Thermoplastic welding - A408

 

Thermoplastic welding
Document Type	Article
Document Identifier	408
Tags	Deposition
Prerequisites	Thermoplastic polymers Fundamentals of composite materials

Introduction

Thermoplastic composite parts provide the unique ability to be welded together, providing many advantages over mechanical fasteners or conventional adhesive bonding. The development of the weld relies mostly on the quality of the contact between the parts, defined as the degree of intimate contact, and the diffusion of the polymer chain across the interface from one side to the other, defined as the degree of healing. A variety of welding techniques exist.

Scope

This page provides an overview of the process of thermoplastic welding. It first covers the theory of welding with its two main parts, intimate contact and healing, and how it differs from adhesive bonding, followed by an introduction to the main methods of welding and typical characterization techniques.

Significance

Thermoplastic welding is highly dependent on the applied temperature and pressure, as well as the process duration. The resulting mechanical properties are related to these process parameters. A good understanding of the welding mechanisms and good control of the process parameters is essential to ensure the completion of the welding process.

Overview

Thermoplastic composites can be joined using typical techniques such as adhesive bonding and mechanical fastening. However, these techniques present limitations in their use with thermoplastic composite parts. Extensive surface preparation is often required to use adhesives, which can be long and labor-intensive. The use of mechanical fasteners usually requires to drill holes in the composite parts, which can lead to delamination, stress concentration points and potential issues of galvanic corrosion .

An alternative joining method is available to thermoplastics by taking advantage of their fusible nature. Unlike thermosets, thermoplastics can be softened (in the case of an amorphous polymer) or molten (in the case of a semi-crystalline polymer) and reprocessed. When two parts in that softened/molten state are brought together under pressure, mobile polymer chains can cross the interface and join the two parts together. By keeping the parts in close contact during cooling, a solid joint is consolidated, and the two parts are joined. This is thermoplastic welding.

To achieve good mechanical properties in a welded joint, it is essential to understand which mechanisms are taking place to ensure to correct process parameters are selected.

Theory of Welding

When two parts to be welded, called adherends or substrates, are placed in close contact, the development of the weld relies mostly on two parameters: the quality of the contact between the parts, defined as the degree of intimate contact, and the diffusion of the polymer chain across the interface from one side to the other, defined as the degree of healing.

The surface of an adherend is not perfectly flat, it is typically made of asperities, or roughness, coming from the manufacturing process. The degree of intimate contact is defined as the fraction of the apparent contact surface that is really in contact at the microscopic level (see Figure 1). It varies from 0 to 1 (or 100%). When parts are brought together, only the top parts of the asperities are touching each other. This is defined as the initial degree of intimate contact. While temperature is increased and pressure is applied on the parts, the asperities start to deform and the contact increases (Figure 1). The degree of intimate contact increases, until eventually reaching 1 when the whole surface is in contact at the microscopic level. This happens when \(t = t_{\mathrm{ic}}\).

Figure 1. Schematics of the evolution of the degree of intimate contact. (a) Surface roughness before contact, (b) at t = 0, (c) during contact evolution, and (d) when complete contact is reached.

The second mechanism is the diffusion of polymer chains through the interface. This can only happen on surface areas where intimate contact has been reached. The mechanism relies on the polymer chain mobility, which is a function of temperature. At higher temperature, chains are more mobile and can cross the interface faster to bridge it and create a weld and “heal” the interface (Figure 2). This movement of the polymer chains across the interface is based on the polymer chain reptation theory, proposed by de Gennes in 1971 . The strength of the interface increases during this process, and this increase in mechanical resistance is typically used to define the degree of healing. This value varies from 0 to 1, with 1 being the maximum strength of the interface, comparable to the bulk material strength.

Figure 2 – Schematics of the evolution of the degree of healing. (a) Polymer chains before contact, (b) at elevated temperature, (c) at t = 0, (d) during healing process, and (e) when complete healing is reached after cooling. It must be noted that steps (b) and (c) can occur in both orders.

Finally, the strength of the joint can also be expressed as the degree of welding. It corresponds to the joined evolution of both intimate contact and healing. It ranges from 0 to 1 and can be calculated as the product of the degree of intimate contact and the degree of healing. The significance of a degree of welding of 1 is that the maximum strength of the weld, equivalent to the strength of the bulk material, has been reached across the complete welding surface.

Common welding methods

Thermoplastic welding methods can be separated in three main categories, depending on how the heat is generated and brought to the welding interface: thermal welding, friction welding and electromagnetic welding . Thermal welding techniques rely on direct heating of the material at the interface. This category regroups methods such as hot tool welding, conduction welding, infrared welding, hot gas welding, and laser welding. Friction welding techniques rely on the relative motion of the parts, and the resulting heat generated by friction at their interface, like what can be observed with metals. This category includes ultrasonic welding, friction stir welding, spin welding and vibration welding, amongst others. In electromagnetic welding, the heat is generated at the welding interface through an electromagnetic phenomenon. Typical methods of this group are resistance welding, induction welding, microwave welding and dielectric welding.

Figure 3 – Summary of the different thermoplastic welding methods

Characterization techniques

There are two main ways to characterize a weld: mechanical testing and non-destructive testing. For mechanical tests, samples are typically tested in single-lap shear or double cantilever beam configuration. These two tests allow to measure the weld shear strength and fracture toughness. The most used non-destructive method for the analysis of a weld is ultrasonic scanning (also known as C-scan). This method can detect voids located at the welding interface corresponding to a lack of contact between the two adherends. However, it cannot assess the degree of healing in the areas where intimate contact has been reached.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

A class of polymer, some common examples include polypropylene and polyethylene.

They soften and melt upon heating (i.e. potentially recyclable), high viscosity when melted, therefore difficult to saturate fibres. Usually needs a lot of pressure and heat to process.

A joining process where a polymeric material (the adhesive) joins two separate structural pieces called adherends. Adhesive bonding is used instead of, or in parallel with mechanical means of joining such as riveting or bolting.

The failure mode characterized by a partial or full separation of adjacent laminae. Delamination is unique to laminated structures, and can be induced by impact load, fatigue load, and quasistatic load. (Also known as interlaminar separation)

Literally "without structure", randomly coiled molecular polymer chains.

A portion of the molecular polymer chains tend to ‘fold up’ into densely packed regions called crystals.