Over the past few decades, the face of combat has changed, and with it, the U.S. Army has had to adapt its vehicles to keep its soldiers safe. In the 1980s and prior, combat vehicles were primarily confronted with head-on threats. Today, however, threats can come from every angle.
So although advancements in material science had previously led to the successful lightweighting of military vehicles, the increased amount of armor now required is driving the need for further weight reductions. In the past 30 years, the U.S. Army successfully reduced the weight of kinetic energy armor by 60 percent, but these new threats have resulted in a net increase in armor and, therefore, combat vehicle weight.
According to a report titled “Lightweight Combat Vehicle S&T Campaign,” the Army’s fleet needs to be “more lethal, expeditionary and agile, with greater capability to conduct operations that are decentralized, distributed and integrated.” To achieve this goal, the United States Army Training and Doctrine Command, or TRADOC, is in the process of identifying the materials and technologies necessary to produce a 30-ton infantry fighting vehicle (IFV) and a 35-ton main battle tank (MBT) by 2030, which will possess similar capabilities and functionality of the Army’s current systems.
“Historically, weight has had a positive correlation with combat vehicle survivability; yet high vehicle weight also decreases the fleet’s ability to be expeditionary, limits global mobility, increases cost and challenges sustainment,” said the report’s authors. “In addition to increased fuel consumption rates and logistical support requirements, heavier vehicles are more difficult to transport by air.
“Put simply, large vehicles require more resources to use, maintain and sustain.”
After a year devoted to assessing its options and reviewing previous successes in lightweighting initiatives, the U.S. Army’s Tank Automotive Research, Development and Engineering Center, or TARDEC, is currently focusing its efforts on a mix of material and joining technologies to meet its 2030 deadlines. Like the automotive community has learned through its own lightweighting trials, TARDEC was confronted with a myriad of material and technology combinations that could potentially achieve the new desired standards.
In regard to the possible joining technologies, the challenge often lies in the high-tech, yet dissimilar materials that must be welded together. To overcome these challenges, thermal friction stir welding (TFSW), scribe friction stir welding (SFSW) and collision welding as well as various weld wire technologies are emerging as the technologies of choice. As TARDEC enhances its understanding and use of these technologies, it aims to establish itself as a joining center of excellence.
Thermal friction stir welding
The overriding goal of implementing TFSW into the production process for newer lighter-weight vehicles lies in TFSW’s ability to join dissimilar materials. In the Army’s case, the need is to successfully join steel to aluminum, specifically high-hardness armor steel to AA6061 aluminum alloy.
According to the U.S. Army’s “Lightweight Combat Vehicle S&T Campaign” report, “the complexity of the material science challenge involves understanding the relationship between five variables: material type, its molecular structure, the application of the material (i.e., iPhone or Tank), material treatments (heating/cooling), and material processes that alter the molecular structure.” Because TFSW uses a laser to preheat the steel parent material to join the high-hardness steel to the aluminum alloy, the technical issue of temperature control can be overcome.
In a separate report produced by the U.S. Army and the U.S. Department of Energy (DoE), titled “Advanced Vehicle Power Technology Alliance Fiscal Year 2014 (FY14) Annual Report,” the authors explained that TFSW had been successful when handling thick plate in a butt joint configuration. However, short tool life and insufficient weld lengths were an issue.
“The data from the study indicated that the differential thermal expansion coefficients of steel and aluminum were significant contributors,” the report explained. “It was hypothesized that more precise temperature control of the parent materials might result in longer weld lengths. This required both a heating and a cooling process, as the laser heating process is designed to heat quickly but not cool quickly.”
To achieve longer tool life, longer weld lengths and better temperature control overall, a custom water-cooled weld fixture was designed and tested. That weld fixture is capable of securing work pieces measuring up to 40 in. long and 36 in. wide.
To date, TFSW has been effective in increasing tool life and weld length – achieving a 36-in.-long weld – but the desired weld strength has remained elusive. According to the report, once the optimal parameters and tooling configurations have been achieved, ballistic shock testing will commence.
Scribe friction stir welding
To carry on with the initiative of joining dissimilar metals, the Army also leveraged another type of friction stir welding known as SFSW. For the SFSW process, the objective was to understand its scalability for increasingly thick material cross sections. Considering the process had effectively welded dissimilar metal sections for the automotive industry, it appeared to be a good candidate for the Combat Vehicle Prototype (CVP) program, as well.
Unlike friction stir welding, which depends on the solid-state mixing of materials where the hot forming temperatures of one exceeds the melting temperature of the other, SFSW disrupts the interface of the two metals in a lap configuration rather than actually mixing them. It does so with the use of a scribe cutter, which is found on the bottom of the pin section of the SFSW tool.
“[The scribe] tool is utilized to cut the high melting temperature material such that it is allowed to deform at much lower temperatures than localized extrusion required for traditional FSW,” the report said. “As such, the SFSW process simultaneously cuts away a portion of the steel while back extruding the aluminum. The process essentially combines these two technologies into a single process allowing for chemical and mechanical bonding to take place at the joint interface.”
To meet the Army’s needs, the SFSW process required the development of custom tooling. To join the thick sections of aluminum and steel, tooling was designed that could penetrate the 25.4-mm-thick aluminum and then cut the 12.6-mm-thick steel below it. It was also designed to allow the scribe cutter to interface with the steel section without allowing the pin to come in contact with the steel.
“This design both serves to manage the heat and direct the flow of the steel during the cutting process,” the report explained. “Additionally, because the main tool body is only in contact with Al, it can be fabricated of low-cost tool steel rather than more costly materials required to traditionally FSW ferrous alloys.”
Eventually, the SFSW process and the custom tooling were able to handle material thicknesses up to 38.1 mm. The tooling and process also resulted in stabilized weld properties and increased strength.
To effectively weld an even broader range of dissimilar materials, a unique method for collision welding, developed at Ohio State University for the automotive industry, is currently being investigated by the Army. Instead of traditional collision welding, which uses explosives to provide the driving force, the method produced at Ohio State – collision welding of dissimilar materials by vaporizing foil actuator – uses magnetic pulse welding.
The benefit is that magnetic pulse welding can be used for smaller-scale projects as opposed to traditional collision welding, which carries stringent regulations for the transport, storage and handling of the explosives. Although the method was found to be successful for more than 10 combinations of magnesium, steel and aluminum alloys, there were, of course, challenges involved.
Elimination of brittle intermetallic compounds at the weld interface, typical with traditional fusion-based welding between aluminum, steel and magnesium, was one of the hurdles. Solid-state welding techniques, like friction stir and collision welding, however, are beneficial in either avoiding or reducing the formation of intermetallic compounds.
In addition to avoiding intermetallic compounds, the collision process accomplished many of the Army’s needs. Those included welding dual-phase 780 and Boron-quenched steel to aluminum alloy 6061-T4, welding galvannealed steel for the first time, and welding wrought and cast magnesium alloys to AA 6061-T4. The collision process also demonstrated the effectiveness of use of interlayers for creating strong bonds with pairs that are relatively difficult to weld.
“The main driver of these experiments is a 0.0762-mm-thick aluminum foil, which is a consumable, replaced after every experiment,” explained the joint Army/DoE report. “When a high, short-duration current pulse, driven by a capacitor discharge, is passed through the foil, the foil vaporizes rapidly due to Joule heating. This phenomenon, also known as electrically exploding foils, creates a high-pressure region around the foil, which, in this case, has a thick steel block on one side and the flyer sheet on the other.
“Therefore, the flyer sheet gets driven to high velocities toward the target plate. Standoff sheets between the flyer sheet and target plate provide the distance over which the flyer is accelerated. Additionally, the height of the standoff sheets and the horizontal distance between them help create an oblique collision, which is necessary for weld creation.”
The ability to weld automotive grade aluminum to high-strength steel and galvannealed steel is incredibly encouraging for the Army’s lightweighting initiatives. Although the tests yielded varied results, six systems have been selected for further study.
Weld wire technologies
As TFSW, SFSW and collision welding have emerged as viable welding technologies for the Army’s lightweighting initiatives, there was also a need to address hydrogen-induced cracking (HIC) also known as cold cracking. According to the Army/DoE report, HIC limits the choices of high-strength steels and filler wire that can be used on lighter-weight military vehicles. So once again, the Army was charged with the task of developing new technologies to address the issue.
“When talking about welding advanced high-strength materials, such as armor materials, there are many challenges,” said Matt Rogers, welding engineer for ground system survivability on the lightweight structures team, TARDEC. “Take our steel armor materials such as MILITARY-DETAIL (MIL-DTL)-12560 or MIL-DTL-46100, which are martensitic materials. They are susceptible to hydrogen-induced cracking. If precautions aren’t taken before and during the welding process, such as preheating, cracks can form.”
As outlined in the report, the overall goal was to develop an innovative welding filler wire that could substantially reduce tensile weld residual stress and mitigate the susceptibility of HIC. Furthermore, the welding wire would need to be used across all armor-steel-protected military vehicles.
“Fabricating HIC-free welded structures can be difficult in field fabrication environment and repair,” the report said. “For certain types of applications, pre-heating, post-weld heat treatment and use of low hydrogen welding practices are mandatory per code/standard specifications. These requirements are often time consuming and represent a significant cost factor in construction. There are also many other applications where it is practically impossible to consistently eliminate the presence of hydrogen, and the hardened microstructure is necessary to maintain the strength of the weld.”
Despite the challenges involved, the Army was able to develop a filler wire that could reduce the high-tensile residual stresses in the weldments of high-strength steels. Additionally, the filler wire enabled “in-welding-process” HIC control, eliminating costly pre- and post-weld heat treatments. It also expanded the use of high-strength steels without the concerns of HIC.
Because the goal was to produce a welding wire that could be used across all armor-steel-protected military vehicles, the Army also hoped to work with an industrial partner to commercialize the production of the filler wire. Today, multiple weld wire manufacturers have expressed interest to do so. A patent application for the weld wire has also been jointly submitted by TARDEC and the Oak Ridge National Laboratory.
High-strength filler wire
To weld high-strength aluminum plate, specifically 6055, while optimizing the corrosion and mechanical properties of weld joints, additional filler wire development projects are also underway. The plan for the new filler wire technology is to be commercialized for application to aluminum high-performance blast shields and vehicle hulls, the report explained.
“High strength 6xxx aluminum alloys, such as 6013 and 6055, have 20 percent-plus higher strength than currently used aluminum alloys,” the report continued. “They offer the potential to significantly improve performance, weight and cost. However, when welding 6xxx high-strength alloys (e.g. 6013), the only commercially available filler wires that can weld these high-strength 6xxx alloys without cracking are 4043, 4047 and 4145. Unfortunately, the welds produced with these filler alloys have limited shear strength and ductility, which in turn limit their capability to withstand blast type of loads.”
To overcome these issues, 200 lbs. of filler wire was developed, which could join 6013 plates while maintaining high strength and ductile weld joints. The chemistry of the weld wire, however, will continue to be optimized and evaluated for corrosion and mechanical property purposes.
As was the case with other filler wire projects, the goal for the new technology is to be leveraged into legacy and future military systems where higher-strength weld wire is needed. As the U.S. military gains weight savings via the stronger weld wire enabling less material to be used for each welded joint, so, too, will the commercial transportation community.
These welding and welding wire technologies only scratch the surface of what was required by the team tasked to enhance the capabilities of combat vehicles. Additional technologies are being investigated, and material science continues to be a focus.
“Composite materials are difficult for military vehicles because of cost, performance, repair/maintenance and joining reasons,” Rogers explained. “That said, we are looking at how to overcome many of the barriers we now face to develop more use of composite materials.”
In addition to the research behind joining and material science technologies, the Army also developed testing and standards to evaluate and implement their findings. As an example, selection criteria for the new MIL-Standard on the welding of armored steel were created as were fatigue and damage models to predict weld life for rolled homogeneous armor weld joints. An analytical simulation of an armor plate weld seam under ballistic impact was also developed among other strength tests.
As global conflicts continue to spread, U.S. combat vehicles will increasingly be subjected to difficult terrains as well as mine and blast threats. The Army’s and industry’s developments being made in response to the changing face of CVP development will not only reap new solutions for weight, performance and cost, but will also continue to enhance our troops’ safety abroad.