Patent Description:
This section provides a general summary of the disclosure, and is not a comprehensive disclosure of tis full scope or all of its features.

Engineering plastic and polymer composites, such as fiber reinforced polymers (FRP), are increasingly used for minimizing weight in airframes, auto-bodies, and marine structures in combination with structural metals. Traditional fastening methods, such as mechanical fastening and adhesive bonding, have significant limitations when joining dissimilar materials. For example, mechanical fastening adds weight and process steps, and is difficult to achieve hermetic sealing in some applications. Similarly, adhesive bonding requires surface preparation and long curing time. Moreover, inadvertent disassembly may occur during operation or service, particularly as adhesives suffer thermal and environmental degradation.

Document <CIT> relates to an aromatic imide polymer laminate material comprising at least one aromatic imide polymer film; and at least one metallic foil which is superimposed on and directly bonded to said aromatic imide polymer film.

<NPL> relates to chemical bonding and reaction at metal/polymer interfaces.

Document <CIT> relates to plastic-metal junctions and methods of making the same.

<NPL>, relates to chemical bonding at a plastic-metal interface, including heating a metal and applying pressure to the plastic-metal joint interface.

To address these limitations, the present teachings provide a method of directly joining a polymer to a metal along a joint interface according to claim <NUM>, and a method of directly joining a polymer to a metal along a joint interface according to claim <NUM>, and, in general a robust technique for directly joining polymeric material to metal through the formation of strong chemical bonds of "C-O-M" (where "M" represents an element in the metal to be jointed) at the interface. Previously, researchers have pursued to directly join metal and polymer through the formation of Van der Waals bonds and/or hydrogen chemical bonds at the polymer interface. A very recent scientific research of the inventors showed that chemical bonds "C-O-M" can be developed at the polymer metal interface instead. The "C-O-M" chemical bonds are more reliable and desirable than the hydrogen bonds or Van der Waals bonds at the interface. The unpublished results of the inventors showed that specific conditions (including surface conditions, interfacial pressure, temperature, and time) need to be created at the joint interface for developing enough "C-O-M" chemical bonds. These lead to the development of the present teachings as all the available joining solutions are not suitable for directly joining polymeric material to metal through the formation of strong chemical bonds of "C-O-M" in practical applications.

It should be understood that in order to develop a C-O-M bond, a polymer surface should contain sufficient carbonyl groups (C=O) because these C=O groups will transfer to C-O-M groups when it meets M atoms in the metal under welding condition. In order to make these reactions sufficient, in some embodiments, dirt and grease should be removed from metal surface to achieve intimate atomic contact between the polymer surface and the metal surface during welding.

As described herein, in embodiments wherein the polymer surface do not contains sufficient carbonyl groups, air pockets can be introduced at the joint interface to in-situ form carbonyl groups though the reaction of polymer surface and the air within the air pockets. The intermediate state of carbonyl groups (C=O) on polymer surface will transfer to C-O-M group.

The developed joining method according to the present teachings has unique advantages in terms of welding speed, process control, and joints quality compare to other conventional joining techniques.

According to the principles of the present teachings, as illustrated in <FIG>, a joining system <NUM> and associated method are provided for directly joining polymer <NUM> (e.g. thermoplastic component) to metal <NUM> along a joint interface <NUM> that is capable of a joining speed as high as <NUM>/min by forming C-O-M chemical bonds at the joint interface <NUM>. It should be appreciated that the present system and method can be achieved without the use or need for adhesive between the polymer <NUM> and the metal <NUM>.

In some embodiments, polymer <NUM> can comprise any polymers suitable for forming quality polymer/metal joints, including but not limited to thermoplastics, polymer composites, or other polymers with thermoplastic surfaces. In some embodiments, polymer <NUM> can be generally planar, tubular, or other prefabricated shapes and have a thickness in the range of about <NUM> to <NUM>.

In some embodiments, metal <NUM> can comprise any metal suitable and sufficiently clean for forming quality polymer/metal joints, including but not limited to steels, alloys of titanium, aluminum, alloys of magnesium, copper, metal matrix composites, and the like. Moreover, metal <NUM> can have a thickness in the range of about <NUM> to <NUM>. In some embodiments, metal <NUM> can be generally planar, tubular, or other prefabricated shapes. In some embodiments, metal <NUM> can comprise distributed air pockets along a surface thereof (i.e. aligned with joint interface <NUM>). In some embodiments, these distributed air pockets can be achieved by (<NUM>) adding a layer of porous structure (inducing metal mesh and metal fragments) between the metal and polymer or (<NUM>) producing in-situ distributed air pocket using an associated air pocket forming system (e.g. distributed 3D surface features, grooves, or protrusions) on the metal surface using an appropriate mechanical engraving, electron beam, chemical agent, and/or electrical discharge system. In some embodiments, the depth or height of the air pockets can be greater than <NUM> microns. In some embodiments, the depth or height of the air pockets can be greater than <NUM> microns.

Generally, in some embodiments, joining system <NUM> and the associated method employ a specially designed welding tool system <NUM> to apply a downward compression pressure and localized heating upon an overlapping region <NUM> of joint interface <NUM>. The resultant joint interface <NUM> can be in form of spot, linear, or curvilinear form and can be along an interface surface generally perpendicular to (see <FIG>, and <FIG>) or inclined relative to (see <FIG> and <FIG>) the direction of the applied force, F. In some embodiments, as will be discussed herein, the welding tool system <NUM> can comprise at least one forging pressure applicator <NUM> (14a, 14b) and at least one heating system <NUM>. As illustrated in <FIG>, in some embodiments, the forging pressure applicator <NUM> and the heating system <NUM> can be integrated into a unitary pressure applicator heating tool <NUM>. Accordingly, unitary pressure applicator heating tool <NUM> is thus configured to apply both a downward compression pressure and localized heating from a singular device. It should be understood that according to the principles of the present method, application of the downward compression pressure can be simultaneous or sequentially sequenced with application of localized heating. The welding temperature needs to be reduced to below the polymer melting temperature before <NUM>% of the melted polymer has pyrolyzed by increasing the travel speed of the welding tool or removing the heating tool from the metal surface.

Conversely, with particular reference to <FIG>, in some embodiments, forging pressure applicator <NUM> and heating system <NUM> can be separate, distinct tools, systems, or members and be operated independently, as will be discussed herein. When forging pressure applicator <NUM> and heating system <NUM> are separate, heating system <NUM> can be positioned in a downstream position, that is in a position that is first, relative to a direction of travel, such that localized heat is applied to metal <NUM> and/or polymer <NUM> and then thereafter compression pressure is applied to metal <NUM> and/or polymer <NUM>.

In some embodiments, heating system <NUM> and//or unitary pressure applicator heating tool <NUM> is configured to heat metal <NUM> and consequently polymer <NUM> at overlapping region <NUM> to a predetermined temperature. In some embodiments, heating system <NUM> and//or unitary pressure applicator heating tool <NUM> can comprise a thermal heating system, an induction heating system, a friction system (e.g. (<NUM>) a bar tool frictionally engaging metal <NUM> to heat metal <NUM> via the relative rubbing of the bar against metal <NUM>; (<NUM>) a bar tool frictionally engaging protective metal layer <NUM> adjacent to metal <NUM> to heat protective metal layer <NUM> via the relative rubbing of the bar against metal <NUM>), a high-rate plastic deformation system, an electric resistance system, a high-energy beam system (e.g. energy beam gun), and the like to provide sufficient thermal energy to heat metal <NUM> to a temperature at or above the glass transition temperature (Tg) of polymer <NUM> and lower than the polymer flash ignition temperature of polymer <NUM> or metal melting temperature of metal <NUM>, whichever is lower. In some embodiments, the welding speed should be higher than <NUM>/min to reduce the high temperature duration time to avoid extensive polymer decomposition at the joint interface.

In some embodiments, forging pressure applicator <NUM> (including 14a and 14b) and/or unitary pressure applicator heating tool <NUM> is coupled to a pressure application system <NUM> for applying a predetermined downward compression pressure to forging pressure applicator <NUM> (including 14a and 14b) and/or unitary pressure applicator heating tool <NUM> that is transferred to metal <NUM> and polymer <NUM>. The predetermined downward compression pressure must be high enough to generate intimate atomic contact at joint interface <NUM> between metal <NUM> and polymer <NUM> (e.g., at or above flow stress of softened polymer at joint interface during wielding and lower than the yield strength of polymer matrix <NUM>). In other words, during welding, a layer of polymer at the joint interface is softened at elevated temperatures. The applied compression pressure should be higher than the flow stress of the softened polymer layer.

Accordingly, it should be understood that unitary pressure applicator heating tool <NUM>, singularly, or forging pressure applicator <NUM> in combination with heating system <NUM> provide heating of metal <NUM> at contact location <NUM> up to a desired temperature while simultaneously or nearly simultaneously pressing against metal <NUM> placed above the polymer <NUM> resulting in intimate atomic contact between metal <NUM> and polymer <NUM> and resulting in strong chemical bonding of metal <NUM> and polymer <NUM> at joint interface <NUM>. A 'strong chemical bond' is understood in the art to be formed from the transfer or sharing of electrons between atomic centers and relies on the electrostatic attraction between the protons in nuclei and the electrons in the orbitals.

As specifically illustrated in <FIG>, in some embodiments, a plurality of forging pressure applicators <NUM> can be used to apply sequential and/or prolonged compression pressure. That is, in some embodiments as illustrated in <FIG>, <FIG>, <FIG>, and <FIG>, forging pressure applicators <NUM>, specifically denoted at 14a, can be configured to be movable relative to metal <NUM> and polymer <NUM> in a moving direction, S. In this way, forging pressure applicator 14a can be moved together with unitary pressure applicator heating tool <NUM> and/or heating system <NUM> while remaining in contact with metal <NUM>. Thus, forging pressure applicator 14a can apply a compression pressure upon metal <NUM> and polymer <NUM> after application of heat from heating system <NUM> or unitary pressure applicator heating tool <NUM>. Moving of the components of the present teachings can be achieved using any conventional drive system operable to achieve the desired rate of movement.

Likewise, in some embodiments as illustrated in <FIG>, <FIG>, forging pressure applicators <NUM>, specifically denoted at 14b, can be configured to be stationary relative to metal <NUM> and polymer <NUM>. In this way, forging pressure applicator 14b can be positioned in a singular location in contact with metal <NUM>. Thus, forging pressure applicator 14b can apply a compression pressure upon metal <NUM> and polymer <NUM> at a localized location after application of heat from heating system <NUM> or unitary pressure applicator heating tool <NUM>.

It should be understood, as illustrated in <FIG>, <FIG>, in some embodiments, a plurality of movable forging pressure applicators 14a and/or a plurality of stationary forging pressure applicators 14b can be used to provide extended application of downward compression pressure upon metal <NUM> and polymer <NUM> during initiation and development of the strong chemical bond between metal <NUM> and polymer <NUM>. Moreover, by using one or more stationary forging pressure applicators 14b, compression pressure can be applied at the overlapping region <NUM> and/or joint interface <NUM> during cooling to ensure proper formation of the strong chemical bond at joint interface <NUM>.

In some embodiments, as illustrated in <FIG>, forging pressure applicator <NUM> and/or unitary pressure applicator heating tool <NUM> can comprise a generally elongated configuration. Accordingly, in the interest of brevity, the shapes thereof will be discussed together. However, it should be understood that the present discussion is applicable to the shape of both forging pressure applicator <NUM> (including 14a and 14b) and/or unitary pressure applicator heating tool <NUM>. Therefore, in some embodiments as particularly illustrated in <FIG>, the elongated configuration can comprise a body portion <NUM> having a distal end <NUM>. In some embodiments, distal end <NUM> of body portion <NUM> can be shaped to define a pressure application surface <NUM> having a predetermined shape. In some embodiments, as illustrated in <FIG>, distal end <NUM> can have a generally flat application surface <NUM>. In some embodiments, as illustrated in <FIG>, generally flat application surface <NUM> can comprise generally straight, defined outer edges <NUM>. On the other hand, in some embodiments, generally flat application surface <NUM> can terminate at a rounded edge portion <NUM> (see <FIG>) or an inclined edge portion <NUM> (see <FIG>). In some embodiments, as illustrated in <FIG>, distal end <NUM> can have a generally curved application surface <NUM> that can terminate at a defined outer edge (not shown) or a rounded edge portion <NUM>. With particular reference to <FIG>, it should be appreciated that body portion <NUM> can define any desired cross-sectional shape desirable for applying heat and/or compression pressure upon metal <NUM> and/or polymer <NUM>, including but not limited to cylindrical, rectangular, square, oval, oblong, ellipsoidal, and the like. In some embodiments, the cross-sectional area of application surface <NUM>, <NUM> can be less than a cross-sectional area of body portion <NUM>.

During operation, welding tool system <NUM> is maintained vertical or near vertical with respect to the surface of metal <NUM> during joining. The strong chemical bond (C-O-M) formed at joint interface <NUM> has been shown as a major contributor to good joint strengths observed. As illustrated in <FIG>, joint interface <NUM> is created between metal <NUM> and polymer <NUM>. As evidenced by the rupture tensile test sample illustrated in <FIG>, failure of the test samples at a location other than joint interface <NUM> confirms the robustness of the present strong chemical bond created in accordance with the present teachings. Finally, as illustrated in the graph of <FIG>, X-ray photoelectron spectroscopy result confirm C-O-Al chemical bonds developed at the joint interface <NUM> in accordance with the principles of the present invention for nylon and an Al alloy.

In some embodiments, a protective layer <NUM> is necessary for protecting metal component <NUM> or welding tools when the metal plate <NUM> is too thin, too soft, or too hard. In some embodiments, the protective layer <NUM> can be light alloy plate with thickness of <NUM>-<NUM>. In some embodiments, the heating can be generated by the friction between the welding tool and the protective layer <NUM>.

According to the principles of the present teachings, several advantages and improvements are realized over existing conventional methods. For example, but not limited to, the present teachings provide an easier to operate mechanism and method with fewer parameters to control, a higher joining speed, that is suitable for automation and robotic applications, has consistent joint quality and high joining strengths, that does not require special surface treatment for many polymer/metal combinations (e.g. the polymer surface contains enough carbonyl groups (C=O)), and requires only minimum energy consumption and is environmental-friendly. However, in some embodiments, such as for some polymer/metal combinations (e.g. the polymer surface does have enough carbonyl groups), additional surface treatment (such as surface texturing, carbonyl group grafting techniques) can further improve joint performance under some service conditions by promoting CO-M bonds.

In particular, in some embodiments, distributed air pockets can be introduced at the interface between the metal and polymer to be joined. In some embodiments, these distributed air pockets can be achieved by (<NUM>) adding a layer of porous structure <NUM> (inducing metal mesh and metal fragments) between the metal and polymer or (<NUM>) producing in-situ distributed air pocket (e.g. distributed 3D surface features, grooves, or protrusions) on the metal surface <NUM> using mechanical engraving, electron beam, chemical agent, or electrical discharge. In some embodiments, specific temperature and pressure environments can be applied to enable the reaction between polymer and the air. The joining temperature at the interface need to be above the glass transition temperature (Tg) of polymer <NUM> and lower than the polymer flash ignition temperature of polymer <NUM> or metal melting temperature of metal <NUM>, or melting temperature of distributed air pocket structure, whichever is lower. The pressure at the interface need to be high enough to generate enough flow of softened polymer materials. In other words, during welding, a layer of polymer at the joint interface is softened at elevated temperatures. In some embodiments, the welding speed should be higher than <NUM>/min to reduce the high temperature duration time to avoid extensive polymer decomposition at the interface. The applied compression pressure should be higher than the flow stress of the softened polymer layer and enable the flow of the softened polymer into the distributed air pocket structure. The welding temperature need to be reduced below the polymer melting temperature before <NUM>% of the melted polymer has pyrolyzed.

In some embodiments, the depth or height of the air pockets can be greater than <NUM> microns to trap enough air
In some embodiments, a layer of porous structure <NUM> is inserted into the interface between metal <NUM> and polymer <NUM> prior to the joining.

In some embodiments, a layer of porous structure <NUM> is joined to metal <NUM> first, and then is placed in between metal <NUM> and polymer <NUM> prior to the joining.

In some embodiments, as illustrated in <FIG>, the molten polymer flows into the porous structure <NUM> and react with the trapped air within porous structure <NUM>, forming interim C=O groups. These C=O groups enables the formation of three-dimensionally distributed C-O-M chemical bonds <NUM> between the polymer and metal as vestigial trapped-air is expelled under localized welding pressure, resulting in three-dimensionally intermeshed joint interface (3D ChemBonds). This is evidenced by the produced polymer metal joins illustrated in <FIG>.

In some embodiments, as illustrated in <FIG>, distributed 3D surface features <NUM> was produced by a rotating tool with one or multiple scribe tips <NUM>. The distributed 3D surface features is then placed to against the polymer <NUM> prior to welding, as illustrated in <FIG>. During welding, the molten polymer flows into the distributed 3D surface features <NUM> and react with the trapped air within distributed 3D surface features <NUM>, forming interim C=O groups. These C=O groups enables the formation of three-dimensionally distributed C-O-M chemical bonds <NUM> between the polymer and metal as vestigial trapped-air is expelled under localized welding pressure, resulting in three-dimensionally intermeshed joint interface (3D ChemBonds).

In some embodiments, as illustrated in <FIG>, three-dimensionally distributed C-O-M chemical bonds <NUM> between the polymer <NUM> and metal <NUM> can be produced at the interface of a spot lap joint.

In some embodiments, as illustrated in <FIG>, three-dimensionally distributed C-O-M chemical bonds <NUM> between the polymer <NUM> and metal <NUM> can be produced at the interface of a lap joint. In some embodiments, the angle α is in the range of <NUM>-<NUM> degree.

In some embodiments, as illustrated in <FIG>, strap joints of polymer <NUM> and metal <NUM> can be produced through a third metal layer <NUM>. The bond between metal <NUM> and the metal layer <NUM> can be achieved by currently available joining solutions (such as resistance welding).

Accordingly, in some embodiments, a joining system is provided for directly joining a polymer to a metal along a joint interface through the formation of strong chemical bonds of C-O-M, where M represents an element in the metal to be joined. The system comprising a heating system configured to heat the metal to a predetermined temperature above the glass transition temperature of the polymer and less than a flash ignition temperature of the polymer and less than a metal melting temperature of the metal; and a forging pressure applicator configured to physically contact at least one of the metal and the polymer and apply compression pressure to the joint interface of the metal and the polymer when the metal is above the glass transition temperature of the polymer and less than the flash ignition temperature of the polymer and less than the metal melting temperature of the metal, the forging pressure applicator applying sufficient compression pressure upon the joint interface of the metal and the polymer to generate intimate atomic contact between the metal and the polymer to create the joint interface. The welding temperature reduced to below polymer melting temperature before <NUM>% of the melted polymer has pyrolyzed.

In some embodiments, dirt and grease is removed from the metal surface for forming C-O-M chemical bonds at the joint interface. In some embodiments, the joint interface comprises distributed air pockets between the metal and the polymer for forming three-dimensional distributed C-O-M chemical bonds at the joint interface.

In some embodiments, the forging pressure applicator is configured to physically contact at least one of the metal and the polymer, capture a porous structure there between, and apply compression pressure to the joint interface of the metal, the porous structure, and the polymer.

In some embodiments, the joining system comprises an air pocket forming system configured to form 3D surface features, grooves, or protrusions on a surface of the metal. In some embodiments, these features are produced by mechanical engraving, electron beam, chemical agent, and/or electrical discharge system.

In some embodiments, the forging pressure applicator is separate from and spaced apart from the heating system. In some embodiments, the forging pressure applicator is integrally formed with the heating system as a unitary member, the unitary member configured to heat the metal and apply the compression pressure.

In some embodiments, at least one of the heating system and the forging pressure applicator is configured to be moved relative to the metal and the polymer to create a linear joint interface.

In some embodiments, at least one of the heating system and the forging pressure applicator is configured to be moved relative to the metal and the polymer to create a curvilinear joint interface.

In some embodiments, the forging pressure applicator is configured to be stationary relative to the metal and the polymer to create the joint interface. In some embodiments, the forging pressure applicator comprises at least two forging pressure applicators. In some embodiments, a first of the at least two forging pressure applicators is configured to be moved relative to the metal and the polymer and a second of the at least two forging pressure applicators is configured to be stationary relative to the metal and the polymer. In some embodiments, the at least two forging pressure applicators are configured to be moved relative to the metal and the polymer. In some embodiments, the at least two forging pressure applicators are configured to be stationary relative to the metal and the polymer.

In some embodiments, the forging pressure applicator is configured to apply compression pressure to the metal and the polymer perpendicularly to the joint interface. In some embodiments, the forging pressure applicator is configured to apply compression pressure to the metal and the polymer at an inclined angle relative to the joint interface. In some embodiments, the forging pressure applicator comprises a body portion terminating at a distal end having a pressure application surface configured to physically contact the metal. In some embodiments, the body portion is elongated and the pressure application surface is flat. In some embodiments, the body portion is elongated and the pressure application surface is curved.

In some embodiments, the heating system is selected from the group consisting of a thermal heating system, an induction heating system, a friction system, a high-rate plastic deformation system, an electric resistance system, and a high-energy beam system.

Claim 1:
A method of directly joining a polymer (<NUM>) to a metal (<NUM>) along a joint interface (<NUM>) through the formation of C-O-M chemical bonds, where M represents an element in the metal (<NUM>) to be joined, the method comprising:
heating the metal (<NUM>) to a predetermined temperature above a glass transition temperature of the polymer (<NUM>) and less than a flash ignition temperature of the polymer (<NUM>) and less than a metal melting temperature of the metal (<NUM>);
physically contacting at least a portion of a surface of the metal (<NUM>);
applying compression pressure to the joint interface (<NUM>) of the metal (<NUM>) and the polymer (<NUM>) when a temperature of the metal (<NUM>) is above the glass transition temperature of the polymer (<NUM>) and less than the flash ignition temperature of the polymer (<NUM>) and less than the metal melting temperature of the metal (<NUM>), the step of applying compression pressure to the joint interface (<NUM>) of the metal (<NUM>) and the polymer (<NUM>) generating intimate atomic contact between the metal (<NUM>) and the polymer (<NUM>) to create the joint interface (<NUM>) comprising substantially C-O-M chemical bonds between the metal and the polymer (<NUM>); and
reducing the temperature of the metal (<NUM>) below the polymer melting temperature before <NUM>% of the polymer has pyrolyzed.