Lightweight materials have become increasingly critical in the transportation manufacturing sectors, including aircraft, automobile, heavy truck, rail, ship, and defense manufacturing industries.
Light metal and alloys possess high strength-to-weight ratios and low density, and are generally defined by low toxicity as opposed to heavy metals, except for beryllium. Lightweight metals consist of aluminum, beryllium, titanium, and magnesium alloys.
Light metals are often used for materials and operations where lightweight and improved performance properties are required. Common applications include chemical process, marine, aerospace, and medical applications.
Lighter vehicles that are designed for consumers, as well as the industry and military sectors, consume less fuel and provide a better performance. In addition to carrying larger loads, lighter vehicles can travel the same distances at reduced cost and release less carbon dioxide.
Aluminum and aluminum alloys are non-ferrous metals that are lightweight and characterized by excellent strength, ductility, and corrosion resistance. Aluminum is a good conductor of heat and electricity and is also used as an alloying element in titanium/steel alloys. Aluminum alloys are also resourceful.
Aluminum is lighter than titanium but not as strong. Aluminum alloys have excellent rigidity and flexibility that increase with decreasing temperature.
Below zero temperature, most of the aluminum alloys display a slight change in properties such as impact strength and tensile strengths. Yield may increase and elongation may slightly decrease. Aluminum alloys do not have a ductile-to-brittle transition.
Similar to aluminum and aluminum alloys, magnesium and magnesium alloys are non-ferrous metals that are characterized by moderate strength, good ductility, low density, and excellent corrosion resistance. The materials have tensile and other properties that rely on several factors such as the condition, composition, heat treatment, details of fabrication, etc.
Magnesium has no definite yield point, and magnesium alloys are susceptible to notches and other stress-raisers, which considerably reduce their endurance limits. Magnesium-base materials posses low modulus of elasticity, and have high unit resilience.
Unlike molten aluminum, molten magnesium does not react with tool steels and thus leads to longer die life. Also, its reduced heat input and low erosion reduce the tendency for thermal fatigue.
Titanium and titanium alloys have high strength-to-weight ratios, good fatigue properties, and excellent corrosion resistance. Titanium is also considered as a refractory metal, which means it is highly resistant to heat and wear. It does not corrode in chlorine or sea water, and it is one of the metals that are resistant to aqua regia.
Commercially pure titanium and titanium alloys are non-magnetic. The thermal conductivity of all titanium alloys is quite low for a metal. Temperature considerably affects the physical properties of titanium. The alloy grades, predominantly the high strength materials, can retain tensile and proof strengths up to relatively higher temperatures when compared to commercially pure grades.
Titanium alloys have excellent high cycle fatigue strengths as opposed to their tensile strengths. The toughness is based on texture, microstructure, strength, and composition, which are interrelated.
Beryllium is a useful alloying element. It is characterized by low density, high rigidity, high strength, structural stability, and reflectivity. Beryllium is distinguished among metals in terms of specific rigidity.
The metal is known to have the second lowest density compared to common structural light metal alloys such as magnesium, aluminum, titanium. Beryllium has high thermal conductivity and specific heat and hence, has excellent specific heat dissipation compared to other metals. The metal has an unusually high Young’s modulus.
Beryllium’s density is 30% less than that of aluminum, whereas its rigidity is 50% greater than that of steel. The metal’s specific rigidity is about four times greater than that of composites and six times greater than that of other alloys or metals. It is resistant to corrosion in normal ambient conditions and at elevated temperatures.
Production of Aluminum and Aluminum Alloys
Aluminum alloys can be developed by rolling, casting, forging, extrusion, and drawing; however varying compositions lend themselves to certain process more readily than others. The hot extrusion of aluminum metal is very important as it allows the production of almost any type of shape in cross section.
In this process, a preheated billet is mounted onto the press container and pressed via a shaped steel die with a ram. While most of the aluminum alloys can be extruded, the 6000 series alloys form the bulk of commercial extrusion, providing an optimum combination of speed and ease of extrusion together with the ability to form thin sections and complex shapes with good surface finish.
Although the extrusion process is versatile, it can provide only 2D shapes. In contrast, casting allows the production of complex 3D products. Over the past few years or so, the development work on the extrusion process has improved the ability of castings to be employed in significantly stressed applications.
Sand, die casting, and permanent mold methods are being used. The choice of the process relies on the quantity of castings needed, property, quality and property considerations, and end use. Earlier, die castings were not used for heat treatment, but this limitation has been overcome with advanced methods.
Magnesium can be cast through a wide range of techniques such as sand casting, permanent mold casting, high-pressure diecasting, and squeeze and semi-solid casting. Diecasting is the most prevalent casting technique for magnesium.
In this procedure, thin-walled, complex components are created at high production rates supported by the low-heat-content per volume of molten metal. Cold chamber and hot chamber machines are used for magnesium.
To ensure optimum performance, higher shot speeds can be used for magnesium, particularly for thin-walled components. Vacuum diecasting is one of the diecasting process variants that can create parts with better properties and lower porosity compared to the traditional diecasting process.
Semi-solid casting methods can also be used to fabricate magnesium, but using magnesium alloy granules instead of liquid magnesium. Semi-solid molding is generally used for smaller components.
When assessing different processes and alloys for magnesium casting, several factored should be taken into account to ensure a high-quality component at a lower cost. These factors include the tooling costs, post-casting operations, and end-use applications.
Magnesium casting can achieve a die life of three to four times more that can be accomplished with aluminum. Magnesium is also known to be the easiest of structural metals to machine, and it is the standard of the cutting tool sector compared to machinability of other metals.
Kroll process is the main production process used for titanium metal. In this procedure, the principle ore rutile is treated with chlorine gas to create titanium tetrachloride, which is subsequently purified and reduced to a metallic titanium sponge through reaction with sodium or magnesium.
This titanium sponge is eventually subjected to an alloying and melting process. However, this process is laborious, making it very expensive. A method was later developed to produce high-quality titanium alloy powders, which was effective for processing this metal into commercial products.
Several steps are involved in the conversion of purified titanium sponge to a form suited for structural purposes. Consolidation into titanium ingot is carried out in an argon or vacuum environment.
Alloying elements, sponge, or sometimes recycled scrap are mechanically compacted and welded into an extended, cylindrical electrode, which is later melted in a copper crucible by passing electricity through it. This ingot is again melted in a similar fashion to ensure even distribution of alloying elements.
Another consolidation process is cold-hearth melting, which is performed within a vacuum or argon chamber comprising of a copper crucible. Heating is achieved with helium/argon plasma torches or a multiple electron-beam, and the molten metal passes over the hearth lip and finally into a water-cooled copper mold.
The cold-hearth process is suitable for isolating high-density contaminants, which collect at the base of the hearth. Consolidated ingots are then processed into mill products such as plate, tubing, bar, wire, billet, etc. by steel facilities.
High purity beryllium hydroxide serves as the input material for important applications of the element such as beryllia ceramics, copper beryllium alloys, and pure beryllium metal manufacturing. To create high-purity beryllium metal, the hydroxide form is first dissolved in ammonium bifluoride and then heated to over 900°C, yielding a molten beryllium fluoride.
After casting the beryllium fluoride into molds, it is combined with molten magnesium in crucibles and then heated. This process separates pure beryllium from the magnesium slag, forming beryllium spheres. Surplus amount of magnesium is then burned off by further treating it in a vacuum furnace, leaving behind 99.99% pure beryllium.
The beryllium spheres are usually changed to powder through isostatic pressing, producing a powder that can be employed in the production of pure beryllium metal shields or beryllium-aluminum alloys.
Lightweight metals are processed in a variety of ways such as melt processing, powder processing, thermo-mechanical processing, forming, coatings, and joining and assembly.
Metal casting, which involves pouring molten metal into a die or mold followed by cooling it to solidify the component, is an ancient process and even today offers great potential to remove weight off the metal structures. There are three advanced melt processing techniques: thin-wall casting, high-integrity casting, and dissimilar-metal casting.
Thin-wall casting is a process where several types of metals, such as aluminum and steel can be cast. However, some complications occur when working with molten metals; such as maintaining proper flow and inhibiting the metal from solidifying prior to filling the mold.
In high-integrity casting, certain products should meet unique standards, without microstructures and porosity that are present throughout the cast part.
In dissimilar-metal casting, two or more metals are used in a single casting which provides considerable benefits. Here, a product can be cast so that parts of it are formed from one type of metal and other parts are formed from a different metal, employing the materials’ various properties where they are most needed.
In this process, heat, pressure, or both are applied to the powder to create hard products with internal mixtures that confer new, valuable properties to the products. The powder is produced by melting the metal and forcing through a nozzle while subjecting it to a water spray or inert gas. This breaks the metal into small droplets, which are eventually cooled down to form a powder.
Next, the metal powder is rolled, extruded, sprayed or pressed to produce blanks for additional shaping or to directly mold into shapes as almost finished components.
Thermo-mechanical processing (TMP) improves the properties of metals, enables lightweight design, and lowers manufacturing costs. TMP involves the concurrent control and use of thermal and deformation processing to acquire components and materials that have improved properties and deliver better performance.
TMP provides controls at each process stage. The shaping time and temperature are simultaneously controlled, together with the amount of deformation at each step of the process in addition to controlling the cooling. This process of controlled heating, forming, and cooling at each step of the process enables engineers to exploit material microstructures in many innovative ways.
TMP provides better control over the end product, potentially removing the need for extra reheating and quenching steps and making the production process less costly and more efficient. TMP can create new, improved properties for aluminum alloys, titanium alloys, and other metals. The process improves materials’ strength, resistance to fracture, and resistance to fatigue-induced degradation.
Forming involves a variety of methods that are used to convert a metal into a usable form. Forming processes consist of bending, extrusion bending, brake bending, roll bending, press drawing, etc.
Bending is often used to form magnesium sheet and extrusions, and the bend radius is a major factor in the bending operation.
Rubber pad forming is most conventional forming process, where a rubber pad is used to replace metal dies during metal forming; hydroforming and marforming are a variation of this process.
Impact extrusion, also called back extrusion/reverse extrusion, has been traditionally used on magnesium alloys and can be used to acquire perfectly symmetrical tubular parts; it can be employed to make components that cannot be made by other techniques.
Spinning involves the use of a spinning tool to produce a component in line with the shape of a mandrel or form block in a lathe; this process is the most economical way for forming products where only a few parts are required. Similar to the spinning process, flow forming is formed in line with the shape of the form block without altering the section thickness of the blank material
Forging is performed at high temperatures for all types of metals; in this process, metals are forged by pounding them with a hammer from various directions until the required shape is obtained
Methods to join lightweight materials, predominantly metals, are becoming more and more important in the development of hybrid structures for engineering applications. Suitable joining techniques are required to consistently join these materials.
Some of the joining methods include adhesive bonding, mechanical fastening, and welding. The processes can be used either separately or in combination in a single method to ensure a durable joint surface. The joining methods come with their own advantages and disadvantages, and the most optimum technique will rely on the application.
Adhesive bonding is a solid state joining method that depends on the formation of intermolecular forces between the workpieces and the polymeric adhesive for the formation of joints. Mechanical fastening uses extra clamping components without combining the joint surfaces.
Traditional welding processes such as gas metal arc welding, gas tungsten arc welding, submerged arc welding, and shielded metal arc welding have been employed to weld dissimilar materials in metal-to-metal joints. However, the high energy inputs of these processes lead to material metallurgical mismatch.
New emerging methods, such as ultrasonic welding, laser welding, friction stir welding, and friction spot welding, are considered to be more viable. Ultrasonic welding is a solid state joining method characterized by low energy input.
In this method, coalescence is initiated by applying localized high-frequency vibration energy with a modest clamping force. Laser welding complements the processing and fabrication of joints, which are difficult to obtain with other welding techniques.
Friction spot joining is similar to linear friction stir welding, but there is no linear movement of the tool during the time of friction spot joining. This process includes three phases such as plunging, stirring, and retracting. Coating is yet another method used for processing lightweight metals.
Recently, UCLA researchers developed a new lightweight metal that contains magnesium infused with thick silicon carbide nanoparticles. The metal holds potential for use in mobile electronics, cars, airplanes, etc. They also developed a new, scalable manufacturing technique that could pave the way for super-strong yet high-performance lightweight metals.
South Korean scientists have developed a new class of steel alloy that is ultra-strong, flexible, and low-cost. It has the same strength-to-weight ratio as that of titanium. In another study, researchers developed a lightweight magnesium-matrix composite that is light enough to float on water yet strong as other composite materials used today. It can tolerate temperatures over 400°C.
A study revealed that new lightweight composite metal foams (CMFs) are more effective at insulating against high heat compared to traditional base metals and alloys. This quality makes these CMFs a potential candidate for use in space exploration, storing and transporting nuclear material, explosives, etc.