Titanium adhesive bonding

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Titanium adhesive bonding is an engineering process used in the aerospace industry, medical-device manufacture and elsewhere. Titanium alloy is often used in medical and military applications because of its strength, weight, and corrosion resistance characteristics. In implantable medical devices, titanium is used because of its biocompatibility and its passive, stable oxide layer. [1] Also, titanium allergies are rare and in those cases mitigations like Parylene coating are used. [2] In the aerospace industry titanium is often bonded to save cost, touch times, and the need for mechanical fasteners. In the past, Russian submarines' hulls were completely made of titanium because the non-magnetic nature of the material went undetected by the defense technology at that time. [3] Bonding adhesive to titanium requires preparing the surface beforehand, and there is not a single solution for all applications. For example, etchant and chemical methods are not biocompatible and cannot be employed when the device will come into contact with blood and tissue. Mechanical surface roughness techniques like sanding and laser roughening may make the surface brittle and create micro-hardness regions that would not be suitable for cyclic loading found in military applications. Air oxidation at high temperatures will produce a crystalline oxide layer at a lower investment cost, but the increased temperatures can deform precision parts. [4] The type of adhesive, thermosetting or thermoplastic, and curing methods are also factors in titanium bonding because of the adhesive's interaction with the treated oxide layer. Surface treatments can also be combined. For example, a grit blast process can be followed by a chemical etch and a primer application.

Contents

Abrasives

Aluminium oxide or Alumina and Silicon carbide are most commonly used to prepare titanium for epoxy bonding. Alumina has a hardness of 9 on the Mohs scale while silicon carbide has a hardness of just under that of diamond. [5] Alumina particle sizes in the 10 to 150 micron range are used depending on the workpiece geometry and blasting capabilities. [5] Silicon carbide particles are typically in the 20 to 50 micron range with texturing occurring at a faster pace than alumina. [5] When silicon carbide hits the titanium surface the operator will see sparks as is common with titanium surfaced golf drivers when they hit the ground surface. Care must be taken if sensitive electronic assemblies are housed within the titanium enclosure. Electrostatic discharge can be mitigated with point ionizers or grounding features in the tools. Glass beads media are used less commonly. They come as spherical particles in the 35-100 micron range. [5] They are a 6 on the Mohs scale and are oftentimes used with water to create a hydrohone slurry. [5] When applied to commercially pure titanium material they will stress relieve the assembly, typically after welding, and create a satin-like finish perfect for laser marking of labels. The surface is also suitable as preparation for assemblies prior to vapor deposition of Parylene coating. [5]

Silicon carbide grit blast on a commercially pure titanium sample - 500X magnification. Titanium Surface Post Grit Blast.png
Silicon carbide grit blast on a commercially pure titanium sample - 500X magnification.

Surface roughness is achieved through the use of a blasting nozzle propelled by compressed air. The focus and velocity of the media created by the nozzle can be varied depending on the roughness requirements and repeatability. Surface roughness measured using Ra, Sa and Sdr is used to characterize the media application and the adhesive bonding strength. Typical Ra values for commercially pure titanium are between 0.2 and 0.75 micro meters. [6] The surface roughness can be tailored to the epoxy viscosity and curing conversion. The roughened surface is rinsed with process water or an alkaline cleaner and is often sealed with a primer application like Silane A-187 or alkoxide. [7] Application of the primer can be achieved through manual means, like a brush. It can also be sprayed on the roughened surface or the whole assembly can be dipped in a primer solution and cured. On commercially pure titanium surfaces that have been roughened with silicon carbide, a silane primer will darken the surface allowing for verification of application.

Implantable medical devices are often manufactured in a cleanroom environment. Typical cleanroom ratings are within the ISO-7 and ISO-8 range or between class 10k and 100k. Abrasives and their application cannot be housed in such cleanrooms. If pass through windows are not available then laser roughening is a good option.

Laser roughening

Grade 1 Ti Laser Roughened Contact Angle Measurement. Contact Angle of Laser Roughened Ti.png
Grade 1 Ti Laser Roughened Contact Angle Measurement.

Laser roughening of titanium surfaces for epoxy bonding is a good option when abrasives and chemical agents are restricted in the manufacturing area. The process is also more repeatable and consistent than the often manual abrasive blasting. Other advantages over abrasives are touch time and maintenance. The drawback of laser roughening is the cost of the equipment and tooling. Also, the laser will heat the material depending on its power output and number of passes. It will remove material from the surface and create regions of hardened material that get relocated within the surface. Neodymium doped yttrium aluminum garnet (Nd:YAG), CO2, green, femtosecond lasers can be used depending on the workpiece and adhesion requirements. YAG or fiber laser markers that anneal the titanium surface are the low cost solution while the femtosecond laser is on the high end of the cost scale. Surface roughness of laser roughened surfaces is best measured using a three-dimensional scanning laser microscope or a non-contact profilometer. XPS and SEM analysis of alloyed titanium, like grade 5, will show the segregation of the aluminum and vanadium. Oftentimes, the laser roughening is done in ambient conditions with or without argon shielding gas. Ambient elements that play no role in bonding like carbon and nitrogen can be ignored from the surface analysis. Laser roughening of grade 5 titanium will show that the vanadium will segregate to the bulk of the alloy and appear at the surface with an increased oxygen level. Lap shear tests have shown that this segregation does not affect surface adhesion. The increase of laser power has shown to increase oxidation of grade 5 titanium which has been correlated to increase bond strength. [8] Also, producing alumina at the surface has shown to improve bonding. Dimples created by multiple laser pulses increase the surface area for adhesion, but the center of the topography will have reduced oxide formation because of the laser-induced plasma. [9] Depending on the grade of titanium and the adhesive used, the laser parameters of power, frequency, and pattern can be tailored to the loading requirements and the aforementioned surface elemental beneficial conditions. Unwanted metal oxide can occur when higher laser powers and multiple passes are employed. These can be removed with a lower powered laser pass or a titanium brush, post roughening. The grain size will affect the surface roughness, hardness, and wettability of the surface. On grade 2 titanium, a smaller grain structure improved these surface prep characteristics. [10] As with abrasives, a silane primer application is used to seal the laser roughened surface.

Commercially pure titanium roughened using a fiber laser .001 inch spacing, 100 inches/second speed - 500X magnification. Laser Roughening.png
Commercially pure titanium roughened using a fiber laser .001 inch spacing, 100 inches/second speed - 500X magnification.

Etchant, chemical and anodizing preparation

Prior to these treatments a solvent degreaser should be used with an alumina grit blast to remove unwanted oxides on the surface. A 1982 study at the Naval Air Development Center compared 11 etchant, chemical and anodizing preparations on grade 5 titanium samples. Once bonded, these samples were exposed to 56 days of 140 degrees F and 100% relative humidity. Crack growth was measured at preselected intervals. The results showed that chromic acid anodizing with fluoride, etchants Turco 5578, Pasa Jell 107C – hydrohone, Pasa Jell 107M – dry hone, Dapcotreat 4023/4000 and alkaline peroxide were superior to phosphate fluoride preparations. [4]

Turco 5578-L is a commonly used etchant and alkaline cleaner for titanium. It is produced by Henkel Technologies and comes in a liquid form so concentrations can be easily modified. It is an anisotropic etchant that avoids hydrogen embrittlement. [6] When used on Grade 5 titanium it will produce an oxide layer 17.5 nm thick and a surface roughness of 3.4 um total height profile (Rt). [7]

In a chromic acid treatment the anodization is typically done at 5 or 10 volts. The 1982 study referred to above stated that the 5 volt performed better than the 10 volt as a function of average crack opening. In the Review of preparation for Ti, Critchlow and Brewis state that the 10 volt anodize showed better durability results. [7] The 10 volt anodize can produce a columnar and cellular oxide layer with a thickness between 80 and 500 nm. [7] The pores and whiskers produced can be penetrated by selecting a low viscosity adhesive, like a 3M 1838 epoxide resin or an Epo-Tek 301 epoxy. The surface oxide can be compromised if it is subjected to high temperatures, above 300 °C, and humidity prior to bonding. [7]

Pasa Jell wet and dry hone are chemical etchants produced by Semco. They create oxide thicknesses of 10-20 nm. [7] It is recommended to degrease the titanium surface and remove any corrosion by way of sanding and/or grit blasting prior to application. Typical application times are 10–15 minutes followed by a rinse with tap water. [11] Application of a corrosion inhibiting primer like BR-127 has shown to produce adhesive joints comparable to ones produced by the chromic acid anodizing process. [7]

Related Research Articles

Ceramic Inorganic, nonmetallic solid prepared by the action of heat

A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing a nonmetallic mineral, such as clay, at a high temperature. Common examples are earthenware, porcelain, and brick.

Aluminium oxide Chemical compound with formula Al2O3

Aluminium oxide is a chemical compound of aluminium and oxygen with the chemical formula Al2O3. It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium(III) oxide. It is commonly called alumina and may also be called aloxide, aloxite, or alundum depending on particular forms or applications. It occurs naturally in its crystalline polymorphic phase α-Al2O3 as the mineral corundum, varieties of which form the precious gemstones ruby and sapphire. Al2O3 is significant in its use to produce aluminium metal, as an abrasive owing to its hardness, and as a refractory material owing to its high melting point.

An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflective surface, the process can also involve roughening as in satin, matte or beaded finishes. In short, the ceramics which are used to cut, grind and polish other softer materials are known as abrasives.

Sandpaper

Sandpaper and glasspaper are names used for a type of coated abrasive that consists of sheets of paper or cloth with abrasive material glued to one face.

A cermet is a composite material composed of ceramic (cer) and metal (met) materials.

Anodizing Electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts

Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.

Grinding wheel

A grinding wheel is a wheel composed of an abrasive compound and used for various grinding and abrasive machining operations. Such wheels are used in grinding machines.

Superalloy

A superalloy, or high-performance alloy, is an alloy with the ability to operate at a high fraction of its melting point. Several key characteristics of a superalloy are excellent mechanical strength, resistance to thermal creep deformation, good surface stability, and resistance to corrosion or oxidation.

Metallography

Metallography is the study of the physical structure and components of metals, by using microscopy.

Fusion bonded epoxy coating, also known as fusion-bond epoxy powder coating and commonly referred to as FBE coating, is an epoxy-based powder coating that is widely used to protect from corrosion steel pipe used in pipeline construction, concrete reinforcing bars and on a wide variety of piping connections, valves etc. FBE coatings are thermoset polymer coatings. They come under the category of protective coatings in paints and coating nomenclature. The name fusion-bond epoxy is due to resigning cross-link and the application method, which is different from a conventional paint.

Plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO), also known as electrolytic plasma oxidation (EPO) or microarc oxidation (MAO), is an electrochemical surface treatment process for generating oxide coatings on metals. It is similar to anodizing, but it employs higher potentials, so that discharges occur and the resulting plasma modifies the structure of the oxide layer. This process can be used to grow thick, largely crystalline, oxide coatings on metals such as aluminium, magnesium and titanium. Because they can present high hardness and a continuous barrier, these coatings can offer protection against wear, corrosion or heat as well as electrical insulation.

Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low alloy steels and high alloy steels. The difference between the two is disputed. Smith and Hashemi define the difference at 4.0%, while Degarmo, et al., define it at 8.0%. Most commonly, the phrase "alloy steel" refers to low-alloy steels.

Cold saw

A cold saw is a circular saw designed to cut metal which uses a toothed blade to transfer the heat generated by cutting to the chips created by the saw blade, allowing both the blade and material being cut to remain cool. This is in contrast to an abrasive saw, which abrades the metal and generates a great deal of heat absorbed by the material being cut and saw blade.

Ultrasonic machining

Ultrasonic machining is a subtractive manufacturing process that removes material from the surface of a part through high frequency, low amplitude vibrations of a tool against the material surface in the presence of fine abrasive particles. The tool travels vertically or orthogonal to the surface of the part at amplitudes of 0.05 to 0.125 mm. The fine abrasive grains are mixed with water to form a slurry that is distributed across the part and the tip of the tool. Typical grain sizes of the abrasive material range from 100 to 1000, where smaller grains produce smoother surface finishes.

Thick-film technology

Thick-film technology is used to produce electronic devices/modules such as surface mount devices modules, hybrid integrated circuits, heating elements, integrated passive devices and sensors. Main manufacturing technique is screen printing (stenciling), which in addition to use in manufacturing electronic devices can also be used for various graphic reproduction targets. The technique is known in its basic form about thousand years – already used during great Chinese dynasties. It became one of the key manufacturing/miniaturisation techniques of electronic devices/modules during 1950s. Typical film thickness – manufactured with thick film manufacturing processes for electronic devices – is 0.0001 to 0.1 mm.

An inclusion is a solid particle in liquid aluminium alloy. It is usually non-metallic and can be of different nature depending on its source.

Eutectic bonding

Eutectic bonding, also referred to as eutectic soldering, describes a wafer bonding technique with an intermediate metal layer that can produce a eutectic system. Those eutectic metals are alloys that transform directly from solid to liquid state, or vice versa from liquid to solid state, at a specific composition and temperature without passing a two-phase equilibrium, i.e. liquid and solid state. The fact that the eutectic temperature can be much lower than the melting temperature of the two or more pure elements can be important in eutectic bonding.

Polytetrafluoroethylene (PTFE), better known by its trade name Teflon, has many desirable properties which make it an attractive material for numerous industries. It has good chemical resistance, a low dielectric constant, low dielectric loss, and a low coefficient of friction, making it ideal for reactor linings, circuit boards, and kitchen utensils, to name a few applications. However, its nonstick properties make it challenging to bond to other materials or to itself.

Aluminium joining

Aluminium alloys are often chosen due to their high strength-to-weight ratio, corrosion resistance, low cost, high thermal and electrical conductivity. There are a variety of techniques to join aluminium including mechanical fasteners, welding, adhesive bonding, brazing, soldering and friction stir welding (FSW), etc. Various techniques are used based on the cost and strength required for the joint. In addition, process combinations can performed to provide means for difficult to join assemblies and to reduce certain process limitations.

Detonation spraying

Detonation spraying is one of the many forms of thermal spraying techniques that are used to apply a protective coating at supersonic velocities to a material in order to change its surface characteristics. This is primarily to improve the durability of a component. It was first invented in 1955 by H.B. Sargent, R.M. Poorman and H. Lamprey and is applied to a component using a specifically designed detonation gun (D-gun). The component being sprayed must be prepared correctly by removing all surface oils, greases, debris and roughing up the surface in order to achieve a strongly bonded detonation spray coating. This process involves the highest velocities and temperatures (≈4000 °C) of coating materials compared to all other forms of thermal spraying techniques. Which means detonation spraying is able to apply low porous and low oxygen content protective coatings that protect against corrosion, abrasion and adhesion under low load.

References

  1. ↑ Linjiang Chai et al. Microstructural characterization and hardness variation of pure Ti surface-treated by pulsed laser. Journal of Alloys and Compounds, January 2018. Pg. 116-122.
  2. ↑ "A New Look at Parylene Conformal Coatings".
  3. ↑ "Titanium submarines return to Russian fleet".
  4. 1 2 S.R. Brown and G.J. Pilla, Titanium Surface Treatments for Adhesive Bonding, Naval Air Development Center Warminster, Pa 1982.
  5. 1 2 3 4 5 6 "Microblasting Nozzles and Abrasive Media" (PDF).
  6. 1 2 S. Zimmermann et al. Improved adhesion at titanium surfaces via laser-induced surface oxidation and roughening. Materials Science & Engineering, August 2012. Pg. 755-760.
  7. 1 2 3 4 5 6 7 G.W. Critchlow and D.M. Brewis. Review of surface pretreatments for titanium alloys. Institute of Surface Science & Technology, February 1995. Pages 161-172.
  8. ↑ Palmieri et al. Laser Ablation Surface Preparation of Ti-6Al-4V for Adhesive Bonding. NASA Langley Research Center; Hampton, VA, United States, 2012.
  9. ↑ J.I. Ahuir-Torres et al. Influence of laser parameters in surface texturing of Ti6Al4V and AA2024-T3 alloys, September 2017. Optics and Lasers in Engineering. Pages 100-109.
  10. ↑ H. Garbacz et al. The effect of grain size on the surface properties of titanium grade 2 after different treatments. Surface & Coating Technology, May 2017. Pages 13-24.
  11. ↑ "SEMCO Pasa-Jell 107 and 107-M bond enhancement for titanium alloys" (PDF).
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