Development of Structural Steels for Powder Bed Fusion - Laser Beam
Doktorsavhandling, 2023

Over the past decade, powder bed fusion – laser beam (PBF-LB) has attracted noticeable attention from both academia and industry. However, there remains a scarcity of approved material for the process, as fewer than 40 alloys are commercially available. Although structural steels are some of the most commonly used materials in traditional manufacturing, they have yet to be developed for PBF-LB as their high carbon content makes them susceptible to cracking. The objective of this thesis was to develop structural steels for PBF-LB by determining the impact of various process parameters on part quality, microstructure and mechanical properties. This involved the production and analysis of various carbon (0.06 to 1.1 wt.% C) and low-alloy steels (AISI 4130, 4140, 4340 and 8620).

In terms of part quality, specimen density was related to the volumetric energy density (VED) and the carbon content of the alloy. Regarding the VED, specimens produced at low VED formed lack of fusion porosity, while specimens produced at high VED formed keyhole porosity. As for the carbon content, increasing the carbon content would reduce lack of fusion porosity at low VED, while lowering the required VED to form keyhole porosity. As for cold cracking, this occurred in structural steels with ≥ 0.38 wt.% C as elevated carbon contents would increase specimen hardness. However, cracking could be mitigated by increasing the VED, laser power or build plate preheating temperature, as each enhanced the level of in situ tempering during PBF-LB. From these findings, process windows were established for each structural steel that produced defect-free and high-density specimens (> 99.8%).

In terms of the microstructure, the as-built specimens were primarily composed of tempered martensite, with retained austenite also observed in alloys with ≥ 0.75 wt.% C. During PBF-LB, martensite formed during layer melting and was initially in a quenched-like state, with carbon atoms segregating to dislocations and martensite lath boundaries. Subsequent tempering of this martensite was due to micro-tempering within the heat affected zone and macro-tempering within the previously solidified material. Although both influenced martensite tempering, micro-tempering had the most significant effect as it reduced martensite hardness by up to ~380 HV. This noticeable reduction in hardness was due to the precipitation of nano-sized carbides at the previously carbon enriched regions of martensite.

Lastly, mechanical testing found that structural steels produced by PBF-LB achieved a high ultimate tensile strength (4140: ∼1400 MPa, 4340: ∼1500 MPa, 8620: ∼1100 MPa), impact toughness (4140:∼90–100 J, 4340:∼60–70 J, 8620:∼150–175 J) and elongation (4140:∼14%, 4340:∼14%, 8620:∼14–15%) that met or exceeded the ASTM standards. Additionally, these specimens displayed limited directional anisotropy due to small grains with weak crystallographic texture, a homogenous microstructure and low levels of internal defects. These findings are meant to highlight that these alloys are not only suitable but actively take advantage of PBF-LB to achieve properties that meet or exceed those of conventionally produced alloys.

tensile properties

in situ tempering

process development

martensite

additive manufacturing

cold cracking

low-alloy steel

structural steel

powder bed fusion – laser beam

carbon steel

Virtual Development Laboratory (VDL), Tvärgata 4C, Chalmers
Opponent: Prof. Thomas Niendorf

Författare

William Hearn

Chalmers, Industri- och materialvetenskap, Material och tillverkning

Effect of Carbon Content on the Processability of Fe-C Alloys Produced by Laser Based Powder Bed Fusion

Frontiers in Materials,;Vol. 8(2022)

Artikel i vetenskaplig tidskrift

Hearn, W; Hryha, E "Influence of Carbon on the Microstructure of Carbon Steels produced by L-PBF"

Hearn, W; Cordova, L; Raza, A; Dahl-Jendelin, A; Hryha, E "Impact of Powder Properties on the Deoxidation and Densification Behavior of Carbon Steel produced by PBF-LB"

Laser-based powder bed fusion of non-weldable low-alloy steels

Powder Metallurgy,;Vol. 65(2022)p. 121-132

Artikel i vetenskaplig tidskrift

In situ tempering of martensite during laser powder bed fusion of Fe-0.45C steel

Materialia,;Vol. 23(2022)

Artikel i vetenskaplig tidskrift

Development of powder bed fusion–laser beam process for AISI 4140, 4340 and 8620 low-alloy steel

Powder Metallurgy,;Vol. 66(2023)p. 94-106

Artikel i vetenskaplig tidskrift

Additive manufacturing, better known as 3D printing, is a technology that has recently entered the public interest. In its simplest form, it refers to any technology that produces a part layer by layer, just like one would when playing with Lego. However, additive manufacturing differs as it can produce geometrically complex parts directly from computer files, affording freedom of design that cannot be achieved with other manufacturing techniques.

One of the recent advances in additive manufacturing has been the production of functional metal-based parts. However, there are still many factors that limit their adoption. One of the biggest is the limited number of metals that are approved for the process. Essentially, metal additive manufacturing is like a restaurant with almost nothing on the menu. Without enough tasty starters, entrees or desserts it is difficult to convince customers to eat at the restaurant, which subsequently makes it difficult for the restaurant to grow. An effective way to deal with this is to develop new materials for the process, which was the focus of the current thesis work.

The material of interest in this thesis was structural steel as it is commonly used in the automotive, railway and construction industries. Despite this wide-spread appeal, structural steels are not often used in metal additive manufacturing as they are difficult to produce without defects. To overcome this issue, one must tune the composition of the material as well as the parameters of the process, just like one would need to adjust the ingredients and recipe when developing a new menu item. Since this is a relatively new topic, a variety of structural steels were examined, in addition to a variety of process parameters. This was done to identify the required conditions for high quality parts and to determine the suitability of the material for the process. After extensive analysis, it was possible to define process windows for high quality structural steel parts. Additionally, results found that the alloys were not only suitable but actively took advantage of additive manufacturing to achieve properties that met or exceeded those of conventionally produced material. This knowledge can serve as a basis for the production of structural steels that can further the adoption of metal additive manufacturing.

Ämneskategorier

Materialteknik

Bearbetnings-, yt- och fogningsteknik

Metallurgi och metalliska material

ISBN

978-91-7905-778-7

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5244

Utgivare

Chalmers

Virtual Development Laboratory (VDL), Tvärgata 4C, Chalmers

Online

Opponent: Prof. Thomas Niendorf

Mer information

Senast uppdaterat

2023-01-25