Applications of Ultrasonic Welding in Mechanical Engineering

Applications of Ultrasonic Welding in Mechanical Engineering

Ultrasonic welding belongs to solid-state welding and is currently mainly used for welding small parts. Its welding quality is reliable and cost-effective. Ultrasonic welding can not only weld softer metal materials such as aluminum, copper, and gold, but also can be used for welding steel materials, tungsten, titanium, molybdenum, and other metals.

Furthermore, it can weld disparate metals with substantial physical property differences, even metals with semiconductors, metals with ceramics, and other non-metals and plastics. Table 6-5 lists the metal materials and their combinations that can be welded with ultrasonic welding.

Table 6-5 Metal Materials and Their Combinations That Can Be Welded With Ultrasonic Welding

AgAlAuBeCuFeGeLiMgMoNiPbPtSiSnTaTiWZr
Zr
W
Ti
Ta
Sn
Si
Pt
Pb
Ni
Mo
Mg
Li
Ge
Fe
Cu
Be
Au
Al
Ag
Note: “●” represents combinations successfully tested abroad, “○” represents combinations successfully tested in our country, and “△” represents combinations successfully tested both domestically and internationally.

Ultrasonic Welding of Like Materials

Welding of Aluminum and Aluminum Alloys

Aluminum and its alloys are the most common materials used in ultrasonic welding, and they best demonstrate the superiority of this method.

Whether it’s pure aluminum, Al-Mg, Al-Mn alloys, or Al-Cu, Al-Zn-Mg, and Al-Zn-Mg-Cu high-strength alloys, they can all be ultrasonically welded in any condition, such as casting, rolling, extrusion, and heat treatment. However, their weldability varies with the type of alloy and heat treatment method.

For lower-strength aluminum alloys, the joint strength of ultrasonic spot welding and resistance spot or seam welding is roughly the same. However, in higher-strength aluminum alloys, the joint strength of ultrasonic welding exceeds that of resistance spot welding. For example, the strength of Al-Cu alloy ultrasonic spot welding is on average 30% to 50% higher than that of resistance spot welding.

The main reason for this increase in joint strength is that the ultrasonically welded material is not affected by melting and high-temperature effects on the heat-affected zone, and the weld spot size is generally larger.

The surface preparation requirements for ultrasonic welding of aluminum and its alloys are lower than other welding methods. Normally, the surface of aluminum is degreased. After heat treatment of aluminum alloys or when the percentage of magnesium in the alloy is high, a thick oxide film is formed. To obtain a good welding joint, this oxide film should be removed before welding.

Welding of Copper and Copper Alloys

Copper and its alloys have good weldability. The surface needs to be cleaned before welding to remove oil stains. The selection of welding parameters and equipment is similar to that when welding aluminum alloys. In motor manufacturing, especially in the manufacture of micro-motors, ultrasonic spot welding is gradually replacing the original brazing and resistance welding methods.

Almost all connection processes can be completed by ultrasonic welding, including the connection of copper wires in general armatures, and the connection of commutators and enameled wires. Table 6-6 shows the welding parameters and joint properties of commonly used aluminum, copper, and their alloys in ultrasonic welding.

Welding of Titanium and Titanium Alloys

Titanium and its alloys have excellent weldability, and the selection range of welding parameters is wide.

Microstructural analysis of the weld spot sometimes reveals a phase transformation from α to β, and there are also weld spot structures that have not undergone phase transformation, but all can achieve satisfactory joint strength.Table 6-7 lists the welding parameters and shear resistance of spot welded joints of titanium alloys.

Table 6-6 Welding Parameters and Joint Properties of Commonly Used Aluminum, Copper, and Their Alloys in Ultrasonic Welding

MaterialsThickness
/mm
Welding ParametersVibration HeadWeld Spot Diameter
/mm
Shear Resistance of the Joint
/N
Static Pressure
/N
Welding Time
/s
Amplitude
/μm
Spherical Radius
/mm
Material GradeHardness
/HV
Pure Aluminum10170. 3 ~ 0. 7200 ~ 3000. 5 ~ 1. 014 ~161045160~1804530
0.8 ~1. 2350 ~ 5001. 0 ~ 1. 514 ~ 1641030
Aluminum Alloy5A030. 6 ~0. 8600 ~ 8000. 5 ~1. 022 ~ 241045 Bearing Steel     GCr15160~180
330~350
41080
0. 3 ~0. 7300 ~ 6000. 5 ~1. 018 ~ 204720
0. 8 ~ 1. 0700 ~ 8001.0 ~1.518 ~ 2042200
2A120. 3 ~ 0. 7500 ~8001.0 ~2. 020 ~ 2210Bearing Steel  
  GCr15
330~35042360
0. 8 ~1. 0900 ~ 11002. 0 ~2. 520 ~2241460
Pure CopperC110000. 3 ~0. 6300 ~7001.5 ~2. 016 ~ 2010 ~ 1545160 ~18041130
0. 7 ~ 1. 0800 ~ 10002.0 ~3.016 ~ 2010 ~ 1545160 ~18042240
1. 1 ~ 1. 31100 ~ 13003.0 ~4.016 ~ 2010 ~ 1545160 ~ 1804

Table 6-7: Ultrasonic Welding Parameters for Titanium Alloy and Shear Resistance of Spot Welded Joints

MaterialsThickness δ
/mm
Welding ParametersVibrator Hardness

HRC
Weld Spot Diameter
/mm
Joint Shear Strength
/N
Static Pressure
/N
Welding Time
/s
Amplitude
/μm
MinMaxAverage
TA30. 24000.316 ~ 18602.5~3.0680820760
0. 254000. 2516 ~ 18602.5~3.070830780
0. 658000. 2522 ~24603.0 ~3.539642004100
TA40. 254000. 2516 ~18602.5~3.069990810
0. 56001. 018 ~ 20602.5~3.017719301840
Note: The spherical radius of the vibration head is 10mm.
① Indicates that the vibration head is equipped with a hardfacing layer.

Welding of High Melting Point Materials

Ultrasonic welding is suitable for high melting point materials such as molybdenum and tungsten. It prevents joint brittleness due to heating, resulting in high-quality, strong welds. Currently, ultrasonic welding can be used to weld molybdenum plates up to 1mm thick.

However, due to the unique physical and chemical properties of molybdenum, tantalum, and tungsten, ultrasonic welding is challenging and requires specific process measures. The vibrating head and workbench should be made of high hardness and wear-resistant materials. Welding parameters should be slightly high, particularly the amplitude and static pressure. Welding time should be short.

Welding between high-hardness metals and metals with poor weldability can be achieved through ultrasonic welding by adding an intermediate transition layer. Soft metal foils are typically chosen as the transition layer material.

For instance, using a 0.062mm thick nickel foil as a transition layer to weld a 0.62mm thick molybdenum plate can achieve a shear strength of 2400N at the weld point. Using a 0.025mm thick nickel foil to weld a 0.33mm thick nickel-based superalloy can achieve a shear strength of 3500N.

Ultrasonic welding can also be used for multi-layer metal structures. For instance, it can weld dozens of layers of aluminum or silver foils at once. It can also weld multi-layer structures using an intermediate transition layer.

As the electronics industry develops, the ultrasonic welding quality of aluminum wires and foils, as well as germanium and silicon semiconductors, is rapidly improving to meet high reliability requirements.

Ultrasonic Welding of Plastics

When welding plastics, the joint surface of the weldment is usually placed at the node of the resonance curve to release the highest local heat and melt the material for welding. Due to this energy concentration effect, ultrasonic welding of plastics is efficient with a small heat affected zone.

An ultrasonic plastic welding machine typically consists of an ultrasonic generator, a welding pressure table, and a welding tool. The welding tool includes an ultrasonic transducer, a modulator, an ultrasonic acoustic pole (also known as an ultrasonic vibrating head), and a base. The ultrasonic vibration frequency for plastic welding is generally 20~40kHz.

Ultrasonic welding machines can be operated semi-mechanically, mechanically, or automatically. Depending on the position of the weldment, plastic ultrasonic welding can be divided into near field and far field types, the former also known as direct ultrasonic welding or contact ultrasonic welding, and the latter as indirect ultrasonic welding.

Near field ultrasonic welding of plastics refers to when the interaction distance between the ultrasonic vibrating head and the plastic welding surface is very small, usually less than 6mm. The entire plastic welding surface in contact with the vibrating head end face melts, achieving welding.

Far field ultrasonic welding of plastics refers to when the interaction distance between the ultrasonic vibrating head and the plastic welding surface is larger, usually more than 6mm. The ultrasonic energy must be transferred through the workpiece to the welding surface, producing mechanical vibrations and heat to achieve welding.

The workpiece itself, which transfers energy between the ultrasonic vibrating head and the welding surface, does not heat up.

Although ultrasonic welding of plastics is a type of fusion welding, it doesn’t rely on surface heat conduction to melt the joint but converts elastic vibration energy into heat via the contact surface. It has several advantages:

1) High welding efficiency, with welding time not exceeding 1s.

2) No need for surface pre-treatment before welding. Moisture, oil, powder, or solution remaining on plastic parts do not affect normal welding, making it particularly suitable for packaging welding.

3) Local melting occurs only on the welding surface during the process, preventing environmental pollution. The weld points are aesthetically pleasing, produce no turbidity, and result in fully transparent welded products.

The weldability of plastic through ultrasonic welding is related to the melting temperature, elastic modulus, friction factor, and thermal conductivity of the plastic material. Most thermoplastics can be welded using ultrasonic waves. Generally, rigid thermoplastics weld better than flexible ones, and amorphous plastics weld better than crystalline ones.

When welding crystalline and flexible plastics, welding should be done in the near field area closest to the vibrating head. Ultrasonic welding is mainly used for welding molded parts, films, plates, and wires, without the need for heating or adding any solvents or adhesives.

Table 6-8 Ultrasonic Weldability of Thermoplastics

Material NameWeldability
Near-field WeldingDistance Welding
Amorphous PlasticsAcrylonitrile Butadiene Styrene (ABS)ExcellentGood
ABS-Polycarbonate AlloyGoodGood
Poly(methyl methacrylate) (PMMA)GoodGood to Average
Acrylic MulticopolymerGoodGood to Average
Butadiene-StyreneGoodAverage
PolystyreneExcellentExcellent
Rubber-Modified PolystyreneGoodGood to Average
CelluloseAveragePoor
Rigid Polyvinyl Chloride (PVC)AveragePoor
PolycarbonateGoodGood
Polyphenylene EtherGoodGood
Crystalline PlasticPolyoxymethyleneGoodAverage
PolyamideGoodAverage
Thermoplastic PolyesterGoodAverage
PolypropyleneAverageBelow Average to Poor
PolyethyleneAveragePoor
Polyphenylene SulfideGoodAverage

Plastic ultrasonic welding has some special requirements for pre-processing the welding face. On the welding face, ultrasonic energy directors with sharp edges are often designed, as shown in Figure 6-18.

Figure 6-18: Energy Director on the Welding Face in Plastic Ultrasonic Welding

The ultrasonic energy director reduces the initial contact area of ultrasonic welding to achieve an ideal initial heating state, precisely controls the flow of material after melting, and prevents overheating of the workpiece.

When welding molded parts, the form and design principles of the ultrasonic energy director depend on the type of plastic being welded and the geometric shape of the molded parts. Figure 6-19 shows examples of welding face designs for amorphous and partially crystalline plastics during ultrasonic welding.

Figure 6-19: Examples of Welding Face Design in Plastic Ultrasonic Welding
  • a) Amorphous Plastic
  • b) Crystalline Plastic

The weld quality of plastic ultrasonic welding is mainly related to the weldability of the parent material, the geometry and tolerance range of the welded parts and weld, the ultrasonic horn, welding parameters, and the adjustment and stable control of the horn insertion depth, among other factors.

During plastic ultrasonic welding, an appropriate ultrasonic horn should be selected for different materials and welding parameters should be strictly controlled.

Ultrasonic Welding of Dissimilar Materials

The weld quality of ultrasonic welding between different types of metal materials depends on the hardness of both materials. Ultrasonic welding performance improves as the hardness of the materials gets closer and the hardness value drops. When welding two materials with significantly different hardness, a high-quality joint can still be formed if one of the materials has lower hardness and better plasticity.

If the plasticity of both materials to be welded is low, an intermediate transition layer can be used for welding. When welding metal materials of different hardness, the material with lower hardness is placed on top to touch the upper sonic pole, and the required welding parameters and power of the welder are selected according to the low hardness weld.

For example, for welding joints of different materials such as copper and aluminum, if conventional thermal fusion welding is used, due to reasons such as a robust oxide film on the aluminum surface, different metal melting points, high metal thermal conductivity, and increased brittleness caused by metal fusion, unstable intermetallic compounds are easily produced, affecting the reliability of the joint quality.

However, using ultrasonic energy for welding does not produce brittle intermetallic compounds, yields a high-quality welding zone, and does not require intermediate processes, improving welding productivity.

Flat solar thermal collectors’ heat-absorbing plates are made using ultrasonic welding. Currently, to increase the heat-absorbing capacity of solar thermal collectors while reducing manufacturing costs, many collectors use copper tubes and aluminum fins welded into heat-absorbing plates.

This type of heat-absorbing plate not only has good corrosion resistance but also prevents secondary pollution of the water during heating and storage, maintaining clean water quality that meets drinking water standards. The structure of the heat absorbing plate’s welded joint is shown in Figure 6-20.

Figure 6-20: Structure of the Welded Joint in the Heat-Absorbing Plate

To reduce the thermal resistance of the fin and flow channel combination, welding is implemented at the contact point of the flow channel axis and the fins. It is a lap joint, where both the fins and the flow channel walls are thin, with thicknesses under 0.5mm, and the weld seam is 2~2.5m long.

When the weldment is working, it heats the flow in the flow channel, and the fins only absorb and conduct heat, so the joint strength and seal requirements are not high. Research shows that using the welding parameters listed in Table 6-9 for ultrasonic welding has advantages like high joint strength, high production rate, low energy consumption, and good working conditions.

Table 6-9: Ultrasonic Welding Parameters for Copper-Aluminum Alloy

MaterialsThickness
/mm
Vibration Frequency
/kHz 
Static Pressure
/N
Welding Time
/s 
Amplitude
/μm
Continuous Welding Speed
/(m/min) 
Steel + Aluminum
(C11300+1200)
0.5 + 0.520300 – 4500.05 – 116 – 2010

When welding aluminum ignition module substrates and copper pads, an ultrasonic automatic welding system can achieve a production rate of 3000 weld points per hour.

By utilizing the ultrasonic welding method to weld the copper-aluminum joints within the coil of an automobile starter motor, issues stemming from the non-conductive oxide layer formed on aluminum joints and wear and tear caused by heat cycles are resolved.

Ultrasonic welding is also effective on metal materials of varying thicknesses, with practically no limit on the thickness ratio of the weldment. For instance, thermocouple wires can be welded onto large objects intended for temperature measurement.

Ultrasonic welding can also be successfully implemented between a 25μm thick aluminum foil and a 25μm thick aluminum plate, resulting in high-quality joints.

When welding different types of metals, the joint structure tends to be more complex. For example, in the joint structure of nickel and copper ultrasonic welding, the softer copper embeds into the nickel material in a serrated pattern, forming a solid-phase connection at the interface.

Ultrasonic welding can connect metal foils and wires to the thermal spray surface of glass, ceramics, or silicon wafers, creating bimetallic joints from materials with vastly different physical properties. This meets the needs of industries such as microelectronics.

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