Demonstrating the capabilities of blue lasers in additive manufacturing
In the five years since the blue industrial laser was introduced to the market, power and brightness have continuously improved.
Applications have also rapidly expanded, beginning with improving the speed and quality of foil welding for lithium ion batteries, then bringing those same advantages to materials processing applications in e-mobility and consumer electronics.
Now, for the first time, we have demonstrated the capabilities of the blue industrial laser in powder bed fusion (PBF) additive manufacturing (AM).
The performance advantages of the blue industrial laser stem from fundamental physics.
Copper, aluminium, stainless steel and many other reflective metals absorb blue light far better than they absorb the infrared (IR) radiation of traditional laser welding sources. IR is often limited to operating in keyhole mode, where the target material is vaporised, ejecting target material and leaving voids. Blue works in conduction mode, melting material with minimal spatter and voids. Those characteristics lead directly to qualitative and quantitative improvements in welding, advantages that transfer directly to AM.
Bringing blue to additive manufacturing
AM is a blanket term referring to a range of part fabrication techniques that create parts by building up raw material into the desired shape. Among the laser-based methods suitable for creating metal parts, two approaches dominate, laser metal deposition (LMD) and PBF. LMD can produce larger parts and is well suited for cladding applications, but PBF produces better dimensional accuracy and smoother surfaces. As a result, PBF accounts for about 95 per cent of applications.
The blue laser’s welding performance leads to the expectation that it would bring the same speed and quality advantages to AM. The first stage in verifying that expectation was to measure build rate efficiency – volume produced per unit time per watt of laser power. Parts produced with a blown-powder LMD machine outfitted with both an IR and blue laser were used to produce test blocks in titanium, stainless steel, copper and other materials. As shown in Figure 1, the build rate efficiency for blue was about 1.4 times to nearly 7 times that of IR.
Figure 1: The additive manufacturing build rate efficiency for the blue laser exceeds that of infrared for a wide range of metals
The next step in demonstrating blue industrial laser AM capabilities was integration into a PBF machine. The PBF process spreads a layer of powder feedstock in a print area. The laser spot heats specific locations, fusing the powder. When one layer is complete, a subsequent layer is added, and the laser fuses the raw material to both adjacent powder on the same layer and to the layer beneath. PBF integration is challenging because the laser source is directed to the build volume through an optical scanner. To maintain the laser spot quality across the working area, the scanner uses a specialised f-theta lens, but such lenses unavoidably introduce aberrations that increase spot size. That means the laser source must have both sufficient power and brightness. The Nuburu AI-series, with a typical beam parameter product of under 5mm.mrad, is the first blue industrial laser capable of integrating into an optical-scanning system.
At SPIE’s Photonics West conference in January, Nuburu reported results for a PBF machine after its standard 200W single-mode IR laser had been replaced with the 200W AI-200. The system delivers a 175µm x 200µm spot to the build volume, creating a power density of 524W/cm2, depositing 20µm thick layers over a 90mm x 90mm addressable area. Test articles in both SS316L powder and pure copper powder were each fabricated in conduction mode, achieving a reduced spot size, less spatter and improved part density. Density is a key performance metric for AM, as ‘solid’ material maximises the mechanical, thermal and electrical properties of the finished part.
Early tests of additive manufacturing with the blue industrial laser demonstrate its ability to quickly fabricate high-quality parts in many metals.
For example, proof-of-principle tensile bar test coupons made from SS316 stainless steel have greater than 98 per cent density, and an ultimate yield tensile strength of 80,000 psi. As-printed stainless steel blocks have a density of 99.8 per cent. For these stainless steel parts, the blue laser build rate is twice that of the standard IR laser. Copper blocks produced with blue have an as-printed density greater than 97.5 per cent, with modifications planned to bring that to greater than 99 per cent. The original IR laser was unable to melt the copper, making quantitative comparison impossible, but displaying an obvious difference in performance.
It’s worth noting these initial results were achieved prior to any process optimisation and with no post-processing. Process parameters can – and should – be adjusted to optimise printing performance. In addition to the standard optical parameters, such as spot size, laser power and scan rate, AM brings in new variables, such as grain size of the feedstock, layer thickness and composition of the atmosphere in the build chamber. It’s reasonable to expect that the advantages of blue industrial laser PBF printing will become even more dramatic as these parameters are tuned.
Beyond the part
New adopters of AM can be taken by surprise at the degree of post-processing required to complete an additively manufactured part. Depending upon the printing method, in addition to steps such as support removal and depowdering, parts may need heat treatment, sintering, hot isostatic pressing, infiltration of filler material and various grinding and polishing treatments. These steps can create shape changes and shrinkage, which are difficult to predict, but – perhaps worse from a global perspective – these steps all take time and resources. The blue laser produces parts not only of high density, but also to near net shape.
Compare this, for example, with binder jet AM, which joins powder with an adhesive-like liquid – a rapid process. Then, however, the part must be heated to drive away the residual binder and join the powder, which shrinks the part, often unevenly. That can require up to 12 hours of process time followed by several hours of hot isostatic pressing. Binder jet printing may also require infiltration of another metal into the gaps. These essential processes all take time.
Nuburu is now demonstrating its blue laser technology for use in powder bed fusion additive manufacturing
Compare that to the blue laser fabrication process. The high density as-printed eliminates the need for extensive processing, and the printed parts exhibit essentially no shrinkage. In addition, conduction mode operation is inherently ‘more gentle’, minimising internal stresses and often obviating the need for annealing or hot isostatic pressing. Blue laser printed parts can reach their final form simply with a surface polish.
Taken together, the higher rate of blue industrial laser AM and the minimal post-processing make a strong economic case for preferring blue laser AM for aluminium, steel, titanium, copper and other industrially important metals.
Living up to expectations
Fundamental physics and a history of excellent performance in welding led to an expectation that the blue industrial laser would excel in AM performance. The advantage was expected to be particularly striking for powder bed fusion, with its capability to produce detailed and accurate parts. Simply by replacing a PBF machine’s standard IR laser scanning system with the high-brightness AI-200 blue industrial laser and its own scanner, the high performance expectations are proving to be well-founded. Part quality is high, production times are low, and post-processing is minimised. Together, these advantages create a compelling case for adopting the blue industrial laser for AM applications.
Dr Mark Zediker is the CEO and founder of Nuburu
Jean-Michel Pelaprat is the CM&SO and co-founder of Nuburu
Andrew Dodd is the VP of global sales at Nuburu