Welding Metallurgy of

Properties and Application of Geopolymers Vol. 841 (/MSF.841 /book) Development and Investigation of Materials Using Modern Techniques Vol. 840 (/MSF.840/book) Superplasticity in Advanced Materials ICSAM 2015 Vols. 838-839 (/MSF.838-839/book) 12th International Conference on High Speed Machining Vols. 836-837 (/MSF.836-837/book) Sintering Fundamentals II Vol. 835 (/MSF.835/book) Advanced Machining Technologies: Traditions and Innovations Vol. 834 (/MSF.834/book) Applied Materials and Technologies Vol. 833 (/MSF.833/book) Emerging Functional Materials: Book (/MSF.841/book) Papers (/MSF.841)

Materials Science Forum

The mass saving potential of light-weight materials, such as Al alloys, is beneficial for fuel economy and reducing CO2 emissions. However, the wide-spread use of these alloys has been long hindered due to the difficulty in fusion joining as well as their high cost. Welding of Al alloys, which are considered to be difficult to weld through conventional arc welding, is now possible by either of low heat input arc welding, high-power density fusion joining, such as laser beam welding and electron beam welding, or friction stir welding. Particularly, friction stir welding can be successfully applied to these materials owing to the fact that no melting takes place in the weld nugget. The aim of this overview is to summarize the developments in the joining of Al alloys over the recent years. This study is also intended to provide guidance for the industry and researchers dealing with joining of these alloys.

Recent developments in joining of aluminum alloys

Rod shaped samples of AlSi10Mg additively manufactured using recycled powder through direct metal laser sintering (DMLS) process showed higher quasi-static uniaxial tensile strength in both horizontal and vertical build directions than those of cast counterpart alloy. In addition, they offered mechanical properties within the range of other additively manufactured counterparts. TEM showed that the microstructure of the as-built samples comprised of cell-like structures featured by dislocation networks and Si precipitates. HRTEM studies revealed the semi-coherency characteristics of the Si precipitates. After deformation, the dislocation density increased as a result of generation of new dislocations due to dislocation motion. The dislocations bypassed the precipitates by bowing around them and penetrating the semi-coherent precipitates. Strengthening of recycled DMLS-AlSi10Mg alloys manufactured in both directions was attributed to Orowan mechanism (due to existence of Si precipitates), Hall-Petch effect (due to eutectic cell walls), and dislocation hardening (due to pre-existing dislocation networks). Due to the slightly different microstructure, the contribution of each strengthening mechanism was slightly different in identical samples made with virgin powder.

Strengthening mechanisms in direct metal laser sintered AlSi10Mg: Comparison between virgin and recycled powders

Aluminum alloys are key materials in additive manufacturing (AM) technologies thanks to their low density that, coupled with the possibility to create complex geometries of these innovative processes, can be exploited for several applications in aerospace and automotive fields. The AM process of these alloys had to face many challenges because, due to their low laser absorption, high thermal conductivity and reduced powder flowability, they are characterized by poor processability. Nowadays mainly Al-Si alloys are processed, however, in recent years many efforts have been carried out in developing new compositions specifically designed for laser based powder bed AM processes. This paper reviews the state of the art of the aluminum alloys used in the laser powder bed fusion process, together with the microstructural and mechanical characterizations.

New aluminum alloys specifically designed for laser powder bed fusion: A review

A new grain orientation spread approach (GOS ≤ 5°) was proposed to study DRX of a Mg-Zn-Zr alloy during hot deformation. DRXed grains possessed random texture while the deformed grains contributed to basal texture. The alloy exhibited rapid DRX at low deformation strains followed by near-saturated behavior as strain increased.

A new grain orientation spread approach to analyze the dynamic recrystallization behavior of a cast-homogenized Mg-Zn-Zr alloy using electron backscattered diffraction

In the current study, cylindrical samples of AlSi10Mg alloy were fabricated using direct metal laser sintering (DMLS) technique in vertical and horizontal directions. The microstructure of the samples was analyzed using scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. It was observed that, by changing the building direction from vertical to horizontal, columnar to equiaxed transition (CET) occurred in the alloy. While 75% of the grains in the vertical sample were columnar, by changing the direction to horizontal, 49% of the grains evolved with columnar shape and 51% of them were equiaxed. Moreover, the texture of DMLS-AlSi10Mg alloy changed due to CET. While {001} fiber texture evolved in the vertical sample, the direction tilted away from the building direction in the horizontal one. Using the fundamentals of solidification and constitutional undercooling, the solidification behavior of AlSi10Mg alloy during DMLS process was modeled. It was observed that, the determinant parameter in CET during DMLS of AlSi10Mg alloy is the angle between the nominal growth rate and h k l > direction of the growing dendrite, which is controlled by the geometry and building direction of the sample. Further TEM studies confirmed that, CET alters the shape and coherency of Si precipitates and dislocation density inside the α-Al dendrites in DMLS-AlSi10Mg alloy.

Columnar to equiaxed transition during direct metal laser sintering of AlSi10Mg alloy: Effect of building direction

Microstructure of deformed DMLS-AlSi10Mg under quasi-static loading was studied using TEM to elaborate the strengthening mechanisms. In addition to Orowan (due to presence of Si precipitates), Hall-Petch (due to eutectic Si walls), and dislocation hardening (due to pre-existing entangled dislocations) mechanisms, Mg2Si precipitates (colonies) contributed to the strength of alloy by impeding dislocation motion. The level of this contribution was evaluated as ~13 MPa by comparing the modeled and measured yield strength.

Contribution of Mg2Si precipitates to the strength of direct metal laser sintered AlSi10Mg

In this paper, maraging steel powder was deposited on top of an H13 tool steel using laser powder bed fusion (LPBF) technique. The mechanical properties, microstructure, and interfacial characteristics of the additively manufactured MS1-H13 bimetals were investigated using different mechanical and microstructural techniques. Several uniaxial tensile tests and micro-hardness indentations were performed to identify the mechanical properties of the additively manufactured bimetal. Advanced electron microscopy techniques including electron backscatter diffraction and transmission electron microscopy were used to identify the mechanism of interface formation. In addition, the microstructure of the additively manufactured maraging steel along with the conventionally fabricated substrate-H13 were studied. It was concluded that, a very narrow interface was formed between the additively manufactured maraging steel and the conventional H13 without forming cracks or discontinuities. The first deposited layers possessed the highest hardness due to grain size refinement, solid solution strengthening, and cellular solidification structure. Finally, under uniaxial tensile loading, the additively manufactured bimetal steel failed from the underlying tool steel, indicating a robust interface.

Amir Hadadzadeh

Papers

Strengthening mechanisms in direct metal laser sintered AlSi10Mg: Comparison between virgin and recycled powders

A new grain orientation spread approach to analyze the dynamic recrystallization behavior of a cast-homogenized Mg-Zn-Zr alloy using electron backscattered diffraction

Columnar to equiaxed transition during direct metal laser sintering of AlSi10Mg alloy: Effect of building direction

Contribution of Mg2Si precipitates to the strength of direct metal laser sintered AlSi10Mg

Additive manufacturing of maraging steel-H13 bimetals using laser powder bed fusion technique