Picosecond laser welding of similar and dissimilar materials

Multi-watt picosecond 1.7 µm Tm-doped fiber laser amplification system

We report on a multi-watt, high-repetition-rate picosecond 1.7 µm Tm-doped fiber (TDF) laser amplification system. The seed oscillator is a figure-9 passively mode-locked TDF laser, which delivers a pulse train with a center wavelength of 1738nm and a fundamental repetition rate of ∼85 MHz. After a pre-amplifier and two stages of TDF amplifiers, the output power can be amplified to 5.2 W at a pump power of 10 W, corresponding to a slope efficiency of 52.1%. The output pulse duration is 33.87 ps and the pulse energy is 61 nJ. These results demonstrated that it is an effective method for achieving high-power ultrafast fiber laser source at 1.7 µm waveband, which would be a promising candidate for diverse applications such as polymer welding, bioimaging, mid-infrared laser generation and medical applications.

Realization of Joints of Aluminosilicate Glass and 6061 Aluminum Alloy via Picosecond Laser Welding without Optical Contact

To achieve laser direct welding of glass and metal without optical contact is hard, owing to the large difference in thermal expansion and thermal conductivity between glass and metal and an insignificant melting area. In this study, the high-power picosecond pulsed laser was selected to successfully weld the aluminosilicate glass/6061 aluminum alloy with a gap of 35 ± 5 μm between glass and metal. The results show that the molten glass and metal diffuse and mix at the interface. No defects such as microcracks or holes are observed in the diffusion mixing zone. Due to the relatively large gap, the glass collapsed after melting and caulking, resulting in an approximately arc-shaped microcrack between modified glass and unmodified glass or weakly modified glass. The shape of the glass modification zone and thermal accumulation are influenced by the single-pulse energy and linear energy density of the picosecond laser during welding, resulting in variations in the number and size of defects and the shape of the glass modification zone. By reasonably tuning the two factors, the shear strength of the joint reaches 15.98 MPa. The diffusion and mixing at the interface and the mechanical interlocking effect of the glass modification zone are the main reasons for achieving a high shear strength of the joint. This study will provide reference and new ideas for the laser transmission welding of glass and metal in the non-optical contact conditions.

Bonding of PMMA to silicon by femtosecond laser pulses

Many devices and objects, from microelectronics to microfluidics, consist of parts made from dissimilar materials, such as different polymers, metals or semiconductors. Techniques for joining such hybrid micro-devices, generally, are based on gluing or thermal processes, which all present some drawbacks. For example, these methods are unable to control the size and shape of the bonded area, and present risks of deterioration and contamination of the substrates. Ultrashort laser bonding is a non-contact and flexible technique to precisely join similar and dissimilar materials, used both for joining polymers, and polymers to metallic substrates, but not yet for joining polymers to silicon. We report on direct transmission femtosecond laser bonding of poly(methyl methacrylate) (PMMA) and silicon. The laser process was performed by focusing ultrashort laser pulses at high repetition rate at the interface between the two materials through the PMMA upper layer. The PMMA-Si bond strength was evaluated as a function of different laser processing parameters. A simple, analytical, model was set up and used to determine the temperature of the PMMA during the bonding process. As a proof of concept, the femtosecond-laser bonding of a simple hybrid PMMA-Si microfluidic device has been successfully demonstrated through dynamic leakage tests.

Picosecond laser microwelding of AlSi-YAG for laser system assembly

We report the successful picosecond laser welding of AlSi and YAG. This material combination is of significant interest to the field of laser design and construction. Parameter maps are presented that demonstrate the impact of pulse energy and focal position on the resultant weld. Weld performance relevant to industrial applications is measured, i.e., shear strength, process yield, and absolute thermal resistance are presented.

Maskless, rapid manufacturing of glass microfluidic devices using a picosecond pulsed laser

Conventional manufacturing of glass microfluidic devices is a complex, multi-step process that involves a combination of different fabrication techniques, typically photolithography, chemical/dry etching and thermal/anodic bonding. As a result, the process is time-consuming and expensive, in particular when developing microfluidic prototypes or even manufacturing them in low quantity. This report describes a fabrication technique in which a picosecond pulsed laser system is the only tool required to manufacture a microfluidic device from transparent glass substrates. The laser system is used for the generation of microfluidic patterns directly on glass, the drilling of inlet/outlet ports in glass covers, and the bonding of two glass plates together in order to enclose the laser-generated patterns from the top. This method enables the manufacturing of a fully-functional microfluidic device in a few hours, without using any projection masks, dangerous chemicals, and additional expensive tools, e.g., a mask writer or bonding machine. The method allows the fabrication of various types of microfluidic devices, e.g., Hele-Shaw cells and microfluidics comprising complex patterns resembling up-scaled cross-sections of realistic rock samples, suitable for the investigation of CO₂ storage, water remediation and hydrocarbon recovery processes. The method also provides a route for embedding small 3D objects inside these devices.

Picosecond laser welding of glasses with a large gap by a rapid oscillating scan

A welding method that utilizes a picosecond laser with a small-scale rapid oscillating scan is presented in this Letter to achieve the welding of glasses with natural stacking contact (gap≈10  μm). The rapid oscillating scan of the laser not only creates enough molten material to fill the gap, but also releases the internal thermal pressure during the welding process. The contraction created by condensation of the welding area can reduce the gap to less than 3 μm, which provides necessary conditions for realizing continuous welding. By using this method, a maximum joint strength up to 64 MPa can be achieved without any defects. The detail mechanism of laser welding with a rapid oscillating scan was revealed in this Letter. This research lays a good foundation for the laser welding of large-gap glass in practical engineering applications.

Interlaced Laser Beam Scanning: A Method Enabling an Increase in the Throughput of Ultrafast Laser Machining of Borosilicate Glass

We provide experimental evidence that the laser beam scanning strategy has a significant influence on material removal rate in the ultrafast laser machining of glass. A comparative study of two laser beam scanning methods, (i) bidirectional sequential scanning method (SM) and (ii) bidirectional interlaced scanning method (IM), is presented for micromachining 1.1-mm-thick borosilicate glass plates (Borofloat® 33). Material removal rate and surface roughness are measured for a range of pulse energies, overlaps, and repetition frequencies. With a pulse overlap of ≤90%, IM can provide double the ablation depth and double the removal rate in comparison to SM, whilst maintaining very similar surface roughness. In both cases, the root-mean-square (RMS) surface roughness (Sq) was in the range of 1 μm to 2.5 μm. For a 95% pulse overlap, the difference was more pronounced, with IM providing up to four times the ablation depth of SM; however, this is at the cost of a significant increase in surface roughness (Sq values >5 μm). The increased ablation depths and removal rates with IM are attributed to a layer-by-layer material removal process, providing more efficient ejection of glass particles and, hence, reduced shielding of the machined area. IM also has smaller local angles of incidence of the laser beam that potentially can lead to a better coupling efficiency of the laser beam with the material.

Improvement of Laser Transmission Welding of Glass with Titanium Alloy by Laser Surface Treatment

Laser surface treatment of the titanium alloy was locally oxidized on the metal surface to improve the joint strength of laser transmission welding of high borosilicate glass with titanium alloy. The results find that the welding strength was increased 5 times. The welding mechanism was investigated by the morphology of the welded parts, the tensile-fracture failure mode, the diffusion of the interface elements, and the surface free energy. The results show that there are many adherents between the titanium alloy and high borosilicate glass after tensile fracture, the welding strength was higher when the laser voltage was 460 V, and the tensile⁻fracture failure mode is mainly ductile fracture. Element-line scanning analysis revealed that elemental diffusion occurred in the two materials, which is an important reason for the high welding strength. Surface free-energy analysis shows that laser surface treatment improves the surface free energy of titanium alloy, promotes the wettability and compatibility, and increases the welding strength of titanium alloy with glass.

Rapid Laser Manufacturing of Microfluidic Devices from Glass Substrates

Conventional manufacturing of microfluidic devices from glass substrates is a complex, multi-step process that involves different fabrication techniques and tools. Hence, it is time-consuming and expensive, in particular for the prototyping of microfluidic devices in low quantities. This article describes a laser-based process that enables the rapid manufacturing of enclosed micro-structures by laser micromachining and microwelding of two 1.1-mm-thick borosilicate glass plates. The fabrication process was carried out only with a picosecond laser (Trumpf TruMicro 5×50) that was used for: (a) the generation of microfluidic patterns on glass, (b) the drilling of inlet/outlet ports into the material, and (c) the bonding of two glass plates together in order to enclose the laser-generated microstructures. Using this manufacturing approach, a fully-functional microfluidic device can be fabricated in less than two hours. Initial fluid flow experiments proved that the laser-generated microstructures are completely sealed; thus, they show a potential use in many industrial and scientific areas. This includes geological and petroleum engineering research, where such microfluidic devices can be used to investigate single-phase and multi-phase flow of various fluids (such as brine, oil, and CO₂) in porous media.

Towards industrial ultrafast laser microwelding: SiO2 and BK7 to aluminum alloy

We report systematic analysis and comparison of ps-laser microwelding of industry relevant Al6082 parts to SiO₂ and BK7. Parameter mapping of pulse energy and focal depth on the weld strength is presented. The welding process was found to be strongly dependent on the focal plane but has a large tolerance to variation in pulse energy. Accelerated lifetime tests by thermal cycling from -50° to +90°C are presented. Welds in Al6082-BK7 parts survive over the full temperature range where the ratio of thermal expansion coefficients is 3.4:1. Welds in Al6082-SiO₂ parts (ratio 47.1:1) survive only a limited temperature range.

Avoiding the requirement for pre-existing optical contact during picosecond laser glass-to-glass welding

Previous reports of ultrafast laser welding of glass-to-glass have indicated that a pre-existing optical contact (or very close to) between the parts to be joined is essential. In this paper, the capability of picosecond laser welding to bridge micron-scale gaps is investigated, and successful welding, without cracking, of two glasses with a pre-existing gap of 3 µm is demonstrated. It is shown that the maximum gap that can be welded is not significantly affected by welding speeds, but is strongly dependent on the laser power and focal position relative to the interface between the materials. Five distinct types of material modification were observed over a range of different powers and surface separations, and a mechanism is proposed to explain the observations.