Guifeng Zhang

Modeling the strengthening response to aging process of heat-treatable aluminum alloys containing plate/disc- or rod/needle-shaped precipitates

Abstract For the heat-treatable aluminum alloys containing plate or rod/needle-shaped precipitates, a previous model was modified to present quantitative relationships between the yield strengths of the alloys and the sizes, volume percentages of precipitates, related to aging temperature and aging time as well as alloy compositions, while the strengthening of the precipitates was coupled with the whole evolution process, i.e. nucleation, growth and coarsening, of the precipitates. It was found that the aging yield strengths have been well predicted by the model for a series of aged Al–Cu binary alloys, 6061 alloys and Al–Zn–Mg alloys. It was also experimentally proved that the model was suitable to evaluate the aging strengthening of the precipitates for an Al–Cu–Mg alloy and an Al–Mg–Si alloy. Furthermore, a detailed discussion has been made to the variation of aspect ratio of precipitates, relative to its strengthening response, with aging parameters and alloy compositions.

An investigation on the effects of side assisting gas flow and metallic vapour jet on the stability of keyhole and molten pool during laser full-penetration welding

This paper reports on a study aiming at separating the effects of side assisting gas flow from a metallic vapour jet on the transient behaviour of a molten pool and a keyhole during laser full-penetration welding. To achieve the research purpose, laser welding process was simulated under three different conditions: in the presence of both side gas flow and metallic vapour jet, in the presence of side gas flow alone and in the presence of metallic vapour jet alone. It was found that the side gas flow not only pushed the molten melt to flow towards the rear part of the molten pool but also formed, on the molten pool surface, a proper pressure distribution which helped maintain both the humping in the rear part of the molten pool and the concave around the upper exit of the keyhole. Under the condition with side gas flow, the swelling formed around the keyhole blocked the side gas flow and on the other hand the side gas flow pushed them to flow backwards to the rear part of the molten pool more effectively, thereby enlarging and stabilizing the keyhole exit. Furthermore, the peak value of the average pressure in the region composed of the molten pool and keyhole decreased step by step with the growth of the concave. Finally, the interaction between the metallic vapour and molten melt was well controlled using the side gas flow, which led to an improvement in the stability of the molten pool and a reduction in spatters and pores.

Effects of shoulder on interfacial bonding during friction stir lap welding of aluminum thin sheets using tool without pin

To separately investigate the potential effects of shoulder on increasing interfacial bonded area and its mechanism, friction stir lap welding (FSLW) of 1.8 mm thick Al sheets without and with insert (copper foil or Al-12Si powders) was conducted using a special tool without pin, respectively. All the FSLW joints (without insert) fractured within top sheet but not along faying surface, suggesting that the shoulder plays an important role comparable or superior to pin in FSLW of thin sheets. Using several specially designed experimental techniques, the presence of forging and torsion actions of shoulder was demonstrated. The fracture surface of the joints with inserts indicates that interfacial wear occurs, which results in the oxide film disruption and vertically interfacial mixing over the area forged by shoulder with a larger diameter than a general pin, especially at the boundary region of weld. The boundary effect can be induced and enhanced by forging effect and torsion effect.

Development of Al-12Si-xTi system active ternary filler metals for Al metal matrix composites

Abstract To improve the wettability of Al metal matrix composites (Al-MMCs) by common filler metals, Al-12Si-xTi (x=0.1, 0.5, 1, 3.0; mass fraction, %) system active ternary filler metals were prepared. It was demonstrated that although the added Ti existed within Ti(Al1-xSix)3 (0≤x≤0.15) phase, the shear strength and shear fracture surface of the developed Al-12Si-xTi brazes were quite similar to those of traditional Al-12Si braze due to the presence of similar microstructure of Al-Si eutectic microstructure with large volume fraction. So, small Ti addition (∼1%) did not make the active brazes brittle and hard compared with the conventional Al-12Si braze. The measured melting range of each Al-12Si-xTi foil was very similar, i.e., 580-590 °C, because the composition was close to that of eutectic. For wettability improvement, with increasing Ti content, the interfacial gap between the Al2O3 reinforcement and filler metal (R/M) could be eliminated, and the amount of the remainder of the active fillers on the composite substrate decreased after sessile drop test at 610 °C for 30 min. So, the wettability improvement became easy to observe repeatedly with increasing Ti content. Additionally, the amount and size of Ti(AlSi)3 phase were sensitive to the Ti content (before brazing) and Si content (after brazing).

Approach to producing in situ reinforcing phase within active transient liquid phase (A-TLP) bond seam of aluminum matrix composite.

To optimize the braze composition design route for aluminum matrix composite, the feasibility of in situ producing reinforcing phase within the transient liquid phase bond seam matrix, by adding active melting point increaser (MPI, e.g., Ti) together with general melting point depressant (MPD, e.g., Cu) into the interlayer, was demonstrated. For SiCp/A356 composite, by comparing the wettability, joint microstructure, joint shear strength, and fracture path for the developed Al-19Cu-1Ti, Al-19Cu, Al-33Cu-1Ti, Al-33Cu (wt pct), and commercial Cu foils as interlayer, the feasibility of in situ producing reinforcing phase within the bond seam by adding Ti was demonstrated. Especially for Al-19Cu-1Ti active braze, small and dispersed ternary aluminide of Al-Si-Ti phase was obtained within the bond seam as in situ reinforcement, leading to a favorable fracture path within SiCp/A356, not along the initial interface or within the bond seam. For the formation mechanism of the in situ reinforcing phase of MPI-containing intermetallic compound within the bond seam, a model of repeating concentration-precipitation-termination-engulfment during isothermal solidification is proposed.

Approach to In - Situ Producing Reinforcing Phase Within an Active-Transient Liquid Phase Bond Seam for Aluminum Matrix Composite

To optimize the braze composition design route for aluminum matrix composite, the feasibility of in situ producing reinforcing phase within the transient liquid phase bond seam matrix, by adding active melting point increaser (MPI, e.g., Ti) together with general melting point depressant (MPD, e.g., Cu) into the interlayer, was demonstrated. For SiCp/A356 composite, by comparing the wettability, joint microstructure, joint shear strength, and fracture path for the developed Al-19Cu-1Ti, Al-19Cu, Al-33Cu-1Ti, Al-33Cu (wt pct), and commercial Cu foils as interlayer, the feasibility of in situ producing reinforcing phase within the bond seam by adding Ti was demonstrated. Especially for Al-19Cu-1Ti active braze, small and dispersed ternary aluminide of Al-Si-Ti phase was obtained within the bond seam as in situ reinforcement, leading to a favorable fracture path within SiCp/A356, not along the initial interface or within the bond seam. For the formation mechanism of the in situ reinforcing phase of MPI-containing intermetallic compound within the bond seam, a model of repeating concentration-precipitation-termination-engulfment during isothermal solidification is proposed.

Influence of Beam Offset on Dissimilar Laser Welding of Molybdenum to Titanium

Dissimilar joining of molybdenum (Mo) to titanium (Ti) is of great significance to the design and fabrication of high-temperature facilities. However, few reports were found about fusion joining of these two metals. The objective of this paper is to assess the feasibility of laser beam welding (LBW) of 2 mm-thick molybdenum and titanium. The effects of laser beam offset on the laser dissimilar joint of pure molybdenum to pure titanium were analyzed in terms of microstructure, chemical composition, microhardness, and tensile behavior. The results showed that the weld appearance improved with the increase of the offset. The fusion zone was strengthened because of the solid solution of these two elements. The mechanical properties of samples increased firstly and then decreased with the increasing of offset. When the laser beam irradiated on the titanium plate and the center of the laser spot was 0.5 mm away from the Mo/Ti interface, the joint performed the highest tensile strength, which was about 70% that of titanium base metal. LBW was demonstrated to be a promising method to join dissimilar Mo/Ti joint.