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The research team whose participates from The State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Department of Materials Science and Engineering, Peking University and Department of Orthopaedics, Peking University Third Hospital Published the paper Hot cracking in ZK60 magnesium alloy produced by laser powder bed fusion process on the magazine Materials Letters. This article mainly identified serious solidification and liquation cracking in ZK60 LPBF samples formed by BLT-S210.

Mg alloys are one of the most promising materials for lightweight structures and biodegradable implants [1]. Compared to general Mg alloys, ZK60 alloy (Mg-5.6Zn-0.5Zr, wt%) has attracted many attentions due to relatively high strength, high stress corrosion resistance and good biocompatibility [2]. However, ZK60 alloy is highly susceptible to hot cracking during welding process [3–6].

A commercial LPBF machine (BLT, Xi’an, China) was used to selectively melt ZK60 spherical powder (Weihao, Tangshan, China) with a mean size of 30 μm [9]. The laser spot size was 70 μm. Considering the high reactivity and high vaporization tendency of magnesium, shielding atmosphere was carefully controlled with oxygen content lower than 80 ppm, and relatively low heat input was used to fabricate 10mm×10mm×10mm cubic samples.

Hot cracking was identified during the LPBF of ZK60 alloy, which was attributed to a wide solidification temperature range, a low eutectic temperature as well as high thermal stress. It was difficult to inhibit cracks and avoid the occurrence of lack of fusion, porosity and serious distortion at the same time just by adjusting laser energy input under the present conditions. The influence of sample size, preheating and scanning strategy on the cracking susceptibility needs to be investigated in the future.

It is reported that as early as 2020, the research group of the Institute of Materials Forming and Manufacturing of Tsinghua University used BLT-S210 to conduct research on magnesium alloy materials.

Remark: Part of the content of this article comes from the paper Hot cracking in ZK60 magnesium alloy produced by laser powder bed fusion process

[1]Y. Liu, Y. Zheng, X.-H. Chen, J.-A. Yang, H. Pan, D. Chen, L. Wang, J. Zhang, D. Zhu, S. Wu, K.W.K. Yeung, R.-C. Zeng, Y. Han, S. Guan, Fundamental Theory of Biodegradable Metals—Definition, Criteria, and Design, Adv. Funct. Mater. 29 (18) (2019) 1805402, https://doi.org/10.1002/adfm.201805402.

[2]N. Fakhar, M. Sabbaghian, A good combination of ductility, strength, and corrosion resistance of fine-grained ZK60 magnesium alloy produced by repeated upsetting process for biodegradable applications, J. Alloys Compd. 862 (2021) 158334, https://doi.org/10.1016/j.jallcom.2020.158334.

[3]D.C. Wagner, X. Chai, X. Tang, S. Kou, Liquation Cracking in Arc and Friction-Stir Welding of Mg-Zn Alloys, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 46 (1) (2015) 315–327, https://doi.org/10.1007/s11661-014-2606-5.

[4]Z.-H. YU, H.-g. YAN, X.-y. YIN, Y. LI, G.-H. YAN, Liquation cracking in laser beam welded joint of ZK60 magnesium alloy, Trans. Nonferrous Met. Soc. China (English Ed. 22 (12) (2012) 2891–2897, https://doi.org/10.1016/S1003-6326(11)61547-1.

[5]Z.H. Yu, H.G. Yan, J.H. Chen, Y.Z. Wu, Effect of Zn content on the microstructures and mechanical properties of laser beam-welded ZK series magnesium alloys, J. Mater. Sci. 45 (14) (2010) 3797–3803, https://doi.org/10.1007/s10853-010- 4434-3.

[6]K. Liu, S. Kou, Susceptibility of magnesium alloys to solidification cracking, Sci. Technol. Weld. Join. 25 (3) (2020) 251–257, https://doi.org/10.1080/ 13621718.2019.1681160.

Based on the research of porous orthopedic implants made of magnesium alloy, the Research Group at Tsinghua University’s Institute of Materials Forming and Manufacturing gave priority to foreign brands of metal 3D printing machine at the stage of machine selection. However, due to restrictive factors such as service and location, the group finally chose the high-quality machine of a national brand. After rounds of technical challenges, BLT was appreciated by Tsinghua University’s Material Forming Laboratory for its high-quality products and professional services. “BLT deserves acknowledgment, and both sides will continue to maintain in-depth cooperation,” Professor Wen commented.

From Trying to Buying

In order to efficiently complete frontier research topics, the Research Group led by Professor Wen at Tsinghua University’s Institute of Materials Forming and Manufacturing collaborated with a leading German university and an international equipment manufacturer. However, due to the long distance between the two sides and the fact that the international partner didn’t have a branch in China, there were many inconveniences in terms of service and communication, so the research group must find a high-quality domestic solution.

After plenty of investigation and surveys, Professor Wen focused on BLT. At the end of December 2019, after an in-depth communication, Professor Wen provided the test material to BLT to try to develop the material forming technology of magnesium alloy in China.

Having understood Professor Wen’s technical requirements in detail, based on the technical characteristics such as the high activity of magnesium alloys, low boiling point, serious slagging, and high requirements on machine safety, process stability, and matching degree of parameters, BLT specially set up an interdepartmental technical team to solve these technological difficulties. On the ground of extensive experimental facts, the special team successfully printed out the verification parts of magnesium alloy which met the technical requirements and developed customized parameters for magnesium alloy forming within just 40 days. Professor Wen said: “I personally went to BLT twice and saw those batches of test parts were all in good conditions.” The Research Group finally chose BLT as its strategic partner and adopted the forming parameters of magnesium alloy generated based on BLT-S210.

Building up Trust with Profession and Service

In 2020, the Research Group moved from the part-validation stage to the official production stage in the massive outbreak of the epidemic. Due to the impact of the epidemic in Beijing at the time, the selection of the site and the installation and commissioning of the machine faced huge challenges. Upon understanding the situation  BLT actively communicated with the Research Group and decided to print at BLT factory in the first place, with the Research Group sending dedicated personnel to the site, so as to meet the periodic production requirements and ensure the project progress.Following the successful advancement of the project, the Research Group also purchased BLT-S210 furnished with forming parameters of magnesium alloy for regular material development, and the technical team of BLT also passed on to the Research Group the technical experience gained when assisting in the research and development.

Better Future with “Magnesium”

Magnesium, which is gentle to the body, is highly absorbable and biocompatible. Medical magnesium alloys have a density and modulus of elasticity close to that of orthopaedic implants, and have a controllable corrosion rate. Professor Wen believes that magnesium alloys have good application prospects in the field of cardiovascular implants and bone repair. Currently, BLT has successfully developed forming parameters for active materials such as magnesium and zinc alloys. 

The Picture shows the parts of new materials printed by BLT machine: molybdenum electrode, nickel-titanium vascular stent

Recently, the team led by Assistant Professor Li Zan at the National Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University generated research results with the help of BLT-S210 and published the paper “Enhanced strengthening and hardening via self-stabilized dislocation network in additively manufactured metals” in Materials Today (Impact Factor: 31.04). The stable ultramicro-dislocation cell structure in additively manufactured metals plays a key role in strengthening and hardening. However, its activity is usually considered to be related to the alloying element. This paper reported that dislocation cells in 3D printed metallic materials would remain self-stabilizing even in the absence of alloying effects.

It was found that Lomer dislocation lock and complex dislocation network were formed in the multiple heating-cooling cycles inherent to the additive manufacturing technology. Such unique dislocation configuration produced enhanced and stable effect of strain hardening, which in turn resulted in the 3D printed metallic material of copper with significantly better mechanical property than that of the copper materials produced in conventional methods. This work demonstrates the ability of laser-selective melting to modulate the dislocation structure to achieve high-performance metallic materials.

It is learned that Professor Li’s team, relying on BLT-S210 purchased in 2020, has published a number of research achievements in leading international journals in the field of metals such as International Journal of Plasticity, Scripta Materialia, Materials Research Letters and so on. “BLT-S210, a specialized machine with small forming size developed specifically for basic research, reduces the powder mass of several kilograms required for conventional metal printing to several hundred grams, which greatly improves the efficiency and versatility of basic research“, Professor Li said. “At the same time, the good stability and convenience in operation of BLT-S210 also provide an important guarantee for the development and creation of new material systems”, Professor Li added.

On September 21st, the Global Suppliers Conference of Airbus Group (“Airbus” for short) was held in Tianjin, China. More than 300 global suppliers of Airbus gathered together in Tianjin to talk about the beautiful future.

Xi’an BLT Laser Technologies Co., Ltd. (“BLT” for short) has begun its cooperation with Airbus since 2013. BLT was invited to participate in the Global Suppliers Conference of Airbus as the only supplier of 3D printing technology. At the conference, BLT attracted high attention with its 3D printing of light-weight and super-large-dimension components displayed in the exhibition area; it also came into focus at the summit forum as an innovative and entrepreneurial enterprise. Jia Xin, the Deputy General Manager of BLT, was invited to attend the theme summit forum, which fully displayed the emphasis placed by Airbus on metal 3D printing.

At the special activity-“Doing Business in China” Summit Forum of Airbus, BLT, AVIC and Aleris had discussion with Airbus concerning the “Business Strategy and Win-win Cooperation in the Chinese Market”, hosted by Francois Mery from Airbus. Director of AVIC Civil Aircraft Industry Department Pang Zhen, VP of Aleris Ingo Kroepfl, and Deputy General Manager of BLT Jia Xin, discussed problems including “how to carry out commercial activities in China; the willingness and path of Chinese suppliers to support the globalization strategy of Airbus, etc.”, and shared experience and inspirations from the actual practice of the company.

BLT Deputy General Manager Jia Xin at the Summit Forum of Innovative and Entrepreneurial Enterprises

After the forum, the Senior Vice President of Airbus led a delegation to visit the exhibition booth of BLT, in order to have a deep understanding of the application of BLT’s metal 3D printing technology in the aerospace field. BLT was under the spotlight of the exhibition with its metal 3D printing of large-size components with a diameter of 600mm. The exhibits with a fillet of only 80mm and the skillful application of lightweight design with topological structure brought more technical inspirations for Airbus.

The Delegation Led by Airbus Senior VP Visited BLT’s Exhibition Booth for In-depth Understanding and Communication