It is proved by examples that the metal and plastic can be highly reliable “welded” by laser.
To be precise, it should be “connection.”.
In figure 1 below, the car door is reinforced with glass fiber plastic.
Fig. 1 Car door reinforced with glass fiber plastic
With the advent of automobile lightweight, metal and plastic connection is widely needed.
How to quickly and effectively connect metal and plastic containing reinforced carbon fiber or glass fiber, this kind of problem urgently needs to be solved.
In order to meet the requirements of assembly line operation in automobile industry, the connection process must be fast, reliable and automated.
The following is a list of the three most common methods of joining composite and metal parts in industry.
Among them, laser-based connection technology is the latest technology.
Many problems still need to be solved in this technology.
These problems are not only related to efficiency, but also to joint strength and aging. This article will discuss these problems in detail.
1) Mechanical connection
It is a fast, economic and stable process.
The disadvantage is that the holes in the composite destroy the fiber distribution and strength of the composite.
Connectors, such as screws, increase the weight of the assembly.
There are a wide selection of adhesives for the connection of various materials.
The disadvantages are that the surface needs to be pretreated, the bonding process takes a long time, the adhesive is needed, and the cost is increased.
3) Laser connection
It is fast and reliable, with high connection strength, and it do not need any auxiliary material.
But its disadvantage is that it applies only to thermoplastics.
1. Fundamentals of laser connection technology
The laser connection of thermoplastic and metal parts is divided into two steps.
In the first step, the microstructure is formed on the surface of metal parts by laser processing.
This is usually done with a fiber laser rated at 1 kW.
The laser scans the surface and processes regular groove and undercut structure (Fig. 2).
Fig. 2 Laser bonding process of plastic and metal
Due to the high power density of the laser beam, the metal is partially melted and evaporated during ablation.
The molten metal is ejected by high-pressure evaporation, and part of it solidifies at the edge of the groove to form undercut structure.
In order to maximize the plastic grip on this surface, the number and density of grooves can be increased, as shown in Figure 3.
Fig. 3 Several laser beam scans to get a good groove shape
There is an alternative, which is realized by using a special pulse ultra-short pulse (USP) laser to form a spongy surface with a conical protuberance.
It can be used on steel, aluminum, silicon and titanium surfaces.
On such a surface, the bonding force of plastics is even better than that of microstructure processed by fiber laser.
The only problem is that the USP laser is too slow.
In step 2, the plastic is heated to melt and then pressed into the metal surface.
After cooling, the plastic is connected to the metal.
There are many ways to heat plastics.
One method is to heat the plastic directly, using hot plate, infrared and other processes, and then press it into the metal surface groove.
Another method is to heat the metal parts and press them on cold plastic.
Heat conduction causes the plastic to melt and then flow into tiny structures.
In the first step, laser micromachining is fast and non-contact.
Therefore, this process is very suitable for inserting into the existing production process, and is suitable for mass production.
2. Connection strength test
In practical application, the joint between metal and plastic will be loaded.
So, what is the maximum pressure of this kind of composite connection?
Where will it break?
Experts from Hoff Laser Technology Research Institute in Germany have conducted a series of stress tests on various materials and answered these questions (Fig. 4).
One of the test contents is as follows:
- The shear tests of 1.5 mm stainless steel plate and 3 mm PP plastic connector with glass fiber were carried out;
- The tensile test of 1.5 mm stainless steel plate and 3 mm PP plastic (excluding glass fiber) connector was carried out.
Fig. 4 Fracture surface of metal and plastic joint after corrosion test
A 1 kW single-mode fiber laser is used to micro process the metal surface, and the spot diameter is about 40um.
A reproducible undercut groove structure is produced on the metal surface by laser.
The plastic is heated by a 3 kW rated power semiconductor laser with a spot size of 7.5 mm × 25 mm.
Clamp the two parts with a pressure of 0.3MPa.
The results are as follows:
- Stainless steel + PP with glass fiber, the connection area is 150 mm2
- Stainless steel + PP, the connection area is 100 mm2
Destructive tests were carried out on 5 samples of each of the above two types.
For stainless steel + PP with glass fiber, shear strength test was carried out.
- When the slot spacing is 400 um, the maximum shear load is 13.1 Mpa;
- The maximum shear load is 15.5 Mpa when the slot spacing is 300 um.
For stainless steel + PP, tensile strength test was carried out.
- The maximum tensile load is 5.1 Pa when the slot spacing is 400 um;
- The maximum tensile load is 9.1 Mpa when the slot spacing is 300 um.
Obviously, the dense groove distribution is helpful to improve the connection strength.
However, it should be noted that dense grooves will increase the micromachining time.
At the same time, a similar experiment was carried out on the magnesium sheet.
All tests have shown that laser-based bonding technology creates a strong and reliable connection between metal and plastic parts.
3. Aging test
Another important issue of automotive products:
Does this connection meet the requirements of climate change resistance and corrosion resistance?
In order to answer this question, experts from Hoff Laser Technology Institute in Germany also conducted other tests.
In the experiment, different metals (steel and aluminum) and different plastics (PP + 30% glass fiber and PP + 40% talc) were connected together by laser.
VW PV 1200 standard was used for climate change test, and the temperature range was 80 ℃ ~ 40 ℃.
The single test cycle is 12 hours, which needs to be repeated twice, 10 times and 30 times respectively.
Before and after the test, the destructive shear strength of the sample was tested.
All the test results were distributed between 8 and 15 MPa.
There is also an interesting phenomenon in the test:
After 30 test cycles, all PP samples filled with talc powder cracked outside the connection area in the strength test.
In other words, the joint is stronger than the bulk PP material.
A similar phenomenon appeared in the corrosion resistance test.
The test was conducted according to VDA 621-415 standard for 7 days.
The test includes salt spray and high humidity test conditions.
Before the test, all samples can withstand 8-5 Mpa shear strength.
Shear test was carried out after the corrosion test, and all PP samples containing talc powder cracked outside the connection area.
The PP sample containing glass fiber breaks at the joint area, but its strength is higher than that before testing.
Corrosion of stainless steel occurs, especially in microstructure.
The laser bonding area also has obvious corrosion penetration, but has no obvious effect on the bonding strength.
There are also signs of corrosion in the microstructure outside the joint area, but not inside the joint area.
Therefore, it can be concluded that the microstructure should not be in an open environment.
The method of laser connecting metal and plastic has been tested and proved to be able to produce high strength and reliable connection.
Climate and corrosion tests have no effect on joint strength.
After some aging, the fracture position is the plastic body, rather than the connection position.