如今，汽车行业对减重的追求正在推动钢、铝和碳复合材料等行业的不断创新。

“今天，我们已进入一个汽车可以真正开始实现减重的时代，”密歇根大学材料科学与工程教授Alan Taub 博士表示，“现在几乎所有新车发布时都会提到实现了5-10%的减重,因为很显然，如今车辆的整备质量已经和燃油经济性直接挂钩了。这也的确是事实，尽管如今车辆的燃油经济性提升仍主要来自对车辆动力系统的改进及对全电动/半电动系统的应用，但仍有 15% 的燃油经济性提升与车辆减重直接相关。”

Taub 博士的经验之谈：车辆每减重 10%，燃油经济性可提高 6％。

在 2019 年塑料工程师学会（Society of Plastics Engineers’）举办的 ANTEC 大会上，曾在通用汽车公司担任研发主管的 Taub 博士对钢、铝和复合物等三大主流汽车材料进行了全面评估。他预计，未来，这些材料在汽车车身结构中的比重将越来越高。

“无论任何新车研发项目的总工程师，你的工作就是尽可能以最低的成本，取得最高的燃油经济性。”Taub 指出，“接着，你得拿出‘平均每加仑英里数可节省的成本’数据。不同厂商的情况略有不同，但普遍减重一磅的经济效益为 2 到 2.5 美元。很显然，哪家供应商能帮助汽车厂商实现减重，哪家供应商就能拿到供应合同。”

STEEL 钢：如今，汽车车身材料规划方面的最大变化是用一系列不断进化的高强度钢种替代之前的低碳钢材料。Taub表示，钢材料将继续扮演汽车架构中的“主力军”，而且如今钢材料的硬度越来越高（因此防撞效果更好）、重量更轻且成本低至每磅 0.5 美元，“具有很高的成本效益”。

不久之前，在车身工程师的概念里，冲压钢的拉伸强度极限还是 300 MPa。但如今，拉伸强度在800 MPa 的钢材比比皆是，甚至还可以达到更高。这些先进高强钢（AHSS）和超高强钢（UHSS）的刚度更高且重量更轻。不难理解，材料越坚固，在相同应用场景下需要使用的材料用量则越少，因此这些钢材在减重方面的效果不言而喻。不过，拉伸强度在1000 MPa 以上的新型钢材料下无法在室温下压印，必须采用热成型技术。热成型技术也称压力淬火，是一种复杂的工艺，需要在模具中完成加热、成型和淬火等过程。

高强度钢材所需的压力淬火工艺会增加成本，但仍不超过“减重所能带来的经济效益，也就是每减重1 磅可节省 2 到 3 美元。”Taub 解释说，目前钢材行业正在推出拉伸强度在 1200 到 1400 MPa 的超高延展性产品，可以在室温下完成冲压成型，而且未来还会推出拉伸强度更高的材料。

Taub 博士向在场塑料工程师介绍到，“我们刚刚讨论的高强度钢材顶多可以帮车辆实现 10% 到 15% 的减重，未来的新材料则可以进一步将该比例提升为 25%。”

ALUMINUM 铝：Taub 博士指出，铝材料并不具有钢材的延展性，因此制造商现在还无法向压制钢材一样将铝板压制成一些更为复杂、更加极端的形状。然而，汽车行业“在制造铝成型零件方面已经取得了很大的进展，而且已经通过采用机械紧固件，甚至是通过新工艺将铝材料点焊到钢材上的方法，解决了部件之间连接紧固的问题，这也是目前铝材相较于钢材的主要劣势之一。

Taub 博士表示，由于密度比同类钢材低 2.5 倍，“铝材已经迅速成为闭合件的首选材料，每减重 1 磅可节省不到2.00 美元。”

然而，由于铝土矿精炼本身就属于能源密集型产业，因此铝材制造的成本本来就比较高，因此最终价格也相对较高。此外，铝材行业已经接近现有轧机产能的极限，这可能会限制这种材料未来的供应保证。根据Taub 博士的说法，“在开展新车项目时，如果需要使用额外的铝材料，则车厂必须直接与铝业公司合作以提高产能，确保铝材料的供应。”

Taub 博士介绍说，先把这些挑战放在一边，福特汽车的铝质皮卡F-150 已经创造了不俗的销售神话，“越来越多的车厂都在讨论扩大铝材料在车身中的应用。”

COMPOSITES 复合材料：Taub 博士表示，“目前市面上最轻的一批车型都采用了碳纤维复合材料。” Taub 博士本人也同时是美国轻质材料制造创新研究所（American Lightweight Materials Manufacturing Innovation Institute）的首席技术官。

目前，碳纤维材料在车身结构中的大规模应用还面临诸多挑战。很多车厂和一级供应商均在探索碳复合材料的小批量应用，但也有厂商选择与流行趋势“背道而驰”。比如 BMW 已经从价值数百万美元的SGL Carbon 合作中抽身，而后者在 BMW 的创新 i3 和 i8 车型的生产中扮演了重要角色。BMW i3 和 i8 车型的零部件采用源自全球，供应商从德国老家一直延伸至美国华盛顿州。

Taub 表示，“汽车车身和底盘结构的选材最终还是会趋向碳纤维复合材料，这是我们的减重的法宝。”

那么，如何才能走到这一步呢？

首先，碳纤维材料必须可以满足所有的冲击标准。对比来说，金属材料在极端冲击条件下会变形，而碳纤维这样的超硬材料在同等条件下更倾向于碎裂。Taub 博士指出，“但好消息是我们已经学会了如何通过建模来优化设计，碳纤维材料吸收能量的能力也因此有了显著提升。”

不过，碳纤维零部件的成型时间较长，这仍然是个麻烦的问题。根据 Taub 博士的说法，复合材料研究人员正在努力将碳纤维底板（这种材料在汽车车身中的最大规模应用）的制作时间降低至1 分钟。“我们已将这一过程从七年前的 8 分钟缩短到今天的大约 4 分钟，未来还有可能继续将其缩短至 1 分钟，”Taub 博士向 SPE 观众介绍说，“我们仍在寻找瞬时固化工艺。”

下一个最值得关注的技术拐点可能是热塑性树脂碳纤维复合材料。这种材料与碳纤维材料的结构特性相同，但成型时间却大大缩短。

目前，材料的前体成本以及将其转化为高强度碳纤维所需的工艺也是材料科学家和工艺工程师关注的焦点。Taub博士介绍说，“为了从各个方面降低成本，我们还有很多工作要做。如今，我们已经将成本从每磅 20 美元降低至每磅 10 美元，未来还将继续为降低至 7 美元的目标而努力。”他补充说，我们还必须建立材料的闭环回收流程：“复合材料将成为最终胜利者，这种材料将逐步在各个方面占据优势，包括成本和回收时间。”

事实上，交通运输行业已经在相当短的时间内完成了从“单一材料密集使用（比如 F-150 皮卡）”到形成全新材料观的转变，即“将合适的材料，用合适的方法，应用至合适的位置”。“现在，设计工程师可以使用 ANSYS 等各种各样的工具，并选择各种各样的材料；组件工程师可以将这个部件制作成最复杂的形状，而制造工程师则会处理这些材料丰富、形状复杂的零部件，”Taub 博士指出，“那么我们不需要连接工程师了吗？恰恰相反，他们可是所有公司都梦寐以求的人。” 

Cost per pound of reduced vehicle mass is helping to drive innovation in steel, aluminum and carbon composites.

We’ve entered an era where true weight reductions in vehicles are occurring,” noted Dr. Alan Taub, professor of Material Science & Engineering at the University of Michigan. “There is no new vehicle launch that doesn’t talk about a 5-10 percent reduction in curb weight because it’s now clearly a part of fuel economy. And while the gains are still coming from powertrain improvements and the introduction of partial and full electrification, about 15 percent of fuel- economy improvements today come from vehicle weight reduction.”

His rule of thumb: Decreasing vehicle weight by 10% yields a 6% improvement in fuel economy.

At the 2019 Society of Plastics Engineers’ ANTEC conference, Dr. Taub, formerly GM’s head of R&D, presented a review of the three major materials groups—steel, aluminum, and composites—that he expects will predominate in vehicle body structures (increasingly in a mixed-materials play) going forward.

“If you’re chief engineer of a new-vehicle program, your job is to deliver the targeted fuel economy at the lowest possible cost,” he noted. “You follow a plot that says, ‘dollars per miles-per-gallon improvement.’ Depending on the OEM, that cost is about $2 to $2.50 per pound saved. Materials suppliers who can deliver that are going to get on the program.”

STEEL: The biggest dynamic in vehicle body materials planning today is the replacement of low-carbon steels with a growing portfolio of high-strength steel grades. Steel remains the Big Kahuna in vehicle structures, and the new grades deliver significantly greater strength (and thus better crash safety) than previous grades, with reduced mass, for a minimum of about $.50 per pound— ”very cost-effective,” Taub said.

Not long ago, body engineers thought 300 MPa [megapascals, a measure of tensile strength] was the limit for stamping steels. Today, stamping 800 MPa material, or even higher grades, is common. These ultrahigh-strength or advanced high strength steel (UHSS and AHSS) grades offer greater stiffness with reduced weight—the stronger the material, the thinner it can be made for the same application. But the new grades above 1,000 MPa are difficult to stamp at room temperature. They require hot forming (also known as press hardening)—a complex operation that heats, shapes, and quenches the sheet while it’s in the die.

Press-hardening adds cost, but those grades are “still well below that $2.00-$3.00 per pound saved threshold,” Dr. Taub explained. He said the steel industry is developing 1,200 to 1,400 MPa products that are sufficiently ductile to be stamped at room temperature—with even stronger grades to follow.

“The high-strength steels we used to say were limited to 10-to-15 percent weight reduction will soon be capable of delivering up to 25 percent reductions,” he told the plastics engineers.

ALUMINUM: Because it’s not as ductile as steel, manufacturers can’t yet stamp aluminum sheet in the same extreme shapes as steel offers, Dr. Taub noted. But the industry “has gotten much better at forming parts out of aluminum. And it has solved the joining problem that put it at a disadvantage to steel spot- welding,” by adopting mechanical fastening and even new processes to spot-weld aluminum to steel.

With a density that is 2.5 times less than comparable steel, “aluminum has quickly become the weight-saving material of choice for closures, where it’s now being delivered at below $2.00 per pound saved,” Dr. Taub said.

But the fundamentally higher cost of making aluminum, due to the energy-intensive nature of refining bauxite, continues to make it a premium-priced play. In addition, the industry is beginning to reach the limit of its rolling-mill capacity—which may cause restrictions of material availability. According to Dr. Taub, “OEMs developing new vehicles with extra aluminum content must team up with one of the aluminum companies to build up that capacity in rolling.”

Those challenges aside, Ford’s aluminum-intensive F-150 is on track for record sales, “and more OEMs are talking about additional aluminum content on vehicles,” Dr. Taub reported.

COMPOSITES: “So far, we know that the lightest- weight vehicle we can make is in carbon-fiber composite,” stated Dr. Taub, who is also CTO of the American Lightweight Materials Manufacturing Innovation Institute.

Making carbon fiber the material of choice for automotive structures faces numerous challenges. While many OEMs and Tier 1s are committed to working on it in low volume projects, BMW has walked away from its multimillion-dollar venture with SGL Carbon that spawned BMW’s novel i3 and i8 models—built on a supply chain that stretched from Germany all the way to northern Washington state.

“I am a believer that the end game in automotive primary body and chassis structure is carbon-fiber composites—it’s the material that can give us the most weight savings,” Dr. Taub said.

So, what’s it going to take to get there?

First, carbon fiber must meet all impact standards—difficult for a super-stiff material that fragments under extreme impact rather than deforms like metals. “The good news is we’ve learned how to model it, and it now gives higher specific energy absorption,” he noted.

Then there’s the molding time to produce a part—still a nagging issue. According to Dr. Taub, composites researchers are working to get the largest part of the vehicle—the floorpan—down to less than 1 minute in the mold. “We’ve reduced that process from eight minutes seven years ago to about four minutes today, and the one-minute cycle is starting to look possible,” he told the SPE audience. “We’re still looking for that instant cure.”

The next technology shift likely will be to thermoplastic-resin carbon fiber composites that offer the same structural properties with much-reduced molding time.

Cost of the material’s precursor, as well as the process used to convert it into high-strength carbon fiber, are also the focus of materials scientists and process engineers. “There is lots of work going on to reduce costs in all areas—we’ve gone from $20 per pound to $10 per pound, and we’re moving toward $7,” Dr. Taub reports. He adds that closed-loop recycling for the material still needs to be established: “That’s the end game for composites. I believe they can win eventually on every other single item including cost and cycle time.”

In a fairly short period, the mobility industry has migrated from single-material-intensive vehicles (i.e., F-150) into the concept of right material, produced the right way and engineered into the right part of the vehicle. “Now, the design engineers can go into ANSYS or whatever tool they use and select their material of choice; the component engineers can make that part in complex shapes, while the manufacturing engineers handle the multiplicity of materials and forms,” Dr. Taub noted. “And the joining engineers? They’re the ones everybody is investing in.”

Author: Lindsay Brooke

Source: SAE Automotive Engineering Magazine