Discussion on the application strength of metal and plastic welding

Overview

With the advent of lightweight automobiles, there is a wide demand for the connection of metals and plastics. How to quickly and effectively connect metals and plastics containing reinforced carbon fiber or glass fiber urgently needs to be solved.

To adapt to the assembly line requirements of the automotive industry. The connection process must be fast, reliable and automated. The following are the three most common industrial methods for connecting composite and metal parts. Among them, laser-based connection technology is the latest technology. There are still many problems to be solved in this technology. These problems are not only related to efficiency, but also to connection strength and aging. This article will discuss these problems in detail.

Mechanical connection. Fast and economical, stable process. The disadvantage is that the holes in the composite material destroy the fiber distribution and strength of the composite material. Connectors (such as screws) increase the weight of the component. Adhesives. There is a wide range of adhesives available for the connection of various materials. The disadvantages are that the surface needs to be pre-treated, the bonding time is long, adhesives are required, and the cost is increased. Laser connection. Fast connection speed, high connection strength, no auxiliary materials are required. The disadvantage is that it is only applicable to thermoplastics.

The Basics of Laser Joining Technology

Laser joining of thermoplastics to metal parts is a two-step process. In the first step, the surface of the metal part is processed with a laser to form a microstructure. This is usually done with a fiber laser rated at 1kW. The laser scans the surface to process regular grooves and interlocking structures.

Due to the high power density of the laser beam, the metal is partially melted and evaporated during the etching process. The evaporated high-pressure jet molten material, part of which solidifies at the edge of the groove to form a bite structure. As shown in Figure 3, the number and density of groove distribution can be increased to maximize the plastic's grip on this surface.

There is another option. Ultrashort pulses using special pulse (USP) lasers can be achieved to form a sponge surface with conical protrusions. It can be used on steel, aluminum, silicon, and titanium surfaces. The joining force of plastics is even better than that of microstructures processed by fiber lasers. The only problem is that the speed of USP lasers is too slow.

In the second step, the plastic is heated and melted and then pressed into the metal surface. After cooling, the plastic is connected to the metal. There are many ways to heat the plastic. One method is to directly heat the plastic, which can be pressed into the grooves on the metal surface by processes such as hot plates, infrared rays, etc. Another method is to heat the metal part and then press it on the cold plastic. Heat conduction causes the plastic to melt and then flow into the tiny structures.

In the first step mentioned above, laser micromachining is fast and contact-free. So this process is very suitable for insertion into existing production processes. It is very suitable for large-scale production.

Joint Strength Testing

Metal-to-plastic joints are subject to loads in real applications. So, what is the maximum stress that such a composite joint can withstand? Where will it break?

Experts at the German Institute for Laser Technology Hof have answered these questions in a series of stress tests on various materials. One of the test contents is as follows:

1.5mm stainless steel metal plate and 3mm glass fiber PP plastic connector were subjected to shear test; 1.5mm stainless steel metal plate and 3mm PP plastic (without glass fiber) connector were subjected to tensile test.

The metal surface was micro-processed by a 1kw single-mode fiber laser with a spot diameter of about 40um. The laser processed a reproducible "bite edge" groove structure on the metal surface. The plastic was heated with a semiconductor laser rated at 3kW, with a spot size of 7.5mm脳25mm. The two parts were clamped with a pressure of 0.3Mpa. The connection results are as follows:

Stainless steel + PP with glass fiber, connection area 150mm^2Stainless steel + PP, the above two types of connection area area 100mm^2 for destructive testing.

Steel + stainless steel with glass fiber PP, shear strength test. The maximum shear load is 13.13.1, the slot spacing is 400um. Mpa; the maximum shear load is 15.5.5, when the slot spacing is 300um. Mpa. Steel stainless steel + PP, tensile strength test. The maximum tensile load is 5.1, when the slot spacing is 400um. pa; when the slot spacing is 300um, the maximum tensile load is 9.1Mpa. Obviously, a dense distribution of grooves contributes to a stronger connection. However, it should be noted that dense grooves increase the micromachining time. At the same time, similar tests were carried out on magnesium sheets. All tests showed that the laser-based joining technology between metal and plastic parts establishes a strong and reliable connection.

Aging test

Another important question for products in the automotive industry: Does the connection meet the requirements for resistance to climate change and corrosion? The experts at the German Institute for Laser Technology Hof conducted additional tests to answer this question.

In the experiment, different metals (steel and aluminum) and different plastics (PP+30% glass fiber and PP+40% talc) were connected by laser. The climate change test was carried out according to the VWPV1200 standard, and the temperature range was 80鈩儈-40鈩?

A single test cycle was 12 hours and needed to be repeated 2 times, 10 times and 30 times. Before and after the test, the samples were tested for destructive shear strength. All the test results were distributed between 8 and 15MPa. In the test, there was also an interesting phenomenon: after 30 test cycles, all PP samples filled with talc failed in the strength test outside the connection area due to cracking. In other words, the connector is stronger than the main PP material.

A similar phenomenon also occurred in the corrosion resistance test. The test was carried out for 7 days according to VDA621-415 standard. This test includes salt spray and high humidity test conditions. All samples can withstand 8-5MPa shear strength before testing. After completing the corrosion test, a shear test was performed and all PP samples containing talc broke outside the connection area. The PP sample contained glass fiber and broke in the connection area, but its strength was higher than before the test.

Stainless steel showed signs of corrosion, especially in the microstructures. There was also noticeable corrosion penetration in the laser connection area, but there was no noticeable effect on the connection strength. The aluminum plate also showed signs of corrosion in the microstructures outside the connection area, but there was no sign of corrosion in the connection area. Therefore, it can be concluded here that the microstructures should be avoided in an open environment when used.

Summary

Joining metal and plastic by laser has been proven through testing to produce high-strength, high-reliability connections. Climate and corrosion tests had no effect on the connection strength. After some aging, the location of the break was the plastic body, not the connection location.