Essay: Ultrasonic welding as a technology

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Literature Review

Introduction

Over the years, in terms of the materials used in building aircraft and other aerospace structure, the aviation industry has evolved from wood and fabrics used by the Wright brothers to metals and composite materials seen today in commercial aircrafts such as Boeing 787 which is made up of 50% composites (Mraz 2003). According to Standridge’s work in 2014, 70% of an aircraft’s weight was once made of aluminum, however, a standard jet constructed today is made of around 20% pure aluminum and most of the non-critical structural material (panels and aesthetic interiors) now consist of even lighter-weight carbon fiber reinforced polymers (CFRPs) and honeycomb materials. Although metals like aluminum and titanium are still widely used in crucial parts (fuselage) of aircrafts, composite materials are also expected to dominate this use of metals over the next decades due to their great mechanical properties relative to their density (Mraz 2003). As metals and composites are still being used in making aircrafts, the need for joints between them becomes obvious for a lighter and stronger build of aircrafts. Currently achieved through mechanical fasteners including rivets and bolts or the use of adhesive, joints between metals and composites may also be achieved through Ultrasonic Welding (USW). Ultrasound waves possess the ability to pass through plastic materials while creating a weld at the joint site of thermoplastics. Until recently, most research has only been done for USW of similar materials but now researchers are trying to explore the application of USW in joining composites together with metals (Balle et al 2009; Wagner et al 2012; Satpathy and Sahoo 2016).

This paper will review ultrasonic welding as a technology, the working principle of an ultrasonic welders, its industrial application and factors that affect its functionality. Joints made from ultrasonic welding and tests results of welding parameters from previous researchers will be critically analyzed and discussed.

Joining Techniques

As previously mentioned, different joining methods are used by engineers for the creation of a joint between similar or dissimilar materials. These methods vary therefore are implemented based on the property and functionality of the material to be joined. Adhesive, rivets, screws, welding, nuts and soldering are but some of the possible options available to engineers. With such a long list of possible joining techniques, choosing the right one for your material becomes a not so obvious decision. A deeper understanding of each class of techniques can serve to aid in one’s selection. Joining techniques ae divided into three main classes: Mechanical, chemical and thermal.

Mechanical Fasteners

Mechanical fasteners are devices used to mechanically join two or more parts together. The use of mechanical fasteners always comes down to the possible need for easy assembly and disassembly of the parts to be joined. Mechanical fasteners include bolts, screws, nuts, rivets, etc. As seen in Dr. Ala Hijazi’s lecture note (Hijazi), mechanical fasteners can be subdivided into permanent (such as rivets in which removing them will cause damage to the part involved) and non-permanent (such as screws, bolts and pins). Their greatest advantage attached to using mechanical fasteners is the easy assembly and disassembly it allows for. John Sprovieri said: “Mechanical fasteners have always been the dominant joining method, and they will remain that way despite competition from adhesive bonding and welding. Many manufacturers simply don’t trust other joining technologies” (Sprovieri 2002). On the other hand, the biggest disadvantage faced is low resistance to vibration (Sprovieri 2002).

Chemical Adhesives

Adhesive bonding is considered one of the oldest bonding processes. In the 1960s, the aerospace industry started using adhesive bonding technology as they were convinced that it was an optimal solution for the manufacture of resistant and lightweight structures (Simon, 1994). Adhesives are used to fasten two surfaces together with the aid of an adhesive, a substance capable of holding two or more substrates together, usually producing a smooth bond between them. This joining technique involves the use of glues, epoxies, or various plastic agents that bond by evaporation of a solvent or by curing a bonding agent with heat, pressure, or time (L. Liu 2010).

Advantages of using this technique include: creation of a smooth joint surface, large area of connection, sealed joints preventing leaks and resistance to galvanic corrosion in the case of dissimilar materials. One should know that as adhesives are used to create permanent joints, disassembly is not easy and can cause damage to the adhered. Other disadvantage include: Adhesives are perishable, they are susceptible to environmental degradation and they require proper surface preparation to produce a strong and durable bond. (Ehrhart et al. 2013)

Thermal Joining methods

As the name implies, this joining technique is achieved with the presence of heat energy. Thermal joining includes welding, soldering and brazing, it is mostly effective on metals with relatively close melting point. EN 14610 defines welding as a “permanent connection of components through application of heat and/or pressure”. The components are connected either by melting or by heating and by applying additional forces (pressure). No other joining process allows such resilient and dense connections with minimal space requirements. There are more than 100 different welding processes depending on the specifics of heat input and pressure application. Besides the usual application of welding as a joining method for metals, there is now an increase in the importance of welding processes in the joining of glass and plastics. Amongst the many types of welding are Arc welding, spot welding, laser welding, ultrasonic welding and electromagnetic welding.

The main advantage this technology brings is the creation of permanent and quality bonds, it is also considered as the most economical way of joining in terms of material usage and fabrication cost. In the case of ultrasonic welding and resistance welding, no fillers are necessary, so no cost is associated with consumables. Of course, once created, welded joints cannot be disassembled. Other disadvantages of thermal joining methods include: high energy cost, high danger risk and being mostly restricted to similar materials.

Ultrasonic Welding Technology

In the 1930s before USW was patented, it was used for grain refinement in molten metals and for the enhancement of resistance welding and by the 1940s, its application was seen in conjunction with arc welding. Ultrasonic metal welding (USMW) process was first demonstrated in the 1950s where it was discovered that sound waves had the ability to create a weld between metals without melting them. Towards the end of the 1950s, UW was used for joining plastics, but it wasn’t until 1965 that Robert Soloff and Seymour Linsley got awarded the patent of ultrasonically welding rigid thermoplastics together (B. Cary 1998).

Sollof, who was then a laboratory manager at Branson instrument accidentally created a weld between two scotch tape dispensers. After continuous testing, he concluded sound waves had the ability to travel between plastic materials and still allowed the welding of the joint area of the thermoplastics. The first application of his discovery was at the toy company “Ideal Toy co” where he showcased the welding ability of an ultrasonic probe attached to an old drill press (Lisa 2017).
This was a revolutionary game changer for companies with products containing high amounts of plastic as it meant that plastics could now be joined with a significantly less amount of heat and energy.

Ultrasonic welding can be simply summarized as a joining technique that uses high-frequency sound waves to achieve a weld between materials. This welding technique locally applies ultrasonic vibrations to workpieces that are being held together by a clamping force or put under pressure in order to create a solid-state joint (Greitmann et al. 2003). Not only does it use high-frequency at relatively low temperatures while requiring less energy than the conventional welding methods, it is extremely fast, more cost-effective and environmentally friendly as it neither requires additive substance including fillers and adhesives nor does it create waste from solvents (Siddiq and Ghassemieh 2008). It is commonly used for joining plastics and especially useful in joining dissimilar materials. Though initially used in creating joints between similar materials like metal-metal joints or polymer-polymer joints, the technology advanced and joints between dissimilar materials were also created. These included metal-glass, metal-ceramic, plastic-glass and of recent, metal-composite joints (Greitmann et al. 2003). More Joints between dissimilar materials for example metals with glass or metals with ceramics were successfully performed at the Institute of Material Science, University of Kaiserslautern (WKK) (Wagner et al 2012).

Components and Working Principle

The main components of an ultrasonic metal welding system can be seen in fig. 1. In fig. 1, the ultrasonic generator (1) converts the 50Hz main voltage into a high-frequency alternating voltage of 20-50KHz depending on the power class of the generator used. In the converter (2), this applied electrical signal is transformed into mechanical oscillations of the same frequency by a reversed piezoelectric effect, hence, the name piezoelectric transducer which the converter is sometimes referred to as. The oscillation amplitude in the welding zone is achieved by an appropriate design of the booster (3) which serves as a modifier either amplifying or reducing the vibrational amplitude of the vibration to be sent to the sonotrode (4) this amplitude typically ranges between 5μm and 40μm and it operates parallel to the joining area in the case of metal welding. During the high-frequency process, the joining parts (5) are pressed pneumatically on an anvil (6) with a clamping force (7) perpendicular to the welding zone and ranging between 100 N and 1500 N (Gutnik et al 2002; Bakavos et al 2010; Wagner et al 2012).

Figure 1 Schematic diagram of USMW system. ( Source: Wagner et al 2012).
When both the machine components are arranged differently and the sonotrode of the welder is designed appropriately as seen in fig. 2 (Wagner et al 2012).

Figure 2 Variants of USMW methods. (Source: Wagner et al 2012).

Forms of Ultrasonic Welding

There are two base USW methods and they depends solely on the direction of oscillation relative to the surface of the joining parts and the nature of the bond to be made. They are: Ultrasonic Metal Welding (USMW) and Ultrasonic Plastic Welding (USPW).

Ultrasonic Metal Welding (USMW)

This technique involves the application of high-frequency vibrations to metallic specimens to hold them together. Its process first involves applying pressure to the workpieces, then subjecting the workpiece to vibrational oscillations parallel to the welding joint interface to create a solid-state weld. This parallel movement not only increases the contact but aids in dispersing both contaminants and an oxide layer due to the scrubbing motion (Lee 2013; Al-Sarraf 2013) as shown in Figure 3. This type of joining can be used for similar and dissimilar metallic materials. The bonding nature of USW in metals is solid-state welding, this means that no fusion or melting of joint is required between workpieces. USMW is used in different fields, such as automotive, electronic components, aviation and ship manufacturing (Ganesh 2013).

Ultrasonic Plastic Welding (USPW)

In the case of polymers, their bonding nature depends on them melting. High-frequency mechanical vibrations in the joining zone creates heat which melts the thermoplastic polymers and after cooling, a weld is created. In USPW, oscillation is perpendicular to the welding zone as illustrated in fig 3 (Troughton 2008).

The welded samples are concurrently pressed together under static force and this force is very important for reliable contact between the sonotrode and the welded workpieces. This force also increases the mechanical energy concentration in the welding region (Gutnik et al 2002).

Figure 3: Comparison between USMW and USPW. (Source: Balle et al 2012)
Both forms of ultrasonic welding can be seen in multiple industries in particular electronics, packaging, medical, aerospace and automotive.

Industrial Applications and Advantages

Aerospace and Automotive industry
The aircraft and automotive industry and engineering in general require weight reduction in order to reduce energy consumption. This reduction can be achieved by using lightweight metals such as titanium, magnesium and aluminum alloys as well as fiber-reinforced polymers (FRP)composites in engineering structures (Wagner et al 2013). Traditional methods include the use of mechanical fasteners which adds to the total mass of the craft hence making it unsuitable.

Electronic and Plastic industries

Due to the continuous miniaturization in industries, a joining process meriting the functionality of miniaturized plastic parts is crucial. Hence the use of ultrasonic welding for joining these plastics rather than the traditional methods due to its use of low thermal and mechanical loads and its high positioning accuracy (Michaeli et al 2008). Unlike mechanical fastening that can damage small parts due to the huge increase in temperature from the drill forming, electronic parts such as diodes and semiconductors, can be assembled using the ultrasonic joining as only a small rise in temperature occurs during the ultrasonic joining process which doesn’t cause any damage to the parts (Troughton 2008).
Medical industry
One of the advantages of UW is that it is both environmentally friendly and clean as the process does not introduce contaminants therefore reducing the chance of infection. Ultrasonic welding devices can be specialized for use in clean rooms hence their introduced application in many medical items and textiles, such as hospital wears, medical chip tests, sterile clothing, masks and textiles. Since Ultrasonic welders can be highly automated, allowing no interference in the biocompatibility of parts, an increase in part quality and decrease in production costs can be realized (Plastics Design Library 1997).

Packaging Industry
Nowadays, the ultrasonic joining technique is used to package many common items in the food industries, such as milk and juice containers. These containers were made from glass and sealed by aluminum foil using the ultrasonic joining because it is quick, healthy and capable of producing hermetic seals. The paper parts to be sealed are coated with plastic, generally polypropylene or polyethylene, and then welded together to create an airtight seal. (Grewell et al 2013).
Other advantages include:
The ultrasonic device is both compact and easily automated. It can also weld through contaminants and oxides that result from the friction.
The ultrasonic energy is generated at the interface zone rather than the top surface as in other welding processes, such as friction stir welding. Hence, the surface deformation or damage is minimal (Patel 2011; Ganesh 2013).
It does not generate fumes, flames, or sparks and, thus, is considered environmentally friendly and a clean joining process (Wagner et al 2012).

Limitations of the Technology
The clearest disadvantage or limitation of the USW technology has to be the fact the thickness of workpieces is currently limited to 3 mm due to the power of equipment being specified (Wagner et al 2012). Thus, the thickness of a workpieces is directly proportional to the ultrasonic power applied for metals. Additionally, in the case of a thicker workpiece, much of the vibrational energy is absorbed in the upper sample where the sonotrode is positions, thus causing the oscillation in the bonding zone to be insufficient to produce a joint. Simultaneously, the upper workpiece cannot withstand the increasing driving force pressure being applied to it. High hardness, stiffness and damping factor of a work piece are material properties to be considered before choosing UW as a weld technique these properties affect the working basis of an ultrasonic welder by decreasing the amount of mechanical energy that is delivered to the bonding region (Gutnik 2002). There is also a high tendency of fatigue of the components and workpieces of an ultrasonic welder due to the cyclic loading a workpiece undergoes during the welding process (Omotayo 2018).
Due to the requirement for at least one of the welded parts to be in contact with the entire surface of the sonotrode, ultrasonic welding can be limited to the weld size. It is best suited to smaller scale weld applications such as thermoplastic inserts for example;
Ultrasonic welding is limited in its applicability to low modulus thermoplastics. Low modulus dissipates the mechanical stresses and heat development is therefore poor (Ageorges, et al., 2001). This is also true for high-thickness substrates.
There are limitations to the geometries that can be welded due to the requirement for the sonotrode access to the weld surface.
The welding technique is a ‘contacting’ process which may induce unwanted stresses into the substrates.
Composite/Metal joints
Joints between composites and metals were initially considered impossible as the difference between the physical and chemical properties of both materials were so variant. Haddadi and Abu-Farha (2015) reported that reaction at the interface and material incompatibilities are the most challenging barriers in joining dissimilar materials.
Graham et al. (2014) reported that the main setbacks of using mechanical fastener is derived from drilling operations that disrupt fiber continuity thus leading to a load carrying capability reduction of the material. This is due to delamination and stress concentration. (Lambiase et al. 2016) also reported that mechanical joining methods such as Self-Pierce Riveting and Mechanical Clinching, bring with them the disadvantages of producing high strain during welding. Moreover, metallic fasteners are not only prone to galvanic corrosion, but they increase the total weight of the structure.
The work done by Mitschang et al. (2013) involved the application of induction welding to join Aluminum magnesium, AlMg3 (AA5754) specimens and Carbon Fiber PA66 in other to obtain a lap shear strength of 15MPa. Goushegir et al. (2014) obtained a lap shear strength of 27MPa on Aluminum Alloy 2024 specimens joined to carbon fiber reinforced (poly-phenylene sulfide) (CF-PPS) by friction spot welding. (Jiao et al. 2017) applied laser welding to join stainless steel to CF-PPS from which they obtained a shear strength of 17.5MPa. In the above-mentioned literature works, thermoplastic matrix composites, mainly based on Polyamide PA66 or PPS, were joined to metals, since thermoset matrices do not melt (Martinez. 2015).
Among the variety of welding techniques studied so far, USW is one of the most promising for the joining of hybrid structures. Works done by Balle et al. (2009) applied ultrasonic welding for joining aluminum to Carbon Fiber Reinforced Thermoplastics (CFRTP) and obtained a lap shear strength of 31.5MPa for AA5754 aluminum joined to carbon fiber reinforced Polyamide PA66 (CF-PA66) and Magin and Balle (2014) successfully applied USW to join metals to thermoplastic matrix composites. They obtained results have proved that hybrid joints with high strength can be achieved when intermolecular contact and mechanical interlocking in the weld zone is promoted by uniform mixing of the metallic and polymeric parts.
Methodical research at the WKK have shown that it is very possible to join metals with brittle materials like glass or ceramic with high precision and reproducibility if a force-controlled USMW system is used. After further research concluded that the ultrasonic plastic welding with an oscillation amplitude acting perpendicular to the welding zone is typically used for joining Glass Fiber Reinforced Polymers (GFRP) or CFRP, but this welding method only realizes a joining between the metal sheet and the polymer matrices of the CFRP (Balle et al 2009; Huxhold et al 2011; Wagner et al 2012). Hence, a mechanical load on the joint is not transmitted directly from the metal to the fibers of the CFRP. Recent investigations at the WKK have shown that ultrasonic metal welding is more suitable for joining CFRP with metal sheets as seen with aluminum alloys or aluminum-plated steel. By using ultrasonic metal welding, the joining is realized in two steps. First, “the transversal ultrasonic oscillation plasticizes and displaces the polymer matrix of the CFRP out of the bonding zone” (Wagner et al 2012). Then, “the transversal oscillation causes a contact between the metal sheet and the fibers of the CFRP” (Wagner et al 2012). As a result, the load can be transmitted directly from the sheet into the fibers. Tensile shear strengths up to 40MPa can be achieved for metal/GFRP, and tensile shear strengths up to 60MPa can be achieved for metal/CFRP joints.

Ultrasonic metal welding has been applied for joining aluminum AA5754 sheets to CF-epoxy laminates modified on top surface with a PA6 film. The following conclusions can be drawn based on the study by Lionetto et al. (2017):
Hybrid ultrasonic welding of CF-epoxy to aluminum is possible by using a semi-crystalline thermoplastic film co-cured as a surface layer of the composite during layup before curing.
An average adhesion strength of 34.8MPa has been obtained on CF/ epoxy-PA6-AA5754 joints ultrasonically welded with a proper combination of welding energy and force.
During ultrasonic spot welding, the strong relative motion of the aluminum sheet at the interface with the CF/epoxy-PA6 sheet enables a very fast increase in temperature underneath the sonotrode tip leading to local melting of PA6 layer. As a consequence of the longitudinal oscillation, the molten polymer is displaced out of the welding zone underneath the sonotrode. At the same time, the metallic sheet is plastically deformed and a direct contact between aluminum surface and carbon fibers is realized.

Welding Parameters

There are a number of parameters can affect the welding process of an ultrasonic welder. The main ones can be summarized as follows:
• ultrasonic frequency
• vibration amplitude
• static force
• power
• energy
• time
• materials being welded
• part geometry
• tooling.

Ultrasonic frequency
UW transducers are designed and tuned to operate at specific frequencies based on their applications, they range from 15 kHz to 300 kHz. Changes in the system resonant frequency can be caused by small dimensional changes which can include system heating during operation, varying static force, and changing tool condition, and the changing effects of the welding load. However, while these conditions of resonance may seem severe, modern power supplies employ feedback control circuitry that automatically counteracts these shifting conditions and manage the driving frequency of the transducer to maintain the resonance of the system (Bloss and Graff 2009).
Vibration amplitude
The vibration amplitude transmitted by the sonotrode to the workpiece is another key parameter affecting welding. It directly ties into the energy delivered to the weld zone. In some welding systems, this amplitude is dependent on the power applied while in other systems, the amplitude is an independent, thus, capable of being set and controlled at the power supply because of added features of the feedback control system. The selection of weld vibration amplitude will depend on the conditions of welding as governed by materials and tooling (Bloss and Graff 2009).
Static force
The static force is also a key parameter of UW. It is the exerted force on the workpiece by both the sonotrode/welding tip and the anvil, in pressing the parts firmly together, creating a close contact between the opposing surfaces. The magnitude of the force will be strongly dependent on the material properties, as well as the size of weld being produced. There is an optimum range of static force, below which welds created will either be too weak or non-existent and above which excessive deformation of the parts may occur (Bloss and Graff 2009).
Power, energy, time
While individually listed as separate weld parameters, these are most conveniently examined in a unified manner, since they are all tied closely together. The power delivered to the transducer from the power supply is converted to ultrasonic power at the weld. However, between the electrical input and the weld, several conversions occur affecting the energy output. The actual power delivered to the weld zone is dependent on several factors that can include: (a) the efficiency of electromechanical conversion of the electrical input to mechanical output by the piezoelectric materials; (b) losses in the bulk materials and at interfaces of the transducer-booster-sonotrode system; (c) power radiated from the weld into the workpieces and the anvil structure. The weld time of ultrasonic metal welds is dependent on different factors like the thickness of the material to be welded or the power input of the system. However, metal welding times are quite short, typically under 1 second in duration (Bloss and Graff 2009).
Material Property
This single category ‘materials’ covers a variant of factors relating to ultrasonic metal welding. Properties of the materials such as its Young’s modulus, yield strength and hardness are a key consideration. Generally speaking, alloys of aluminum, copper, nickel, magnesium and gold are most easily ultrasonically welded due to their relative softness. With increasing alloy hardness, ultrasonic welding difficulty increases. The material surface characteristics come next, within this includes surface finish, oxidation, coatings and contaminants. It becomes obvious how important material properties, both physical or chemical can be a deciding factor in UW (Bloss and Graff 2009).
Part Geometry
As the word implies, the shape and dimensions of the workpiece, specifically the thickness plays a significant role in UW. Simply put, the thinner the parts, of whatever material, the better the chances of achieving ultrasonic welds. Increasing part thickness, will require larger welding tip areas which in turn generally requires an increase in welding power. Maximum thicknesses that can be achieved will obviously depend on the material being welded and the power levels available from a welder. The very vibrations that create an ultrasonic weld can, when transmitted away from the weld and into the surrounding part, affect the making of the weld itself, as well as affect previously made welds. Typically, issues in this area can be corrected by modifying part dimensions and stiffness (Bloss and Graff 2009).

Literature Review Summary
Ultrasonic welding technique has proven to be fast, economical and environmentally friendly. It has also produced bonds stronger than traditional methods that involve the use of adhesives and some mechanical joining techniques. This joining method has the potential to revolutionize how metals and composite structures are used in the Aviation industry. The feasibility of using USW for fibre reinforced composites has been proven by several studies. However, the process requires optimization in other for it to be actively applied in the manufacture of aerospace structures.
If fully optimized, USW could pave a pathway to lightweight aircrafts as the use of rivets that tremendously add to the total mass of any aircraft could become absolute. Using the Boeing 747 as an example, at least 3 million mechanical fasteners are used with about 40,000 rivets on each wing of the airplane (Hill 2011). With just a half decrease in the number of rivets used on the Boeing 747, a proportionally decrease in the total fuel energy consumed by the plane can be achieved.
Maximizing the Optimization of this technology relies on finding solutions to the current limitations it faces and the alteration of welding parameters to aide faster and stronger bonds.

Project Aim and Objectives
The aim of the present experimental research is to study the feasibility then applicability of USW in creating a joint between low cost Aluminum alloy and CFRP. Furthermore, the joints will be tested to correlate joint performance with ultrasonic welding parameters. This will be achieved by replicating previous works of the likes of Wagner and Balle in other to analyze the bonds created through non-destructive testing. After which a comprehensive and in-depth understanding of the effects of the welding parameters on the optimization of the technology may occur. The study will evaluate optimum values of each parameters to create the strongest bond.

Methodology
First, the feasibility and applicability of USW (USMW and USPW) to create a metal-composite joint is evaluated by means of an experimental study. In this experimental study, the materials used, equipment, welding process and resulting joints will be analyzed. Also, the mechanical properties of those joints will be analyzed through testing. This analysis will include; optimizing each welding parameters, non-destructive testing at the joint site to further investigate material surface interaction and lastly, lap-shear testing of the resulting joints will be performed to compare joint strength

Experimental Set-up

Materials

Metal
Aluminum alloy sheet AA5754, sometimes referred to as AlMg3 was chosen based on the knowledge gotten from previous comparable researches on ultrasonic metal welding carried out on carbon fiber reinforced thermoplastic composites, such as CF-PA66, as reported by Wagner et al. (2013). This Aluminum alloy is made up of 2.6–3.6% by weight of magnesium with a density of 2.67 g/cm3. It is also resistant to corrosion, especially to seawater and industrially polluted atmospheres and it is often used in the automotive industry for structural panels, as reported by Miller et al. (2000).
Composite Laminate
Two carbon fiber reinforced polymers were also chosen for the study. IM78852 carbon fiber reinforced epoxy laminate was chosen as the thermoset composite material due to its industrial standards and widespread use in the aerospace industry. It is manufactured to Hexcel aerospace grade specification HS-CP-5000. The epoxy exhibits good impact resistance and damage tolerance for a wide range of applications allowing it to operate in environments of up to 121℃(250℉). The composite material was prepared from a carbon fiber-epoxy prepreg, supplied by Hexcel corporation under the product name Hexcel IM7/8852. The prepreg matrix was a Mid-toughened, high Strength, damage-resistant, structural epoxy matrix. As recommended in the product datasheet, its curing conditions included: Full vacuum (1 bar), 7 bar gauge autoclave pressure, safety value of 0.2 bar when the autoclave pressure reached approximately 1 bar gauge, heat at 110℃ for 60 minutes then 180℃ for 120 minutes, cool down then vent autoclave pressure when component reach 60℃ or below. The carbon fiber content reported in the Hexcel product datasheet of the prepreg was 60% by volume.
Stretch Broken Carbon Fiber/Polyamide 12 (SBCF/PA 12) was chosen as the thermoplastic composite material. SBCF is an improved aligned, discontinuous carbon-fiber system developed by Hexcel Corporations. The unique advantages of the material lie in its combined properties of improved formability without the large mechanical performance knock-downs typical of discontinuous systems (i.e. random short fiber composites). The matrix PA12 is a semi-crystalline thermoplastic polymer with a crystallinity of about 41% for solidification rates between 1 and 100℃ per minute. It was selected on the basis of various criteria such as viscosity, wetting, adhesion, durability and costs.
The chosen dimensions for all test piece as recommended by ASTMD5868 was 100 × 25 mm3 with varying thicknesses of 1 mm and 2 mm for the aluminum alloy and composite sheets respectively, since the use of joining partners of different thickness is common in the design of hybrid metal-CRFP structures. For the AA5754, a thickness of 1 mm is of industrial relevance while a thickness higher than 1.6 mm for the CFRP is also relevant due to the minimum wall thickness criteria, as seen in the aircraft industry for damage tolerance and repairing issues.
Ultrasonic welding equipment
The welding equipment consists of an USMW machine, USPW machine and the welding jig that is used to clamp the test samples.
The ultrasonic metal welding and ultrasonic plastic welding will be carried out with both the Branson Ultraweld L20 system and the Branson 2000Xc machine respectively. Both machines are available in the Bernal institute composite laboratory of the University of Limerick (UL).
In combination with the available power supply, the Branson L20 system has a maximum power of 6,000 Watts. The machine’s Advanced Control Unit (ACU) allows a user to select one of three welding drivers: energy, time or displacement of the sonotrode. When a set value for one of these drivers is set reached, the machine stops the vibration phase. Other welding parameters such as the force and power are recorded every millisecond. Collected data from the welder can be further processed using AcuCapture or Microsoft Access.

Figure 4: Schematic diagram of the Branson Ultraweld L20 System (left) and Branson Ultraweld L20 System (right). (Source: Bernal Institute Composite Laboratory, UL)

The Branson 2000Xc combines precise and consistent high-quality welds, fast cycle times, and the process control needs of today’s manufacturing environment. It features multiple weld modes including weld by time, peak power, energy, distance and ground detect. It has a maximum power of 4,000 Watts with it’s a vibration frequency of 20KHz. As the latest addition to the Branson 2000X

Figure 5 ACU for both the Branson 2000Xc and Ultraweld L20 System (left) and Branson 2000Xc welder (right). (Source: Bernal Institute Composite Laboratory, UL)
series, it is considered the best plastic welder with special features like being fully electronic and with password-protected settings.
The welding jig used in this research is a construct by a final year undergrad student of UL, it was developed at the University of Limerick. This jig is dedicated to welding single-lap shear specimens and can be used to weld other specimens following the ASTM D5868-01 standard.

Figure 6 Welding clamp vise (left) and sample jig (right). (Source: University of Limerick)

Further Work
Future work simply involves performing all objectives mentioned in 5 (Project Aim and Objectives).

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