Segregations in Steels during Heat Treatment – A Consideration along the Process Chain

Segregation is an unavoidable phenomenon in continuous or ingot casting of steel for thermodynamic reasons. If costly processes that explicitly reduce segregation are not carried out, the segregations remain until the final product. Therefore, an understanding of the development and effects of segregation along the entire process chain is necessary. The focus in this publication is on the treatment of segregations in low alloy steels. First, the characterisation of segregation is presented. An overview of the formation and development of segregations during primary shaping and forming processes is given. The focus is on segregation-related effects during heat treatment with regard to inhomogeneous microstructure and negative heat treatment results such as distortion or hardening cracks. In a short outlook, the influence of segregation on the component behaviour is described. ◼

Inhomogeneous Microstructure and its Thermal Stability of AlSi10Mg Lattice Structure Manufactured via Laser Powder Bed Fusion

The inhomogeneous microstructure and its change by annealing for an AlSi10Mg lattice structure with a body-centered cubic unit cell additively manufactured via laser powder bed fusion (LPBF) were investigated. The as-built lattice structure exhibited a cellular microstructure consisting of a number of primary α-Al phases decorated with α-Al/Si eutectic structure. The developed microstructure varied depending on the locations of the node and strut parts of the lattice structure. At the location near the bottom surface of the node part, the cellular microstructure became coarser and more equiaxed than those at the location near the top surface. At the location near the bottom surface of the strut part, the columnar α-Al phases were often elongated along the direction of the strut part. After the annealing at 300 °C for 2 h, numerous Si particles finely precipitated within the primary α-Al phases and coarsening of the eutectic Si phases occurred. After the annealing at 530 °C for 6 h, the microstructural characteristics changed significantly. A significant coarsening of the Si particles and the formation of Fe-containing intermetallic phase (β-AlFeSi) with a plate-shaped morphology occurred. The microstructures became homogeneous in the whole area of the lattice structure annealed at 530 °C for 6 h.

Effect of Cr Addition on Microstructure and Properties of AuGa Solder

In order to meet the service requirements of electronic devices working at 300 °C in the fields of energy resource prospecting and space exploration, Cr element was added to modify AuGa solder to improve its high-temperature performance. The results showed that the addition of 0.3 wt.% Cr element reduced the loss of Ga element in the smelting and casting process, and effectively improved the problem of the inhomogeneous microstructure of the solder matrix. On the basis of maintaining the good wettability of the solder, the addition of trace chromium effectively restrains the excessive flux of the solder, and the presence of chromium improves the oxidation resistance of the solder. Furthermore, Cr element optimized the interface morphology and improved the mechanical properties of the solder joint. The shear strength of the AuGa-0.3Cr/Ni joint was 87.2 MPa, which was 13.1% higher than that of the joint without Cr element. After 240 h of aging, the shear strength of the AuGa-0.3Cr joint was still the peak value at 84.1 MPa, which was 16.3% higher than that of the AuGa joint.