Page 32 - MetalForming-Oct-2018-issue

Lubrication Challenges
Austenitic stainless steels
Bake-hardening steels Dual-phase steels
Deep-drawing steels
Micro-alloyed steels
TRIP-steels Duplex-steels
6xxx
7xxx
5xxx
Magnesium alloys
50 40 30 20 10
100 200 300
Yieldstrength􏰀y [MPA]inoptimisedhardenedcondition
400 500
Fig. 1—The banana diagram is a common way to present the tradeoff between a metal or alloy’s ductility (as percent elongation) and strength. This diagram compares magne- sium alloys and 5xxx-, 6xxx- and 7xxx-series aluminum alloys with various types of steel. (Figure courtesy of R. Schneider, B. Heine and R.J. Grant (2014). Mechanical Behaviour of Commercial Aluminium Wrought Alloys at Low Temperatures, Light Metal Alloys Applications, Dr. Waldemar A. Monteiro (Ed.), InTech, DOI: 10.5772/58362. Available at www.intechopen.com/books/light-metal-alloys-applications/mechanical-behaviour-of- commercial-aluminium-wrought-alloys-at-low-temperatures.)
shows the body of a 2017 Chrysler Paci- fica as being composed of 28-percent mild, 23-percent HSS and 49-percent advanced-high-strength (AHSS) and ultra-high-strength steels (UHSS), while the 2019 Chevrolet Silverado’s body is 25-percent mild, 46-percent HSS and 29-percent AHSS and UHSS.
Mild steel has a low carbon content —a few tenths of a percent to a few hundredths of a percent. Mild steel is ductile, machinable and easily welded, but it has a low tensile strength, typi- cally less than 200 MPa. Thinner sheets of steel can make a lighter vehicle, McClure says, but it takes a stronger steel to achieve the same or better crash worthiness. These HSS have tensile strengths in the range of 210 to 550 MPa. In this range, the steel is ductile enough to be shaped, but strong enough to meet performance stan- dards. Some advanced third-generation steels have tensile strengths exceeding 1000 MPa, but they still have “decent
Domestic OEMs use different strategies for lightweighting to meet CAFE standards.
formability,” he says (Fig. 1).
Steel generally loses ductility as it
gains strength, making it harder to form, McClure says. In addition, less- ductile steel tends to spring back after it is formed, making dimensional accu- racy difficult. Solving this problem is an active area of research, he adds. HSS are more expensive and harder to work with, which drives up produc- tion costs. Higher tool temperatures from working with these steels and the higher stresses involved in forming parts requires tools, tool coatings and lubricants that can stand up to these conditions.
Aluminum Ramping Up
High-strength aluminum presents some of the same ductility and spring-
CAFE numbers up. For example, Ford’s best-selling vehicle, the F-150 pickup truck, uses aluminum body panels, and it will be transitioning this over to other vehicles over the next 4 to 5 years, Budai says. The F-150’s cab and box are made from “high-strength military- grade aluminum alloys,” according to Ford’s website. That, and the high- strength steel in the frame, reduce the pickup’s weight by 700 lb. as compared to the previous generation of trucks. Other manufacturers are heading in that direction as well, but maybe not as quickly as Ford, Budai says.
OEMs are looking for strong mate- rials with the desired degree of ductility and wear resistance. “We’re seeing more aluminum alloys containing tita- nium in specific engine components— power train valves, camshafts, pins, crankshafts, and parts such as exhaust and intake valves,” Budai says. His col- league, Yixing (Philip) Zhao, senior research scientist and innovation team leader, research and technology, at Houghton, adds that newer aluminum alloys have higher strength, different
compositions and harder surfaces. The aerospace industry is the biggest user of titanium right now, Zhao says, but the knowledge that fluid developers gain in developing lubricants for this market eventually transfers to other industries.
Steel, Still Around
The auto industry is changing quick- ly, with newer vehicles made using thinner, stronger steel panels. Moving to high-strength steels is “a huge change in the way cars are made,” McClure says. In 2005, mild steel com- prised about 85 percent of the mass of a typical auto body, with the rest being high-strength steel. By 2015, typical auto bodies were about 40-percent mild steel and more than half high- strength steel (HSS). Lighter metals such as aluminum and magnesium comprise the remainder, as much as 10 percent, but these metals are gain- ing, he says. Since then, the percentage of HSS has only increased, according to the Steel Market Development Insti- tute (SMDI). For example, SMDI data
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Fracture elongation [%] in annealed condition