Solid-state bonding is paramount in joining dissimilar materials, often combined with metal forming, such as forging, rolling and extrusion, to fabricate composites with a designed structure. Owing to its superior mechanical properties and low density, the carbon steel/Ti composite structure is attractive in automotive and aerospace industries but is difficult to fabricate due to the decarburization of carbon steel's faying surface. In this work, a carbon steel/Ti6Al4V composite structure was first fabricated via a forge-bonding method with the assistance of a Ni coating layer. Results show that the Ni layer on the faying surfaces precludes the decarburization of the steel and the growth of the TiC phase so as to improve the bonding quality. Meanwhile, the in-situ eutectic reaction between the Ni layer and β-Ti was observed. As a result, a seamless bonded interface with the total elimination of the Ni layer can be obtained. The bonding strength was examined to establish the relationship between bonding windows, microstructure and the resulting mechanical properties. An experimentally validated thermal-mechanical finite element model was also developed to understand the process-dependent interface evolution. This work opens a new avenue for fabricating carbon steel/Ti6Al4V composite, extending the flexibility of achieving complex structures by combining solid-state bonding with other metal-forming technologies.

Thermally-induced cracking typically occurs during the cooling stage of various manufacturing processes, and is commonly seen in multiphase or the joints of dissimilar materials due to mismatch in their thermo-mechanical properties, such as thermal expansion, elastic-plastic deformation and, in some cases, phase transformation. However, the underlying cracking mechanism associated with local microstructure is still elusive. To improve the mechanistic understanding of thermal cracking, this work uses the diffusion-bonded 9Cr-1Mo steel as an example to study the key microstructural variables, such as interfacial phases, voids, grain boundary migration and crystallographic orientations. Meanwhile, a temperature-dependent crystal plasticity model coupled with a cohesive zone model is developed to provide more insights into the thermal-induced stress distribution at the grain scale. It is found that the stress at the void-free boundary of martensite and ferrite is dominated by shear, and its magnitude is insufficient to nucleate cracks. Whereas voids at phase boundaries can induce significant tensile stress, resulting in cracking at the phase boundaries as well as diffusion-bonded interfaces. Also, the occurrence of interfacial grain boundary migration plays an important role in local stress distribution. These microstructure features and their evolution are experimentally observed and used to verify the developed crystal plasticity models. These findings enhance the understanding of the influence of microstructure features on thermal cracking and provide a guide to designing and fabricating the microstructure with improved thermal crack resistance in various manufacturing processes.

To solve the poor printability issue of high-performance aluminum alloys in additive manufacturing processes, a solid-state hybrid additive manufacturing (AM) technology, combining extrusion and roll bonding operations, is proposed, in which the extruded aluminum layers are soundly bonded through the hot-rolling operation. In this feasibility and early-stage research, a lab-scale extrusion-roll-bonding prototype machine is designed and built. Aluminum alloy, AA1060, owing to its relatively low loading requirements for the prototype, is used for the proof of the concept. The extrusion-roll-bonding tests have been undertaken at different temperatures and rolling reductions to identify optimal processing parameters. The corresponding bonding quality was estimated using an optical microscope (OM). Also, miniature AM tensile samples were tested to evaluate the tensile properties of the bonding interfaces. It is of great interest to see that the AM material did not fracture at the bonding interface and its stress-strain response is similar to that of the original material. In-depth analyses on the interfacial grain and oxide distribution and their effects on the interfacial strength were undertaken using a transmission electron microscope (TEM) and scanning electron microscope (SEM)/electron backscatter diffraction (EBSD). This study demonstrates the proposed solid-state AM technique can successfully manufacture aluminum alloys at their solid states, achieving comparable mechanical properties to their wrought states. This new solid state AM technique may enable a wide range of alloys with poor printability to be used in various industries to produce safety-critical and large, structural components with simple geometry.

Understanding the interaction of micro-voids and grain boundaries is critical to achieving superior mechanical properties for safety-critical parts. Micro-voids and grain boundaries may interact during advanced manufacturing processes such as sintering, additive manufacturing and diffusion bonding. Here, we show imparted benefits on mechanical properties by achieving grain boundary migration across voids. The micromechanisms and quantitative analysis of grain boundary migration on local deformation were studied by integrated in-situ EBSD/FSE and crystal plasticity finite element modelling. It is revealed that a migrated grain boundary does not alter the activated slip systems but precludes grain boundary-multislip interaction around interfacial voids to alleviate stress concentrations. The stress mitigation caused by grain boundary migration is almost the same as that caused by void closure under the example diffusion bonding thermal-mechanical process used in this study. This new understanding sheds light on the mechanistic link between GND hardening, grain boundary migration and the corresponding material tensile behaviour. It opens a new avenue for achieving superior mechanical properties for metallic parts with micro-defects such as those generated in diffusion-bonded, sintered and additive manufactured components.

The process of Hot Form and Quench of aluminum alloys, called Direct HFQ®, has been developed and applied to manufacture high-strength panel components, in which aluminum alloy sheet is heated to solution heat treatment temperature, quickly transferred to cold press dies, simultaneously formed and quenched, and subsequently artificially aged. For Direct HFQ, however, forming occurs at high temperatures, which results in high workpiece/die friction and wear, and hence high tooling and maintenance costs. In the present study, a novel Indirect HFQ for aluminum alloys has been proposed, in which alloy sheet in the O temper is formed at room temperature, then heated to solution heat treatment temperature, and quickly transferred to cold press dies for shape calibration and quenching, followed by artificial aging. In order to compare Indirect HFQ with Direct HFQ, AA6082 sheet specimens have been deformed uniaxially using the two HFQ techniques to a given strain or fracture. Mechanical properties of the deformed specimens have been measured, and differences in mechanical properties after the two HFQ processes have been quantified. Their microstructures have also been characterized to explain those differences. In addition, both HFQ techniques have been applied to form a B-pillar sectional component. It has been found that grain growth occurs in alloy deformed uniaxially to a strain higher than or equal to 10% during Indirect HFQ process, and the degree of grain growth decreases with increasing deformation. The grain growth during Indirect HFQ leads to a lower yield strength (up to ∼8%) and tensile strength (up to ∼12%) than that of the alloy processed using Direct HFQ. In addition, the alloy has a lower ductility and formability during Indirect HFQ than Direct HFQ.

Solid-state dissimilar bi- or multi-metallic bonding is promising for achieving lightweight or multifunctional components in automotive, nuclear power and aerospace industries. To understand how to achieve a high-quality bonding interface between dissimilar materials, aluminium alloy (Al)–steel (Fe) bimetal gears manufactured under hot forge bonding were systematically investigated. In this work, comprehensive analyses of forge bonding mechanics, microstructure features, bonding interface behaviours and resulting mechanical properties were undertaken using ex/in-situ experiments and finite element modelling. The results revealed that the bonding behaviour and microstructure evolution were significantly affected by the mechanical property mismatch between the two dissimilar workpieces (AA6082 and E355). This mismatch could be effectively adjusted by setting different forging temperatures. The interfacial bonding strengths of AA6082 and E355, manufactured at low and high temperatures, were observed to be governed by interdiffusion and oxide particles, respectively. Balancing interdiffusion and oxide breaking appears to be key to achieving optimized interface strength for dissimilar bimetallic forge bonding technology.

Stainless steel 316L is a widely used structural material in the nuclear industry because of its excellent corrosion resistance and mechanical properties. However, very little research can be found on its viscoplastic behaviour and microstructure evolution at warm and hot deformation conditions, which hinder the possible application of advanced manufacturing technologies for producing complex parts, such as superplastic forming or hydroforming. The aims of this study are to explore stainless steel 316L’s viscoplastic behaviour, to determine its strain rate sensitivities, and to reveal its underlying microstructure evolution; this will allow appropriate manufacturing (forming) technologies and the optimal forming condition to be determined. Hence, isothermal tensile tests at 700 °C, 800 °C, 900 °C, and 1000 °C at strain rates of 0.01 s−1 and 0.001 s−1 have been conducted. Moreover, the corresponding microstructure evolution, including the grain orientation and geometrically necessary dislocation density, has been revealed by the electron backscatter diffraction method. The data show the viscoplastic behaviour of stainless steel 316L under various thermomechanical deformation conditions and how microstructure evolution influences the viscoplastic flow stress.

During plastic deformation, the change of structural states is known to be complicated and indeterminate, even in single crystals. This contributes to some enduring problems like the prediction of deformed texture and the commercial applications of such material. In this work, plane strain compression (PSC) tests were designed and implemented on single crystal pure aluminum to reveal the deformation mechanism. PSC tests were performed at different strain rates under strain control in either one-directional or two-directional compression. The deformed microstructures were analyzed according to the flow curve and the electron back-scattered diffraction (EBSD) mappings. The effects of grain orientation, strain rate, and strain path on the deformation and mechanical response were analyzed. Experimental results revealed that the degree of lattice rotation of one-dimensional compression mildly dependents on cube orientation, but it is profoundly sensitive to the strain rate. For two-dimensional compression, the softening behavior is found to be more pronounced in the case that provides greater dislocations gliding freeness in the first loading. Results presented in this work give new insights into aluminum deformation, which provides theoretical support for forming and manufacturing of aluminum.