</sup> |
| Example conditions (Ti-6Al-4V) | 900 °C, 0.3 kgf/mm², 2 h, 10⁻³ Pa vacuum<sup>[8](https://www.jstage.jst.go.jp/article/isijinternational1966/25/6/25_6_513/_pdf/-char/ja)</sup> |
| SPF/DB savings | Typically 10–50% weight savings and 25–40% cost reduction versus conventional fabrication<sup>[9](https://www.matec-conferences.org/articles/matecconf/pdf/2015/02/matecconf-icnft2015_01005.pdf)</sup> |
| Example joint strength | Ti-6Al-4V to Ti-22Al-25Nb: 894 MPa tensile, nearly equal to the base alloy<sup>[10](https://www.sciencedirect.com/science/article/pii/S1003632621655814)</sup> |

## How it works

Bonding proceeds because real surfaces touch only at asperity peaks. The accepted sequence has four basic stages: development of intimate physical contact, formation of the metallic bond, interdiffusion, and recrystallization and/or grain growth across the interface.<sup>[7](https://cdn.ymaws.com/titanium.org/resource/resmgr/ZZ-WCTP1973-VOL1/1973_Vol.1-5-Pressure_Requir.pdf)</sup> In the first stage, pressure must be high enough to plastically deform surface irregularities, which on machined parts range from roughly 10 to 1,000 microinches in roughness.<sup>[11](https://ntrs.nasa.gov/api/citations/19660010173/downloads/19660010173.pdf)</sup> At the end of this stage the bonded area is still less than 10%, with voids and oxide remaining between isolated bonded regions; creep and diffusion then close the voids.<sup>[2](https://www.phase-trans.msm.cam.ac.uk/2005/Amir/bond.html)</sup>

Temperature dominates the kinetics. Atom migration across the interface follows an exponential dependence of the form \( D = A e^{-a' \cdot R \cdot T} \),<sup>[11](https://ntrs.nasa.gov/api/citations/19660010173/downloads/19660010173.pdf)</sup> and an increase of about 20 K can double the diffusion coefficient and drastically raise the creep rate at a given bearing pressure.<sup>[12](https://pdfs.semanticscholar.org/ce94/91e458ed96688444c870159782343684d687.pdf)</sup> [Grain boundary](https://www.edgepedia.org/grain-boundary) diffusion predominates at low and medium temperature, while volume diffusion takes over at high temperature.<sup>[12](https://pdfs.semanticscholar.org/ce94/91e458ed96688444c870159782343684d687.pdf)</sup> Residual microporosity can be removed by holding at temperature without pressure, driven by surface-energy vacancy diffusion, with the void shrinkage rate proportional to the inverse square of the pore radius.<sup>[7](https://cdn.ymaws.com/titanium.org/resource/resmgr/ZZ-WCTP1973-VOL1/1973_Vol.1-5-Pressure_Requir.pdf)</sup> For many steels, copper, titanium, tantalum, columbium, and zirconium, the oxide films dissolve or decompose at bonding temperature, so they do not block joining.<sup>[2](https://www.phase-trans.msm.cam.ac.uk/2005/Amir/bond.html)</sup>

## How it is done

A diffusion bonder is essentially a vacuum hot press with independently controlled pressure and temperature, using radiation, conduction, or induction heating; its limitations include stringent surface preparation and very high capital investment. An industrial vacuum hot-press workflow has four steps: stacking the parts in air, evacuating the furnace, plastically deforming the interface irregularities under simultaneous pressurization and heating, and completing the bond by volume diffusion, interface migration, and void disappearance.<sup>[3](https://www.ihi.co.jp/en/technology/techinfo/contents_no/__icsFiles/afieldfile/2023/09/14/Vol56No2_12.pdf)</sup> Bonding is generally carried out at 50–80% of the melting point, with pressure chosen to avoid excessive deformation and vacuum chosen to prevent oxidation; inadequate vacuum or poor surface condition produces void defects.<sup>[3](https://www.ihi.co.jp/en/technology/techinfo/contents_no/__icsFiles/afieldfile/2023/09/14/Vol56No2_12.pdf)</sup>

Typical conditions illustrate the windows. Ti-6Al-4V plates 5–40 mm thick were bonded at 900 °C, 0.3 kgf/mm², for 2 hours in 10⁻³ Pa vacuum.<sup>[8](https://www.jstage.jst.go.jp/article/isijinternational1966/25/6/25_6_513/_pdf/-char/ja)</sup> A 1973 analysis of Ti-6Al-4V found the required bonding pressure drops sharply as bond time increases to about 3–4 hours at 1650–1740 °F, with virtually no further reduction beyond that.<sup>[7](https://cdn.ymaws.com/titanium.org/resource/resmgr/ZZ-WCTP1973-VOL1/1973_Vol.1-5-Pressure_Requir.pdf)</sup> For 316H stainless steel, phase-field modeling points to optimal temperatures of 1050–1100 °C and adequate pressures of 7–20 MPa, with bonding stagnant at or below 1000 °C and grain coarsening above 1100 °C.<sup>[13](https://www.sciencedirect.com/science/article/abs/pii/S0921509325012754)</sup> Quality is verified by microstructure, tensile testing, and ultrasonic inspection.<sup>[3](https://www.ihi.co.jp/en/technology/techinfo/contents_no/__icsFiles/afieldfile/2023/09/14/Vol56No2_12.pdf)</sup>

## Origin

Forms of diffusion bonding have been used for millennia, most notably to produce patterned blades such as those of the samurai.<sup>[14](https://-sage-cnpereading-com-443.webvpn1.xju.edu.cn/doi/10.1177/0954405413498584)</sup> A 1966 NASA technical report describes the process as an established joining technique already used to produce reliable bonds in nuclear reactor fuel elements, with aerospace structural applications projected.<sup>[11](https://ntrs.nasa.gov/api/citations/19660010173/downloads/19660010173.pdf)</sup> Studies of diffusion welding of Ti-6Al-4V published in 1968–1970 in the Welding Research Supplement preceded the 1973 pressure-requirements analysis.<sup>[7](https://cdn.ymaws.com/titanium.org/resource/resmgr/ZZ-WCTP1973-VOL1/1973_Vol.1-5-Pressure_Requir.pdf)</sup> Research on diffusion welding of titanium alloys led to the technique being put into practical aircraft production in Japan, with a CCV research aircraft flying safely in August 1983.<sup>[8](https://www.jstage.jst.go.jp/article/isijinternational1966/25/6/25_6_513/_pdf/-char/ja)</sup>

## Variants

**SPF/DB** combines superplastic forming with diffusion bonding in a single step to produce complex hollow or honeycomb structures. A patented concurrent SPF/DB process achieves bonding at much lower normal pressures than hot isostatic pressing vessels, because the pressure originates from the superplastic forming operation itself.<sup>[14](https://-sage-cnpereading-com-443.webvpn1.xju.edu.cn/doi/10.1177/0954405413498584)</sup><sup> • </sup><sup>[15](https://pmc.ncbi.nlm.nih.gov/articles/PMC10342841/)</sup> In the four-sheet process, two core sheets with a welded cell pattern and two face sheets are welded together along the periphery, then inert gas pressure at elevated temperature forms the sandwich into an integrally stiffened structure.<sup>[16](https://saemobilus.sae.org/articles/advancements-superplastic-forming-diffusion-bonding-titanium-alloys-heat-critical-aerospace-applications-2020-01-0033)</sup>

**Transient liquid phase (TLP) bonding**, commonly called [diffusion brazing](https://www.edgepedia.org/diffusion-brazing), places a thin interlayer with a lower melting point between the substrates; on heating the interlayer melts and its solute diffuses into the base materials, causing isothermal solidification, so the finished bond has a higher melting point than the bonding temperature.<sup>[5](https://www.mdpi.com/2075-4701/10/5/613)</sup><sup> • </sup><sup>[17](https://link.springer.com/article/10.1007/s10853-011-5561-1)</sup> The slow stage is controlled by the solid-phase interdiffusion coefficient.<sup>[2](https://www.phase-trans.msm.cam.ac.uk/2005/Amir/bond.html)</sup> Partial transient liquid phase (PTLP) bonding, a variant used mainly to join ceramics, uses thin low-melting layers on each side of a much thicker refractory core.<sup>[17](https://link.springer.com/article/10.1007/s10853-011-5561-1)</sup> Simpler bonding aids, such as interface foils or coatings, can facilitate bonding or prevent brittle phases between dissimilar materials, provided they do not form a low-temperature liquid eutectic.<sup>[1](https://www.asminternational.org/results/-/journal_content/56/ASMHBA0001350/BOOK-ARTICLE/)</sup>

## Applications

Diffusion bonding is used where a monolithic, parent-metal joint or a lightweight laminated structure justifies its cost. Laminated diffusion welding of titanium reduced the buy-to-fly ratio (weight of raw material bought vs. final part) from approximately 8 for machined forgings to approximately 3.<sup>[8](https://www.jstage.jst.go.jp/article/isijinternational1966/25/6/25_6_513/_pdf/-char/ja)</sup> SPF/DB structures serve as ducts, aircraft wing access panels, rudders, nozzles, engine casings, and blades; the largest reported parts reach 3000 mm × 1500 mm with four layers, and a rib-stiffened fuselage panel of that size gave 10% weight saving, 30% production-time reduction, and 70% fewer components versus conventional processing.<sup>[9](https://www.matec-conferences.org/articles/matecconf/pdf/2015/02/matecconf-icnft2015_01005.pdf)</sup> A United States Department of Energy project led by the University of Michigan is qualifying diffusion-bonded Alloy 617 and type 316H compact heat exchangers for ASME BPVC Section III, Division 5 nuclear service.<sup>[4](https://www.osti.gov/servlets/purl/2368568)</sup> The process is also a candidate for joining titanium alloys to aluminum, stainless steel, and magnesium, where fusion welding produces undesirable intermetallic compounds,<sup>[18](https://mdpi-res.com/d_attachment/jmmp/jmmp-04-00039/article_deploy/jmmp-04-00039.pdf?version=1592389328)</sup> and for joining and repair of nickel-based superalloy components via TLP bonding.<sup>[17](https://link.springer.com/article/10.1007/s10853-011-5561-1)</sup>

When the applied pressure is sufficient, joints can match the parent metal. An analytical model for Ti-6Al-4V predicts complete closure of bond surfaces, and full parent-metal strength, when the ratio of applied to required pressure exceeds 1; below that condition bonds show porosity and sub-parent strength.<sup>[7](https://cdn.ymaws.com/titanium.org/resource/resmgr/ZZ-WCTP1973-VOL1/1973_Vol.1-5-Pressure_Requir.pdf)</sup> Measured examples include 894 MPa tensile strength for a Ti-6Al-4V/Ti-22Al-25Nb joint bonded at 950 °C, 15 MPa, and 100 min,<sup>[10](https://www.sciencedirect.com/science/article/pii/S1003632621655814)</sup> and base-metal-equivalent tensile strength for SUS304 stainless steel when surface pressure was raised enough to eliminate interface voids.<sup>[3](https://www.ihi.co.jp/en/technology/techinfo/contents_no/__icsFiles/afieldfile/2023/09/14/Vol56No2_12.pdf)</sup>

## Limitations and alternatives

The main defects are residual voids from insufficient pressure or vacuum, surface oxides, and interfacial compounds. On titanium alloys heated in air, an α-case oxide layer several microns thick forms and suppresses atomic diffusion, hindering bonding.<sup>[15](https://pmc.ncbi.nlm.nih.gov/articles/PMC10342841/)</sup> Large differences in diffusivity of major alloying elements across the interface produce Kirkendall pores that degrade mechanical properties,<sup>[19](https://www.mdpi.com/2075-4701/13/8/1438)</sup> and thick intermetallic phases significantly decrease bond strength in dissimilar joints.<sup>[18](https://mdpi-res.com/d_attachment/jmmp/jmmp-04-00039/article_deploy/jmmp-04-00039.pdf?version=1592389328)</sup> For diffusion-bonded Alloy 617, elevated-temperature creep-fatigue properties were significantly reduced compared with the base metal, with failure occurring at the weakest interface.<sup>[4](https://www.osti.gov/servlets/purl/2368568)</sup>

[Titanium alloys](https://www.edgepedia.org/titanium-alloys) and steels whose oxides dissolve at bonding temperature bond readily. Stable oxides on aluminum and its alloys are the classic obstacle: early work in 1966 and 1995 showed a minimum deformation of about 40% is needed to disrupt the oxide and produce reasonable solid-state bond strengths,<sup>[20](http://www.phase-trans.msm.cam.ac.uk/2005/surface.science.pdf)</sup> and thick surface oxide also makes aluminum difficult to SPF/DB.<sup>[9](https://www.matec-conferences.org/articles/matecconf/pdf/2015/02/matecconf-icnft2015_01005.pdf)</sup> The protective oxide layer on [austenitic stainless steel](https://www.edgepedia.org/austenitic-stainless-steel) makes bonding it non-trivial,<sup>[5](https://www.mdpi.com/2075-4701/10/5/613)</sup> and passivation layers of different composition on austenitic, ferritic, and martensitic stainless steels affect the local diffusion coefficient, so different bonding temperatures are needed; martensitic grades bond easily at low deformation, while ferritic and austenitic grades require much more extreme parameters.<sup>[21](https://onlinelibrary.wiley.com/doi/10.1002/adem.201700367)</sup> [Refractory metals](https://www.edgepedia.org/refractory-metals) are difficult because solid-state joining needs more than 50% of the homologous melting temperature plus high pressure, and with reasonable effort tungsten or tantalum cannot be diffusion bonded.<sup>[19](https://www.mdpi.com/2075-4701/13/8/1438)</sup><sup> • </sup><sup>[12](https://pdfs.semanticscholar.org/ce94/91e458ed96688444c870159782343684d687.pdf)</sup> Dissimilar pairs suffer from differences in thermal expansion, melting point, and metallurgical compatibility, giving brittle intermetallics and high residual stresses, so interlayers are often required.<sup>[22](https://www.ams.org.cn/EN/Y2026/V62/I5/835)</sup>

Compared with fusion joining, diffusion bonding produces less distortion and no residual stresses or thermal gradients at the bond site.<sup>[23](https://iopscience.iop.org/article/10.1088/1757-899X/1013/1/012011/pdf)</sup> In [vacuum brazing](https://www.edgepedia.org/vacuum-brazing), by contrast, the filler does not fully diffuse into the base materials, isothermal solidification does not occur, and no relevant pressure is applied, which allows much simpler equipment.<sup>[5](https://www.mdpi.com/2075-4701/10/5/613)</sup> [Hot isostatic pressing](https://www.edgepedia.org/hot-isostatic-pressing) (HIP) applies isostatic argon pressure instead of a uniaxial ram; one review gives HIP conditions of several hundreds to 2000 °C and several tens to 200 MPa,<sup>[5](https://www.mdpi.com/2075-4701/10/5/613)</sup> while another source describes aerospace HIP diffusion bonding at up to about 1000 °C and around 100 atmospheres.<sup>[14](https://-sage-cnpereading-com-443.webvpn1.xju.edu.cn/doi/10.1177/0954405413498584)</sup> [Friction stir welding](https://www.edgepedia.org/friction-stir-welding), high-energy beam welding, brazing, and additive manufacturing are the other major routes for dissimilar and complex structures.<sup>[22](https://www.ams.org.cn/EN/Y2026/V62/I5/835)</sup> The process's main economic limitation is cost: it is expensive and only widespread in aerospace engineering.<sup>[5](https://www.mdpi.com/2075-4701/10/5/613)</sup>

Recent work targets those limits. In one 2024 study, field-assisted sintering technology joined INCONEL 617 at 800 °C under 10 MPa for 30 min, well below the 1120–1200 °C needed in conventional vacuum bonding, giving a crack- and void-free faying surface about 10 μm thick.<sup>[24](https://beta.iopscience.iop.org/article/10.1088/2053-1591/ad6237)</sup> Impulse pressure-assisted bonding, using varying rather than constant pressure, has been claimed to reduce overall processing and bonding time.<sup>[19](https://www.mdpi.com/2075-4701/13/8/1438)</sup>

## References

1. [Fundamentals of Diffusion Bonding (ASM International, 1993)](https://www.asminternational.org/results/-/journal_content/56/ASMHBA0001350/BOOK-ARTICLE/)
2. [Diffusion bonding (University of Cambridge phase-trans course material by A. Shirzadi)](https://www.phase-trans.msm.cam.ac.uk/2005/Amir/bond.html)
3. [Development of Diffusion Bonding Process by IHI Vacuum Hot Press](https://www.ihi.co.jp/en/technology/techinfo/contents_no/__icsFiles/afieldfile/2023/09/14/Vol56No2_12.pdf)
4. [Advancing Diffusion Bonded Compact Heat Exchangers for High Temperature Applications (DOE-NE project report)](https://www.osti.gov/servlets/purl/2368568)
5. [Diffusion Bonding and Transient Liquid Phase (TLP) Bonding of Type 304 and 316 Austenitic Stainless Steel, A Review (Metals, 2020)](https://www.mdpi.com/2075-4701/10/5/613)
6. [Fundamentals of solid-state diffusion bonding (Welding and Joining Matters, 2024)](https://oro.open.ac.uk/99929/1/Issue%209%20Shirzadi%20pages%2013-15.pdf)
7. [C. H. Hamilton, 'Pressure Requirements for Diffusion Bonding Titanium' (Titanium Conference, 1973)](https://cdn.ymaws.com/titanium.org/resource/resmgr/ZZ-WCTP1973-VOL1/1973_Vol.1-5-Pressure_Requir.pdf)
8. [The Application of Diffusion Welding to Aircraft Titanium Alloys (Transactions ISIJ, Vol. 25, 1985)](https://www.jstage.jst.go.jp/article/isijinternational1966/25/6/25_6_513/_pdf/-char/ja)
9. [Superplastic forming and diffusion bonding: Progress and trends (MATEC Web of Conferences, ICNFT 2015)](https://www.matec-conferences.org/articles/matecconf/pdf/2015/02/matecconf-icnft2015_01005.pdf)
10. [Microstructure and mechanical properties of Ti-6Al-4V/Ti-22Al-25Nb joint formed by diffusion bonding (Trans. Nonferrous Metals Society of China)](https://www.sciencedirect.com/science/article/pii/S1003632621655814)
11. [Diffusion Bonding (NASA Technical Report, 1966)](https://ntrs.nasa.gov/api/citations/19660010173/downloads/19660010173.pdf)
12. [Diffusion Bonding: Influence of Process Parameters and Material Microstructure (book chapter, Semantic Scholar copy)](https://pdfs.semanticscholar.org/ce94/91e458ed96688444c870159782343684d687.pdf)
13. [Phase-field modeling of diffusion bonding in 316H stainless steel: Impact of processing conditions on grain morphology and bonding quality (2025)](https://www.sciencedirect.com/science/article/abs/pii/S0921509325012754)
14. [Preliminary study of improving the speed and cost of diffusion bonding of metal sheets (Proc. IMechE, 2013/2014; DOI 10.1177/0954405413498584, accessed via university proxy mirror)](https://-sage-cnpereading-com-443.webvpn1.xju.edu.cn/doi/10.1177/0954405413498584)
15. [One-Step Hybrid Bending/Diffusion Bonding Process and Analysis of the Bonding Characteristics of Titanium Alloy Sheets (2023, PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC10342841/)
16. [Advancements of Superplastic Forming and Diffusion Bonding of Titanium Alloys for Heat Critical Aerospace Applications (SAE 2020-01-0033)](https://saemobilus.sae.org/articles/advancements-superplastic-forming-diffusion-bonding-titanium-alloys-heat-critical-aerospace-applications-2020-01-0033)
17. [Overview of transient liquid phase and partial transient liquid phase bonding (Journal of Materials Science)](https://link.springer.com/article/10.1007/s10853-011-5561-1)
18. [Current Trends in Dissimilar Diffusion Bonding of Titanium Alloys to Stainless Steels, Aluminium and Magnesium (J. Manuf. Mater. Process. 2020, 4, 39)](https://mdpi-res.com/d_attachment/jmmp/jmmp-04-00039/article_deploy/jmmp-04-00039.pdf?version=1592389328)
19. [Evaluation of Tungsten, Steel Solid-State Bonding: Options and the Role of CALPHAD to Screen Diffusion Bonding Interlayers (Metals, 2023)](https://www.mdpi.com/2075-4701/13/8/1438)
20. [Interface evolution and bond strength when diffusion bonding materials with stable oxide films (Shirzadi & Wallach)](http://www.phase-trans.msm.cam.ac.uk/2005/surface.science.pdf)
21. [Challenges of Diffusion Bonding of Different Classes of Stainless Steels (Advanced Engineering Materials, Wiley)](https://onlinelibrary.wiley.com/doi/10.1002/adem.201700367)
22. [Acta Metallurgica Sinica review of dissimilar material joining technologies (2026)](https://www.ams.org.cn/EN/Y2026/V62/I5/835)
23. [Solid State Diffusion Bonding Process – A Review (IOP Conf. Series: Materials Science and Engineering, 2021)](https://iopscience.iop.org/article/10.1088/1757-899X/1013/1/012011/pdf)
24. [Solid-state diffusion bonding for INCONEl 617 superalloy using field-assisted sintering technology (FAST) guided by CALPHAD approach (Materials Research Express, 2024)](https://beta.iopscience.iop.org/article/10.1088/2053-1591/ad6237)

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*Topic: Encyclopedia › Technology and the built world › Engineering and manufacturing › Manufacturing processes and fabrication › Welding, soldering, and joining*

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