Abstract
Metal additive manufacturing (AM), also known as 3D printing, is a disruptive manufacturing technology in which complex engineering parts are produced in a layer-by-layer manner, using a high-energy heating source and powder, wire or sheet as feeding material. The current paper aims to review the achievements in AM of steels in its ability to obtain superior properties that cannot be achieved through conventional manufacturing routes, thanks to the unique microstructural evolution in AM. The challenges that AM encounters are also reviewed, and suggestions for overcoming these challenges are provided if applicable. We focus on laser powder bed fusion and directed energy deposition as these two methods are currently the most common AM methods to process steels. The main foci are on austenitic stainless steels and maraging/precipitation-hardened (PH) steels, the two so far most widely used classes of steels in AM, before summarising the state-of-the-art of AM of other classes of steels. Our comprehensive review highlights that a wide range of steels can be processed by AM. The unique microstructural features including hierarchical (sub)grains and fine precipitates induced by AM result in enhancements of strength, wear resistance and corrosion resistance of AM steels when compared to their conventional counterparts. Achieving an acceptable ductility and fatigue performance remains a challenge in AM steels. AM also acts as an intrinsic heat treatment, triggering ‘in situ’ phase transformations including tempering and other precipitation phenomena in different grades of steels such as PH steels and tool steels. A thorough discussion of the performance of AM steels as a function of these unique microstructural features is presented in this review.
Highlights
Overview of additive manufacturing of alloys and steelsAdditive manufacturing (AM), commonly known as 3D printing, has recently gained huge interest in both academia and industry, with its market value expected to reach $21 billion by the end of 2020 [1]
If we consider the cost of a final product as a function of material cost, tooling cost, equipment cost, and overhead cost, further work and progress is needed for AM to outperform traditional manufacturing in terms of material cost, equipment cost and overhead cost, as described in detail in [12]
A similar conclusion has been made by Baek et al [74] who report a higher resistance to hydrogen embrittlement under high-pressure H atmosphere for AM 304L austenitic stainless steel compared to its conventional counterpart, which is mainly discussed based on the stability of the austenite phase that does not transform to martensite phase under load stress
Summary
Additive manufacturing (AM), commonly known as 3D printing, has recently gained huge interest in both academia and industry, with its market value expected to reach $21 billion by the end of 2020 [1]. If we consider the cost of a final product as a function of material cost, tooling cost, equipment cost, and overhead cost, further work and progress is needed for AM to outperform traditional manufacturing in terms of material cost, equipment cost and overhead cost, as described in detail in [12]. Another critical issue is the fact that despite AM’s physical metallurgy commonalities to phenomena observed during casting, welding, powder metallurgy and thermo-mechanical processing, many of the established textbook theories for traditional manufacturing might fail in AM [13]. An overview of how current AM steels compete with conventionally processed steels in terms of performance would, provide invaluable insight for the ongoing research in AM of steels
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