Patent Description:
Ultrasonic welding of thermoplastics is widely used in many industries to weld together two parts to form an assembly in a short time without introducing additional consumables such as fasteners, adhesives, or solvents. The ultrasonic welding process is fast, economical and easily automated, and is commonly controlled by time, energy, or weld distance to achieve welds of predicted quality. However, given real-life variations in part dimensions, as well as the limited ability to control the rate of molten material displacement with pneumatically driven ultrasonic welders, none of these control modes can assure the consistency of the weld. The present disclosure is directed to solving these and other problems.

<NPL> already discloses investigations into the residual melt coat thickness as a new characteristic for process optimization in the case of ultrasonic welding including a correlation between the residual melt coat thickness and the weld strength.

Further, <CIT> discloses a method and an apparatus for welding parts by bringing a part into contact with a horn by providing relative motion between the part and the horn. The horn applies high frequency energy to the part. At least one of the weld force, weld speed, horn position, and any combination of the weld force, weld speed, weld amplitude, and horn position can be programmed and varied during welding. For example, these variables can be programmed versus part position for a continuous weld or versus time for a plunge weld.

One aspect of the present invention relates to a method as defined in claim <NUM>.

Another aspect of the present invention relates to a method for producing welds as defined in claim <NUM>.

Additional features and benefits of the present invention are apparent from the detailed description and figures set forth below.

While the disclosure is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, and alternatives falling under scope of the invention as defined by the appended claims.

Referring generally to <FIG>, an exemplary assembly <NUM> includes a first part <NUM> and a second part <NUM> to be welded together using a welding process according to the present invention. The first part <NUM> includes an energy director <NUM> (<FIG>) configured to aid in welding the first part <NUM> to the second part <NUM>. As shown in <FIG>, prior to welding, the first part <NUM> and the second part <NUM> are placed against each other such that the energy director <NUM> abuts an upper surface of the second part <NUM>. The welding process imparts energy (e.g., heat energy, vibration energy, or electromagnetic energy) and pressure (e.g., by clamping the first part <NUM> and the second part <NUM> together) to the first part <NUM> and the second part <NUM> such that the two parts are welded together. The energy director <NUM> is fully consumed, or melted, during the welding process. The welding process used to weld or join the first part <NUM> and the second part <NUM> can be, for example, a servo-controlled ultrasonic welding process, a pneumatic-controlled ultrasonic welding process, a laser welding process, or a hot plate or hot bar welding process.

As shown in <FIG>, the welding process forms a weld joint <NUM> between the first part <NUM> and the second part <NUM>. The weld joint <NUM> has a melt layer thickness <NUM> where a portion of the first part <NUM> and a portion of the second part <NUM> melt during the welding process due to the imparted energy. As discussed in further detail herein, the melt layer thickness <NUM> can be accurately controlled by adjusting one or more weld process parameters (e.g., a weld velocity, a dynamic hold velocity, a dynamic hold distance, an amplitude, a trigger force, a melt-detect percentage, a static hold time, or any combination thereof). As also discussed in further detail herein, the melt layer thickness <NUM> is closely correlated to a strength of the weld joint <NUM>. While the first part <NUM> and the second part <NUM> are shown in <FIG> as having a generally circular shape, more generally, the first part <NUM> and the second part <NUM> can have any suitable shape (e.g., triangular, rectangular, square, polygonal etc.) Moreover, the first part <NUM> and the second part <NUM> can be the same or different shapes or sizes. The first part <NUM> and the second part <NUM> can comprise any suitable material for welding (e.g., a thermoplastic material).

Referring to <FIG>, an illustrative ultrasonic welding machine is described, which can be used for the various implementations of the present disclosure (e.g., to form the assembly <NUM> as shown in <FIG>). For example, this illustrative ultrasonic welding machine can include an ultrasonic welding stack <NUM> that is mounted for controlled vertical movement by a bidirectional, electrically powered linear actuator <NUM> (<FIG>). The stack <NUM> will be described in more detail below in connection with <FIG>. The actuator <NUM> can be mounted within a main housing <NUM>, which also supports an auxiliary housing <NUM> that contains the power supply and electronic controls for the welding press. In a variation of this concept, the housing <NUM> and auxiliary housing <NUM> can be combined into one structure. The workpieces W1 and W2 (<FIG>) to be welded can be mounted in a stationary fixture below the ultrasonic stack <NUM>, and the actuator <NUM> can advance the stack <NUM> downwardly against the upper workpiece W1. The lower end of the stack <NUM> is pressed downwardly against the workpiece W1 to press the upper workpiece W1 against the lower workpiece W2 while applying mechanical vibrations to the workpiece W1 to effect the desired welding that joins the two workpieces W1 and W2 together. The main housing <NUM> is mounted on a frame that includes a vertical column <NUM> extending upwardly from a base <NUM> that carries a fixture for receiving and supporting the workpieces to be welded. The housing <NUM> is typically adjustably mounted on the column <NUM> to allow the vertical position of the entire housing <NUM> to be adjusted for different workpieces. A control panel <NUM> is provided on the front of the base <NUM>.

The ultrasonic welding stack <NUM> (see <FIG>) includes an electromechanical transducer <NUM> which converts electrical energy into mechanical vibrations, a booster <NUM> to alter the gain (i.e., the output amplitude) of the mechanical vibrations produced by the transducer <NUM>, and a horn <NUM> to transfer the mechanical vibrations from the booster <NUM> to the parts to be welded. As shown in <FIG>, the transducer <NUM> can include a connector <NUM> for attaching a high voltage coaxial cable <NUM> that delivers a high-frequency electrical signal for exciting the transducer <NUM>.

This signal can be supplied by a separate ultrasonic signal generator (not shown). An alternative method of connection can also be utilized to permit easier removal and installation of the transducer. The transducer <NUM> can generate the ultrasonic vibrations as a Langevin piezoelectric converter that transforms electrical energy into mechanical movement. Power applied to the transducer <NUM> can range from less than <NUM> Watts up to <NUM> Watts at a typical frequency of <NUM>.

The transducer <NUM> can be made from a number of standard piezoelectric ceramic elements separated by thin metal plates, clamped together under high pressure. When an alternating voltage is applied to the ceramic elements, a corresponding electric field is produced which results in a variation in thickness of the ceramic elements. This variation in thickness induces a pressure wave that propagates through the material and is reflected by the ends of the metal mass of the transducer. When the length of the assembly is tuned to its frequency of excitation, the assembly resonates and becomes a source of standing waves. The output amplitude from a <NUM>-kHz transducer is typically about <NUM> microns (<NUM> inches). This amplitude needs to be amplified by the booster <NUM> and the horn <NUM> to do useful work on the parts W1 and W2. The booster and horn act as an acoustic waveguide or transformer to amplify and focus the ultrasonic vibrations to the work piece.

The primary function of the booster <NUM> is to alter the gain (i.e., output amplitude) of the stack <NUM>. A booster is amplifying if its gain is greater than one and reducing if its gain is less than one. Gains at <NUM>-kHz typically range from less than one-half to about three. The horn <NUM> cannot normally be clamped because it must be free to vibrate and thus only the transducer <NUM> and the booster <NUM> are secured. Thus, a secondary function (and sometimes the sole purpose) of the booster <NUM> is to provide an additional mounting location without altering the amplification of the stack when secured in a press. The neutral or coupling booster is added between the transducer and horn and mounted in the press by a mounting ring which is placed at the nodal point (where the standing wave has minimal longitudinal amplitude).

The horn <NUM> has three primary functions. First, the horn <NUM> transfers the ultrasonic mechanical vibrational energy (originating at the transducer <NUM>) to the thermoplastic work piece (W1 and W2) through direct physical contact, and localizes the energy in the area where the melt is to occur. Second, the horn <NUM> amplifies the vibrational amplitude to provide the desired tip amplitude for the thermoplastic workpiece and welding process requirements. Third, the horn <NUM> applies the pressure necessary to force the weld when the joint surfaces are melted.

The horn is precision machined and is typically designed to vibrate at either <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The higher the frequency, the shorter the acoustic wavelength, and consequently the smaller the horn. The tuning of a horn is typically accomplished using electronic frequency measurement. Horns are usually manufactured from high-strength aluminum alloys or titanium, both of which have excellent acoustical properties to transmit the ultrasonic energy with little attenuation.

There are many different horn shapes and styles depending on the process requirements. Factors which influence the horn design are the materials to be welded and the method of assembly. The horn must amplify the mechanical vibration so that the amplitude is sufficient to melt the thermoplastic workpieces at their interface, and the gain of the horn is determined by its profile. The amplitude at the tip of the horn typically ranges from <NUM> to <NUM> microns peak to peak (<NUM> to <NUM> thousandths of an inch) at <NUM>. In an alternate variation, the horn can be designed so that it takes the form of a booster and combines the functions of stabilization and welding. In this variation, the booster is eliminated and the horn is secured in the press in the position of the booster mounting ring area. As the frequency increases, the vibration amplitude decreases. Higher frequencies are used for seaming of thin materials and delicate parts that do not require a lot of amplitude. Since the horn becomes smaller at higher frequencies, closer spacing can also be achieved.

Plastic welding is the most common application of ultrasonic assembly. To perform ultrasonic plastic welding, the tip of the horn is brought into contact with the upper workpiece W1, as shown in <FIG>. Pressure is applied and ultrasonic energy travels through the upper workpiece, increasing the kinetic energy (or heat) at the contact point of the two workpieces. The heat melts a molded ridge of plastic on one of the workpieces, and the molten material flows between the two surfaces. When the vibration stops, the material solidifies forming a permanent bond.

Although an illustrative ultrasonic weld is discussed above for purposes of the present application, any ultrasound weld can be used for the systems and methods of the present disclosure. Additional discussions of the physical components and control systems of weld processes can be found, for example, in Klinstein, et al. Generally, the most reliable control mode for the ultrasonic weld process is the "weld by distance" option, which is available with most computer-controlled welding equipment. Normally, weld distance is monitored by an encoder and controlled by the processor that determines the movement of the stack. For weld joint designs using an energy director style interface, the optimum weld distance for strength and repeatability of the weld is typically set at a value based on the size and height of the energy director. However, this setting may not guarantee that the weld will be strong and free of excessive flash. For instance, if an energy director (e.g., the energy director <NUM> in <FIG>) is not melted, but rather partially deformed during the weld process, the deformation will be interpreted by the welding process controller as part of the weld distance, even though the movement occurred without melting. The result of this is a "cold weld," during which there is more physical deformation than actual melting of the energy director. Possible reasons for "cold weld" formation include energy director molding inconsistencies, excessive trigger force, part misalignment, incorrect frequency selection, inadequate amplitude and many others. Well-qualified weld processes, utilizing appropriate limits on key process parameters, will catch some of these poorly formed welds. However, data from the welder will often not show this lack of weld, as the energy level, weld time and power level recorded by the process controller may not differ greatly than the same values associated with a good weld.

Alternatively, reaching the programmed weld depth can also result in excessive flash, which is undesirable. Excessive flash formation can occur when the energy director is shorter than expected, as would occur in a part with inadequately filled energy director details (resulting from molding short shots), or with an energy director that has been damaged in handling. In this case the part is collapsed beyond the energy director creating an unintended melt, which results in excessive flash around the joint area. Some of these welds can be identified as a suspect by closely monitoring weld energy set as a secondary weld control. This approach has varying levels of success and is notably dependent on the specific weld setup.

Consistent (e.g., repeatable) welds with high weld joint strength are desirable. In some cases, it is desirable for the weld joint strength to be the same as or greater than the parent material strength. In other words, the weld joint is stronger than the constituent material of the parts that are assembled together. To achieve high-strength weld joints, an operator of the welding system (e.g., the machine in <FIG>) needs to adjust one or more weld process settings. Some methods for optimizing these weld process settings for increased weld strength include destructive testing, such as tensile, bending, pressure testing, and others, adjusting one or more process settings based on the test results, and then repeating the process. Such methods for achieving a desired weld strength are time consuming and inefficient because many sample parts must be made and destructively tested.

In ultrasonic welding applications (e.g., ultrasonic servo-driven welding processes), some of the weld process settings (e.g., weld velocity) can be adjusted to accurately control a melt lack thickness. For example, unlike pneumatic-controlled welding systems, a servo-controller welding system (e.g., that is the same as, or similar to, the welding system shown in <FIG>) can be programmed to assure that an initial melt layer thickness exists prior to starting the downward movement of the press. That is, the press can be held in position upon initiation of welding until a decrease in force is detected. This prevents the deformation of the energy director prior to the application of the weld force from counting towards the weld distance. When the force drop reaches a predetermined value (e.g., programmed as a melt-percentage), this is indicative of the presence of an initial molten layer. Thereafter, downward movement of the press continues.

Additionally, servo-controller ultrasonic welding systems allow the operator to control the propagation of the melt layer during the weld cycle, by precisely varying the velocity of molten material displacement throughout the process, for example. Other weld process settings can be adjusted during the "hold phase" to directly control the flow squeeze rate of the molten material, and thus the thickness of the melt layer. For example, a dynamic hold feature allows controlling of the squeeze flow rate of molten material and the collapse distance after ultrasonic vibration has ceased. This allows for precise control of the material displacement during recrystallization and solidification at the end of the weld. In other words, the thickness of the melt layer can be accurately predicted based on the values of one or more weld process settings. Accordingly, it would be advantageous to establish a relationship between the melt layer thickness and the weld strength so that the weld strength of a given weld can be predicted based on the predicted melt layer thickness.

Referring to <FIG>, a method <NUM> for optimizing a welding process to produce weld joints having a predetermined (e.g., sufficient) weld strength is illustrated. The inventors have discovered that there is a close correlation between the melt layer thickness of the weld joint and the failure load, or strength, of the weld joint. For example, an increased melt layer thickness of the weld joint can correspond to an increase in the failure load (strength) of the weld joint. Because one or more weld process settings can be adjusted to accurately control the melt layer thickness, it is advantageous to correlate a desired strength of the weld joint (e.g., that is equal or very close to the parent material strength) with a melt layer thickness, and in turn form production assemblies (e.g., for mass production) with high strength weld joints by adjusting the weld process settings that control melt layer thickness.

Step <NUM> of the method <NUM> includes forming a first plurality of sample assemblies (e.g., that are the same as, or similar to, the assembly <NUM> of <FIG>) by welding parts together (e.g., parts that are the same as, or similar to, the first part <NUM> and/or the second part <NUM> of <FIG>) using a welding process (e.g., a servo-controller ultrasonic welding process that uses an ultrasonic welding machine that is the same as, or similar to, the machine of <FIG>). Sample assemblies are used for testing (e.g., destructive testing) and are not mass produced for sale to consumers. The first plurality of sample assemblies can include any suitable number of sample assemblies (e.g., two, ten, fifteen, fifty, one-hundred, etc.).

In one non-limiting, exemplary implementation of step <NUM>, a first plurality of sample assemblies can be formed using the weld process settings shown below in Table <NUM>. More generally, any suitable ultrasonic amplitude value, trigger force value, melt-detect percentage value, weld distance value, weld velocity value, static hold value, or any combination thereof can be used for the welding process to form the first plurality of sample assemblies during step <NUM>. The weld velocity value can be a constant weld velocity (e.g., about <NUM>/s, about <NUM>/s, about <NUM>/s, etc.) or a linearly profiled weld velocity (e.g., linearly increasing from <NUM>/s to <NUM>/s, linearly decreasing from <NUM>/s to <NUM>/s, etc.).

Step <NUM> of the method <NUM> includes varying the value of a first weld process setting subsequent to forming the first plurality of sample assemblies during step <NUM>. The varied weld process setting can be, for example, the weld velocity, a dynamic hold distance, or a dynamic hold velocity. In one non-limiting, exemplary implementation of step <NUM>, the value of the weld velocity is varied. For example, if the weld velocity has a linearly increasing profile between <NUM>/s and <NUM>/s during step <NUM>, the weld velocity can be varied to a weld velocity with a linearly decreasing profile between <NUM>/s and <NUM>/s during step <NUM>. During step <NUM>, the other weld process settings (e.g., ultrasonic amplitude, trigger force, melt-detect percentage, weld distance, static hold time, dynamic hold distance, dynamic hold velocity, etc.) of the weld process are held constant (i.e., are not varied).

Step <NUM> includes forming a second plurality of sample assemblies according to the weld process using the varied first weld process setting from step <NUM>. In other words, the first weld process setting has a first value when the first plurality of sample assemblies is formed during step <NUM>, and a second value that is different than the first value when the second plurality of sample assemblies are formed during step <NUM>. The second plurality of sample assemblies formed in step <NUM> can include the same number of assemblies as the first plurality of sample assemblies formed during step <NUM>, or a different number of sample assemblies. However, the first plurality of sample assemblies and the second plurality of sample assemblies are the same assembly (e.g., both include the same first part <NUM> and second part <NUM> of <FIG>).

As shown in <FIG>, step <NUM> and step <NUM> can be repeated one or more times to vary the weld process setting a plurality of times (e.g., two times, six times, ten times,. n times), thereby forming a plurality of sample assemblies for each variation of the weld process setting. For example, in one non-limiting, exemplary implementation of the method <NUM>, steps <NUM> and <NUM> are repeated three times such that five different pluralities of sample assemblies are formed. Step <NUM> of the method <NUM> includes measuring a melt layer thickness of a weld joint in each of the plurality of sample assemblies formed during step <NUM> and step <NUM>. Specifically, step <NUM> includes cross-sectioning at least one of the first plurality of sample assemblies formed during step <NUM> and cross-sectioning at least one of the second plurality of sample assemblies formed during step <NUM>. These cross-sections can then be polished and examined (e.g., by a human user) under a microscope to measure the melt layer thickness of the weld joint.

In some implementations, step <NUM> includes averaging the measured melt layer thickness across two or more sample assemblies in one or both of the first plurality of sample assemblies from step <NUM> and the second plurality of sample assemblies from step <NUM>. For example, if the first plurality of sample assemblies formed during step <NUM> and the second plurality of sample assemblies formed during step <NUM> each include ten sample assemblies, step <NUM> can include cross-sectioning five sample assemblies from each group and averaging the measured melt layer thickness. Averaging the measured melt layer thickness can aid in providing more accurate results and account for uncontrollable variations in the welding process and/or the sample assemblies.

Referring generally to <FIG>, in some implementations of the method <NUM>, steps <NUM>-<NUM> are repeated three times such that five different pluralities of sample assemblies are formed with the varied first weld process setting. In the non-limiting examples of <FIG>, the weld velocity is varied. Referring to <FIG>, an exemplary cross-sectioned sample assembly <NUM> including a first part <NUM>, a second part <NUM>, and a weld joint <NUM> is shown. The weld joint <NUM> has a melt layer thickness <NUM> that can be measured by examining the cross-sectioned sample assembly <NUM> (e.g., under a microscope). In this example, the sample assembly <NUM> is one of a first plurality of sample assemblies that were formed according to the welding process with a weld speed of about <NUM>/s (e.g., during step <NUM>) and the measured melt layer thickness <NUM> is about <NUM> microns.

Referring to <FIG> an exemplary cross-sectioned sample assembly <NUM> including a first part <NUM>, a second part <NUM>, and a weld joint <NUM> is shown. The weld joint <NUM> has a melt layer thickness <NUM> that can be measured by examining the cross-sectioned sample assembly <NUM>. In this example, the sample assembly <NUM> is one of a second plurality of sample assemblies that were formed according to the welding process with a weld speed of about <NUM>/s (e.g., during step <NUM>) and the measured melt layer thickness <NUM> is about <NUM> microns.

Referring to <FIG> an exemplary cross-sectioned sample assembly <NUM> including a first part <NUM>, a second part <NUM>, and a weld joint <NUM> is shown. The weld joint <NUM> has a melt layer thickness <NUM> that can be measured by examining the cross-sectioned sample assembly <NUM>. In this example, the sample assembly <NUM> is one of a third plurality of sample assemblies that were formed according to the welding process with a weld speed of about <NUM>/s (e.g., when step <NUM> is repeated a first time) and the measured melt layer thickness <NUM> is about <NUM> microns.

Referring to <FIG> an exemplary cross-sectioned sample assembly <NUM> including a first part <NUM>, a second part <NUM>, and a weld joint <NUM> is shown. The weld joint <NUM> has a melt layer thickness <NUM> that can be measured by examining the cross-sectioned sample assembly <NUM>. In this example, the sample assembly <NUM> is one of a fourth plurality of sample assemblies that were formed according to the welding process with a weld speed having a linearly decreasing profile between <NUM>/s and <NUM>/s (e.g., when step <NUM> is repeated a second time) and the measured melt layer thickness <NUM> is about <NUM> microns.

Referring to <FIG> an exemplary cross-sectioned sample assembly <NUM> including a first part <NUM>, a second part <NUM>, and a weld joint <NUM> is shown. The weld joint <NUM> has a melt layer thickness <NUM> that can be measured by examining the cross-sectioned sample assembly <NUM>. In this example, the sample assembly <NUM> is one of a fifth plurality of sample assemblies that were formed according to the welding process with a weld speed having a linearly increasing profile between <NUM>/s and <NUM>/s (e.g., when step <NUM> is repeated a third time) and the measured melt layer thickness <NUM> is about <NUM> microns.

Referring back to <FIG>, step <NUM> of the method <NUM> includes measuring a failure load in the first plurality of sample assemblies from step <NUM> and the second plurality of sample assemblies from step <NUM> (and any additional pluralities of sample assemblies formed when step <NUM> and step <NUM> are repeated one or more times). The failure load is the load (e.g., applied forced) at which a sample assembly fails (e.g., fractures) under stress (e.g., tensile stress). Thus, the failure load is indicative of the tensile strength of the sample assembly. In some implementations, step <NUM> includes applying a tensile strength to the sample assemblies (e.g., using a pull test fixtures). Further, in some implementations, step <NUM> includes determining an average and/or standard deviation for the measured failure load across multiple ones of each of the plurality of sample assemblies being tested (e.g., averaging the failure load for fifteen sample assemblies in each plurality of sample assemblies).

Referring back to the example implementation of step <NUM> illustrated in <FIG>, sample assemblies in each of the plurality of sample assemblies were subjected to a tensile test to determine the failure load. Specifically, fifteen samples from each of the plurality of sample assemblies were subjected to the tensile test. Table <NUM> (below) summarizes the relationship between weld velocity, melt layer thickness, and failure load.

Step <NUM> of the method <NUM> includes selecting one melt layer thickness from the plurality of measured melt layer thicknesses (step <NUM>) that corresponds to a predetermined weld strength. As discussed herein, the inventors have discovered that the melt layer thickness of a weld joint closely correlates to the strength of the weld joint. Each of the plurality of sample assemblies are associated with (<NUM>) a measured melt layer thickness (step <NUM>) and (<NUM>) an average failure load (step <NUM>). As discussed herein, the average failure load is indicative of the strength of the weld joint. If the average failure load is equal to a parent material strength of the parts comprising the sample assemblies (e.g., the first part <NUM> and/or the second part), this means that the strength of the weld joint is at least as strong as the parent material strength. A weld joint strength that is at least as strong as the parent material strength is desirable in welding applications. Thus, by examining the measured average failure loads (from step <NUM>) and determining which measured average failure load(s) correspond to a predetermined weld strength (e.g., that is equal to or greater than the parent material strength), an associated melt layer thickness (from step <NUM>) can be selected.

In some implementations, step <NUM> can include generating a graph that plots the measured average failure load and the measured melt layer thickness (e.g., on a y-axis) versus the varied first weld process setting (e.g., on an x-axis). Referring to <FIG>, an exemplary graph illustrating the relationship between melt layer thickness, failure load, and first weld process setting (weld velocity) for the examples shown in <FIG> is illustrated. As shown, the measured average failure load and the measured melt layer thickness are linearly related to one another. That is, as the melt layer thickness increases, the average failure load (weld strength) increases. However, at some point as these values continue to increase, the average failure load approaches or becomes equal to the parent material strength of the sample assemblies (e.g., the material strength of the first part <NUM> and/or the second part <NUM> of assembly <NUM>). At this point, any further increase in melt layer thickness will not impact the overall strength of the assembly given the limitations of the parent material strength. Thus, it is generally desirable to select the melt layer thickness whose associated failure load is equal to or very close (e.g., to within <NUM>% or <NUM>%) to the parent material strength.

For example, in the example of <FIG>, the melt layer thickness (<NUM> microns) of the fifth plurality of sample assemblies (<FIG>) is selected because the associated average failure load (<NUM> N) is the greatest value on the graph. This result was created using the profiled weld velocity that linearly increases from <NUM>/s to <NUM>/s. A slower velocity during melt initiation and a faster velocity in the middle and end of the weld increases weld strength, shortens weld time, and reduces surface marking. In other words, a linearly increasing weld velocity profile causes generation of a larger, more consistent melt layer.

Alternatively, the predetermined weld strength used for selecting the melt layer thickness in step <NUM> can be a predetermined percentage of the parent material strength. For example, the predetermined percentage can be between about <NUM>% and about <NUM>% of the parent material strength (e.g., about <NUM>% of the parent material strength, about <NUM>% of the parent material strength, about <NUM>% of the parent material strength, about <NUM>% of the parent material strength, about <NUM>% of the parent material strength, about <NUM>% of the parent material strength, about <NUM>% of the parent material strength, at least about <NUM>% of the parent material strength, at least about <NUM>% of the parent material strength, at least about <NUM>% of the parent material strength, at least about <NUM>% of the parent material strength, at least about <NUM>% of the parent material strength, etc.).

Step <NUM> of the method <NUM> includes forming a production assembly using the welding process settings associated with the selected melt layer thickness from step <NUM>. Unlike the sample assemblies described herein, a production assembly is a welded assembly that is produced (e.g., mass-produced) for consumers rather than for testing purposes (e.g., destructive testing). As described herein, various weld process settings can be adjusted (e.g., weld velocity, dynamic hold distance, dynamic hold velocity, etc.) such that the production assembly has a weld joint with a melt layer thickness that substantially the same as the selected melt layer thickness from step <NUM> (e.g., the melt layer thickness of the production assembly is within ± <NUM>%-<NUM>% of the selected melt layer thickness from step <NUM>). As such, the production assembly will have the predetermined weld strength.

In some implementations, steps <NUM>-<NUM> of the method <NUM> can be repeated using the selected melt layer thickness from step <NUM> to vary another weld process setting. For example, if the value of the weld velocity is varied during step <NUM>, the method <NUM> can be repeated one or more times to also vary additional weld setting parameters such as, for example, dynamic hold velocity and dynamic hold distance.

Referring to <FIG>, an exemplary graph is illustrated showing a relationship between force and distance (see y-axis) plotted against time (see x-axis). In the particular examples described herein, the profiled weld velocity between <NUM>/s and <NUM>/s produced the best results. As shown by the graph in <FIG>, the application of moderate forces at the later stages of the process generated a steady linear displacement rate. A steady melt rate creates a homogenous molecular structure and stronger weld.

Referring to Table <NUM> below, the method <NUM> can be repeated using the weld velocity associated with the selected melt layer thickness from step <NUM> (in this example, <NUM>/s to <NUM>/s) while varying the dynamic hold distance weld process setting (e.g., between <NUM> microns and <NUM> microns) and/or the dynamic hold velocity weld process setting (e.g., between <NUM>/s and <NUM>/s). Step <NUM> can then be repeated to measure failure loads for each plurality of sample assemblies formed using the varied weld process settings.

As shown in Table <NUM>, all sample assemblies formed using the linearly profiled <NUM>/s to <NUM>/s weld velocity failed through the parent material regardless of the dynamic hold velocity value and/or the dynamic hold distance value.

Referring to <FIG>, an exemplary graph is illustrated showing force versus distance for a <NUM> dynamic hold distance, a <NUM>/s dynamic hold velocity, and a <NUM>/s dynamic hold velocity. Referring to <FIG>, an exemplary graph is illustrated showing force versus distance for a <NUM> (<NUM> microns) dynamic hold distance, the <NUM>/s dynamic hold velocity, and the <NUM>/s dynamic hold velocity. The graphs in <FIG> and <FIG> show that a weld formed with a <NUM>/s dynamic hold velocity had, in general, a steeper slope of the force/time curve during the dynamic hold cycle than a weld formed with a <NUM>/s dynamic hold velocity.

Referring to <FIG> an exemplary cross-sectional image of a sixth sample assembly <NUM> including a first part <NUM>, a second part <NUM>, a weld joint <NUM>, and a melt layer thickness <NUM> is shown. The sixth sample assembly <NUM> of <FIG> was formed using a <NUM>-micron dynamic hold distance and a <NUM>/s dynamic hold velocity.

Referring to <FIG>, an exemplary cross-sectional image of a seventh sample assembly <NUM> including a first part <NUM>, a second part <NUM>, a weld joint <NUM>, and a melt layer thickness <NUM> is shown. The seventh sample assembly <NUM> of <FIG> was formed using a <NUM>-micron dynamic hold distance and a <NUM>/s dynamic hold velocity.

Referring to <FIG>, an exemplary cross-sectional image of an eighth sample assembly <NUM> including a first part <NUM>, a second part <NUM>, a weld joint <NUM>, and a melt layer thickness <NUM> is shown. The eighth sample assembly <NUM> of <FIG> was formed using a <NUM>-micron dynamic hold distance and a <NUM>/s dynamic hold velocity.

In some implementations, the method <NUM> can further include examining (e.g., using a microscope) cross-sections of sample assemblies (e.g., that are the same as, or similar to, <FIG>) formed using the varied dynamic hold distance and/or dynamic hold velocity. In the examples of <FIG>, the melt layer thickness remained unchanged after reaching a minimum dynamic hold distance of <NUM> microns regardless of increased dynamic hold distance values. As material solidifies, further increase in force required to gain additional weld collapse does not produce any added material displacement, but instead results in increased residual stress in the weld joint. These results (<FIG>) show that some material displacement occurs during the dynamic hold phase, resulting from moderate force application to the molten material is beneficial to weld strength and a small pull strength standard deviation. As material solidifies, a further increase in force to gain additional hold collapse does not produce any added material displacement and results in increased residual stress in the weld.

Associating a specific weld velocity profile and dynamic hold settings with formation of a homogeneous melt layer in the interface of the assembly can facilitate selection of optimum welding parameters. Because the thickness of the melt layer closely correlates with the strength of the weld join, the thickness of the melt layer is a major predictor of the resulting weld quality. The capabilities of servo-driven ultrasonic welding machines in controlling material flow during every stage of the welding cycle enable an operator to empirically establish a defined range of melt layer characteristics correlated to known weld strength. This allows the operator to reuse the best velocity profile and dynamic hold parameters to generate an optimum melt layer thickness for a given joint geometry. Considering high repeatability and accuracy of servo-driven ultrasonic welders, maintaining these settings in manufacturing process should result in anticipated melt layer thickness and joint strength. The methods described herein for selecting weld process settings and controlling the welding process offer users a more robust way to assure weld quality in manufacturing operations.

While the method <NUM> has been described herein as being used with a servo-controlled ultrasonic welding process, more generally, any of the methods disclosed herein can be used with any other welding process, such as, for example, a non-servo-controlled ultrasonic welding process, a pneumatically-driven ultrasonic welding process, a laser welding process, an infrared welding process, or a hot plate welding process.

Claim 1:
A method for determining welding parameters to produce a weld joint (<NUM>) having a predetermined strength, the method comprising:
forming by a servo-controlled ultrasonic welding process a plurality of sample assemblies using one of a weld velocity or a dynamic hold velocity as a varying weld process setting;
measuring a plurality of melt layer thicknesses (<NUM>) of weld joints (<NUM>) for the plurality of sample assemblies (<NUM>);
measuring a plurality of failure loads of weld joints (<NUM>) for the plurality of sample assemblies (<NUM>), each of the measured plurality of failures loads being associated with one of the measured plurality of melt layer thicknesses (<NUM>);
selecting a first failure load from the plurality of measured failure loads responsive to determining that the first failure load corresponds to a predetermined weld strength; and
selecting a first melt layer thickness (<NUM>) from the plurality of measured melt layer thicknesses (<NUM>) that is associated with the selected first measured failure load,
wherein measuring the plurality of failures loads includes applying a tensile strength test to at least some of the plurality of sample assemblies (<NUM>).