Khalid Salim Alhawari

Semisolid Metal Processing Techniques for Nondendritic Feedstock Production

Semisolid metal (SSM) processing or thixoforming is widely known as a technology that involves the formation of metal alloys between solidus and liquidus temperatures. For the procedure to operate successfully, the microstructure of the starting material must consist of solid near-globular grains surrounded by a liquid matrix and a wide solidus-to-liquidus transition area. Currently, this process is industrially successful, generating a variety of products with high quality parts in various industrial sectors. Throughout the years since its inception, a number of technologies to produce the appropriate globular microstructure have been developed and applied worldwide. The main aim of this paper is to classify the presently available SSM technologies and present a comprehensive review of the potential mechanisms that lead to microstructural alterations during the preparation of feedstock materials for SSM processing.

Microstructural evolution and mechanical properties of thixoformed A319 alloys containing variable amounts of magnesium

The effects of Mg content on the microstructure and tensile properties of thixoformed A319 alloys were studied. The samples were thixoformed at 50% liquid content and some of the thixoformed samples were subjected to the T6 heat treatment. The samples were then examined by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD) analysis as well as tensile tests. The results showed that magnesium was able to refine the eutectic silicon in the samples. It was also observed that a compact Al9FeMg3Si5 phase was formed when the magnesium content was 1.0% and 1.5%. The results also revealed that as the magnesium content in the alloy increases, the tensile strengths of the thixoformed alloys also increase. The ultimate tensile strength, yield strength and elongation to fracture of the thixoformed A319 heat treated alloy were 298 MPa, 201 MPa and 4.5%, respectively, whereas the values of the thixoformed heat treated alloy with 1.5% Mg content were 325 MPa, 251 MPa and 1.4%, respectively. Thixoformed A319 alloy showed a dimple fracture behaviour, while thixoformed A319 alloys with 1.5% Mg showed a mixed mode fracture behaviour, where dimple and cleavage ruptures were seen on the fracture surface of the samples.

Study on thixojoining process using partial remelting method

Cold-work tool steel is considered to be a nonweldable metal due to its high percentage content of carbon and alloy elements. The application of a new process of the semisolid joining of two dissimilar metals is proposed. AISI D2 cold-work tool steel was thixojoined to 304 stainless steel by using a partial remelting method. After thixojoining, microstructural examination including metallographic analysis, energy dispersive spectroscopy (EDS), and Vickers hardness tests was performed. From the results, metallographic analyses along the joint interface between semisolid AISI D2 and stainless steel showed a smooth transition from one to another and neither oxides nor microcracking was observed. Hardness values obtained from the points in the diffusion zone were much higher than those in the 304 stainless steel but lower than those in the AISI D2 tool steel. The study revealed that a new type of nonequilibrium diffusion interfacial structure was constructed at the interface of the two different types of steel. The current work successfully confirmed that avoidance of a dendritic microstructure in the semisolid joined zone and high bonding quality components can be achieved without the need for force or complex equipment when compared to conventional welding processes.

Microstructural evolution during DPRM process of semisolid ledeburitic D2 tool steel

Semisolid metal processing is a relatively new technology that offers several advantages over liquid processing and solid processing because of the unique behaviour and characteristic microstructure of metals in this state. With the aim of finding a minimum process chain for the manufacture of high-quality production at minimal cost for forming, the microstructural evolution of the ledeburitic AISI D2 tool steel in the semisolid state was studied experimentally. The potential of the direct partial remelting (DPRM) process for the production of AISI D2 with a uniform globular microstructure was revealed. The liquid fraction was determined using differential scanning calorimetry. The microstructures of the samples were investigated using an optical microscope and a scanning electron microscope equipped with an energy dispersive spectroscopy analyser, while X-ray phase analysis was performed to identify the phase evolution and the type of carbides. Mechanical characterisation was completed by hardness measurements. The typical microstructure after DPRM consists of metastable austenite which was located particularly in the globular grains (average grain size about 50 μm), while the remaining interspaces were filled by precipitated eutectic carbides on the grain boundaries and lamellar network.

Dry sliding wear behaviour of thixoformed hypoeutectic Al–Si–Cu alloy with different amounts of magnesium

Abstract In this study, the dry sliding wear characteristics of Al–6Si–3Cu–(0.3–2) Mg–aluminium alloys produced by the thixoforming process were examined. The cooling slope technique was employed to produce feedstock alloys before they were thixoformed at 50% liquid fraction. A pin-on-disc rig was used to carry out the wear tests at 50 N load, 1 m/s speed and 9 km distance. The results revealed that adding magnesium to Al–Si–Cu alloy led to precipitate Al5Cu2Mg3Si5 and Mg2Si intermetallic phases. The platelet β-Al5FeSi was transformed to a Chinese script π-Al8Mg3FeSi6 phase with the addition of magnesium. The thixoformed alloys showed a fine globular primary phase microstructure surrounded by uniformly distributed silicon and refined fragmented intermetallic phases. Moreover, in comparison with alloys produced by the conventional casting method, the morphology and size of the primary Mg2Si particles were modified, and the Chinese script morphology of π-Al8Mg3FeSi6 changed to a compact shape. The hardness of the thixoformed Al–Si–Cu alloys increased continuously with the increase in magnesium content. The wear resistance of the thixoformed alloys improved with the addition of magnesium content up to 1.5%, above which the trend reversed. The dominant wear mechanism was found to be a combination of abrasion and adhesion in the low-Mg alloys and delamination with some abrasion in the high-Mg alloy.

Microstructural evolution during semisolid processing of Al–Si–Cu alloy with different Mg contents

Abstract A series of Al–6Si–3Cu–(0.3–2)Mg alloys were produced by a conventional casting process. Cooling slope technique was employed to produce feedstocks before they were thixoformed at 50% liquid fraction. The effect of Mg on the microstructure of Al–Si–Cu aluminium alloys under as-cast and semisolid conditions was investigated. It was found that by adding Mg to Al–Si–Cu alloy, some of the Al 2 Cu phase and silicon were consumed to form Al 5 Cu 2 Mg 3 Si 5 and Mg 2 Si phases. The needle-like β -Al 5 FeSi phase transformed to Chinese-script-like π -Al 8 Mg 3 FeSi 6 with the addition of Mg. In the as-cast alloys, the primary α (Al) was dendritic, but as the Mg content increased, the phase became less dendritic. Moreover, the Mg addition considerably modified the size of the α (Al) phase, but it had no significant effect on the silicon morphology. In the thixoformed alloys, the microstructure showed a fine globular primary phase surrounded by uniformly distributed silicon and fragmented intermetallic phases. The eutectic silicon was modified from a flaky and acicular shape to fine fibrous particles. The effect of Mg on eutectic silicon during semisolid processing was evident. The primary Mg 2 Si particles were modified from big polygonal particles to become smaller and more globular, whereas the morphology of the Chinese-script-like π -Al 8 Mg 3 FeSi 6 changed to a compact shape. The results also exhibit that as the Mg content in the A319 alloy increased, the hardness, yield strength and ultimate tensile strength of the thixoformed alloys significantly improved, but the elongation to fracture dropped.

Trend and Development of Semisolid Metal Joining Processing

The semisolid metal joining (SSMJ) process or thixojoining process has recently been developed based on the principles of SSM processing, which is a technology that involves the formation of metal alloys between solidus and liquidus temperatures. Thixojoining has many potential benefits, which has encouraged researchers to carry out feasibility studies on various materials that could be utilized in this process and which could transform the production of metal components. This paper reviews the findings in the literature to date in this evolving field, specifically, the experimental details, technology considerations for industrialization, and advantages and disadvantages of the various types of SSMJ methods that have been proposed. It also presents details of the range of materials that have been joined by using the SSMJ process. Furthermore, it highlights the huge potential of this process and future directions for further research.

Effects of Cu on the Microstructures and Tensile Properties of Thixoformed Al-Si Alloys

In this study, the effects of copper content on the microstructures and tensile properties of thixoformed Al-5Si-xCu-0.5Fe (x =1.0, 2.0 and 3.0 wt. %) were investigated. For this study, three different alloys having various amounts of copper were prepared using cooling slope casting before thixoforming. The semi-solid liquid range for the alloys were estimated using the diffrential scanning calorimetry (DSC) analysis. The samples were thixoformed at 40% liquid fraction. Some of these samples were treated with a T6 aging process. The thixoformed and thixoformed T6 samples were then characterized by optical microscopy, scanning electron microscope (SEM) and energy dispersive X-ray (EDX) as well as tensile tests. The different phases formed in the thixoformed and thixoformed T6 samples were throughly investigated.The results indicate that as copper content increases, the tensile strength also increases, which might due to precipitation hardening. The thixoformed T6 alloys attained an ultimate tensile strength (UTS) as high as 303 MPa when Cu content is 3 wt%.

A Study of Microstructure Properties of AISI D2 Tool Steel in Partial Re-Melting Method

Due to the growing demand for cold-work tool steel in various industrial applications, it is crucial to improve the fabrication technique, because complex shapes involve an extensive and costly workshop effort. Hence, a one-step net-shaping process, such as the semi-solid forming, could offer great benefits. With the aim of finding a minimum process chain for the manufacturing of a high-quality production, the microstructural evolution of the ledeburitic AISI D2 tool steel in the semi-solid-state was studied experimentally via the Direct Partial Re-Melting Method (DPRM). Samples were heated in an argon atmosphere up to 1255°C, which corresponded to about 16% of liquid fraction, and held for 0 minute. The microstructural observation after DPRM showed that the equiaxed austenite grains are observable within a small liquid matrix. The microstructure also contains primary, non-dissolved carbides with a new, precipitated eutectic.

Microstructure evolution and mechanical properties of rheocast A319 aluminum alloy using cooling slope

A319 aluminum alloys are commonly used in automotive industry due to a combination of good fluidity and mechanical strength. In this present work, cooling slope (CS) rheocasting process was employed to produce A319 billets with near spherical morphology of primary Al phase. The dendritic primary phase in the cast A319 alloy had readily transformed into non-dendritic when the ingots were cast over a cooling plate from pouring temperatures between 620°C and 640°C and with cooling lengths of between 300 mm and 400 mm. The shear driven flow of the solidifying melt on the cooling slope wall promotes heterogeneous nucleation of α-Al phase and subsequent separation from there due to shear driven flow of the solidifying melt produced nearly spheroidal morphology of the primary phase in the microstructure. The results show that the best combination of pouring temperature and cooling length was found to be 630°C and 400 mm respectively. The hardness of the rheocast ingots improved to 85.3 HV from 81.8 HV in as-cast condition.