如今，汽車行業(yè)對(duì)減重的追求正在推動(dòng)鋼、鋁和碳復(fù)合材料等行業(yè)的不斷創(chuàng)新。
“今天，我們已進(jìn)入一個(gè)汽車可以真正開始實(shí)現(xiàn)減重的時(shí)代，”密歇根大學(xué)材料科學(xué)與工程教授Alan Taub 博士表示，“現(xiàn)在幾乎所有新車發(fā)布時(shí)都會(huì)提到實(shí)現(xiàn)了5-10%的減重,因?yàn)楹茱@然，如今車輛的整備質(zhì)量已經(jīng)和燃油經(jīng)濟(jì)性直接掛鉤了。這也的確是事實(shí)，盡管如今車輛的燃油經(jīng)濟(jì)性提升仍主要來自對(duì)車輛動(dòng)力系統(tǒng)的改進(jìn)及對(duì)全電動(dòng)/半電動(dòng)系統(tǒng)的應(yīng)用，但仍有 15% 的燃油經(jīng)濟(jì)性提升與車輛減重直接相關(guān)。”
Taub 博士的經(jīng)驗(yàn)之談：車輛每減重 10%，燃油經(jīng)濟(jì)性可提高 6％。
在 2019 年塑料工程師學(xué)會(huì)（Society of Plastics Engineers’）舉辦的 ANTEC 大會(huì)上，曾在通用汽車公司擔(dān)任研發(fā)主管的 Taub 博士對(duì)鋼、鋁和復(fù)合物等三大主流汽車材料進(jìn)行了全面評(píng)估。他預(yù)計(jì)，未來，這些材料在汽車車身結(jié)構(gòu)中的比重將越來越高。
“無論任何新車研發(fā)項(xiàng)目的總工程師，你的工作就是盡可能以最低的成本，取得最高的燃油經(jīng)濟(jì)性。”Taub 指出，“接著，你得拿出‘平均每加侖英里數(shù)可節(jié)省的成本’數(shù)據(jù)。不同廠商的情況略有不同，但普遍減重一磅的經(jīng)濟(jì)效益為 2 到 2.5 美元。很顯然，哪家供應(yīng)商能幫助汽車廠商實(shí)現(xiàn)減重，哪家供應(yīng)商就能拿到供應(yīng)合同。”
STEEL 鋼：如今，汽車車身材料規(guī)劃方面的最大變化是用一系列不斷進(jìn)化的高強(qiáng)度鋼種替代之前的低碳鋼材料。Taub表示，鋼材料將繼續(xù)扮演汽車架構(gòu)中的“主力軍”，而且如今鋼材料的硬度越來越高（因此防撞效果更好）、重量更輕且成本低至每磅 0.5 美元，“具有很高的成本效益”。
不久之前，在車身工程師的概念里，沖壓鋼的拉伸強(qiáng)度極限還是 300 MPa。但如今，拉伸強(qiáng)度在800 MPa 的鋼材比比皆是，甚至還可以達(dá)到更高。這些先進(jìn)高強(qiáng)鋼（AHSS）和超高強(qiáng)鋼（UHSS）的剛度更高且重量更輕。不難理解，材料越堅(jiān)固，在相同應(yīng)用場景下需要使用的材料用量則越少，因此這些鋼材在減重方面的效果不言而喻。不過，拉伸強(qiáng)度在1000 MPa 以上的新型鋼材料下無法在室溫下壓印，必須采用熱成型技術(shù)。熱成型技術(shù)也稱壓力淬火，是一種復(fù)雜的工藝，需要在模具中完成加熱、成型和淬火等過程。
高強(qiáng)度鋼材所需的壓力淬火工藝會(huì)增加成本，但仍不超過“減重所能帶來的經(jīng)濟(jì)效益，也就是每減重1 磅可節(jié)省 2 到 3 美元。”Taub 解釋說，目前鋼材行業(yè)正在推出拉伸強(qiáng)度在 1200 到 1400 MPa 的超高延展性產(chǎn)品，可以在室溫下完成沖壓成型，而且未來還會(huì)推出拉伸強(qiáng)度更高的材料。
Taub 博士向在場塑料工程師介紹到，“我們剛剛討論的高強(qiáng)度鋼材頂多可以幫車輛實(shí)現(xiàn) 10% 到 15% 的減重，未來的新材料則可以進(jìn)一步將該比例提升為 25%。”
ALUMINUM 鋁：Taub 博士指出，鋁材料并不具有鋼材的延展性，因此制造商現(xiàn)在還無法向壓制鋼材一樣將鋁板壓制成一些更為復(fù)雜、更加極端的形狀。然而，汽車行業(yè)“在制造鋁成型零件方面已經(jīng)取得了很大的進(jìn)展，而且已經(jīng)通過采用機(jī)械緊固件，甚至是通過新工藝將鋁材料點(diǎn)焊到鋼材上的方法，解決了部件之間連接緊固的問題，這也是目前鋁材相較于鋼材的主要劣勢之一。
Taub 博士表示，由于密度比同類鋼材低 2.5 倍，“鋁材已經(jīng)迅速成為閉合件的首選材料，每減重 1 磅可節(jié)省不到2.00 美元。”
然而，由于鋁土礦精煉本身就屬于能源密集型產(chǎn)業(yè)，因此鋁材制造的成本本來就比較高，因此最終價(jià)格也相對(duì)較高。此外，鋁材行業(yè)已經(jīng)接近現(xiàn)有軋機(jī)產(chǎn)能的極限，這可能會(huì)限制這種材料未來的供應(yīng)保證。根據(jù)Taub 博士的說法，“在開展新車項(xiàng)目時(shí)，如果需要使用額外的鋁材料，則車廠必須直接與鋁業(yè)公司合作以提高產(chǎn)能，確保鋁材料的供應(yīng)。”
Taub 博士介紹說，先把這些挑戰(zhàn)放在一邊，福特汽車的鋁質(zhì)皮卡F-150 已經(jīng)創(chuàng)造了不俗的銷售神話，“越來越多的車廠都在討論擴(kuò)大鋁材料在車身中的應(yīng)用。”
COMPOSITES 復(fù)合材料：Taub 博士表示，“目前市面上最輕的一批車型都采用了碳纖維復(fù)合材料。” Taub 博士本人也同時(shí)是美國輕質(zhì)材料制造創(chuàng)新研究所（American Lightweight Materials Manufacturing Innovation Institute）的首席技術(shù)官。
目前，碳纖維材料在車身結(jié)構(gòu)中的大規(guī)模應(yīng)用還面臨諸多挑戰(zhàn)。很多車廠和一級(jí)供應(yīng)商均在探索碳復(fù)合材料的小批量應(yīng)用，但也有廠商選擇與流行趨勢“背道而馳”。比如 BMW 已經(jīng)從價(jià)值數(shù)百萬美元的SGL Carbon 合作中抽身，而后者在 BMW 的創(chuàng)新 i3 和 i8 車型的生產(chǎn)中扮演了重要角色。BMW i3 和 i8 車型的零部件采用源自全球，供應(yīng)商從德國老家一直延伸至美國華盛頓州。
Taub 表示，“汽車車身和底盤結(jié)構(gòu)的選材最終還是會(huì)趨向碳纖維復(fù)合材料，這是我們的減重的法寶。”
那么，如何才能走到這一步呢？
首先，碳纖維材料必須可以滿足所有的沖擊標(biāo)準(zhǔn)。對(duì)比來說，金屬材料在極端沖擊條件下會(huì)變形，而碳纖維這樣的超硬材料在同等條件下更傾向于碎裂。Taub 博士指出，“但好消息是我們已經(jīng)學(xué)會(huì)了如何通過建模來優(yōu)化設(shè)計(jì)，碳纖維材料吸收能量的能力也因此有了顯著提升。”
不過，碳纖維零部件的成型時(shí)間較長，這仍然是個(gè)麻煩的問題。根據(jù) Taub 博士的說法，復(fù)合材料研究人員正在努力將碳纖維底板（這種材料在汽車車身中的最大規(guī)模應(yīng)用）的制作時(shí)間降低至1 分鐘。“我們已將這一過程從七年前的 8 分鐘縮短到今天的大約 4 分鐘，未來還有可能繼續(xù)將其縮短至 1 分鐘，”Taub 博士向 SPE 觀眾介紹說，“我們?nèi)栽趯ふ宜矔r(shí)固化工藝。”
下一個(gè)最值得關(guān)注的技術(shù)拐點(diǎn)可能是熱塑性樹脂碳纖維復(fù)合材料。這種材料與碳纖維材料的結(jié)構(gòu)特性相同，但成型時(shí)間卻大大縮短。
目前，材料的前體成本以及將其轉(zhuǎn)化為高強(qiáng)度碳纖維所需的工藝也是材料科學(xué)家和工藝工程師關(guān)注的焦點(diǎn)。Taub博士介紹說，“為了從各個(gè)方面降低成本，我們還有很多工作要做。如今，我們已經(jīng)將成本從每磅 20 美元降低至每磅 10 美元，未來還將繼續(xù)為降低至 7 美元的目標(biāo)而努力。”他補(bǔ)充說，我們還必須建立材料的閉環(huán)回收流程：“復(fù)合材料將成為最終勝利者，這種材料將逐步在各個(gè)方面占據(jù)優(yōu)勢，包括成本和回收時(shí)間。”
事實(shí)上，交通運(yùn)輸行業(yè)已經(jīng)在相當(dāng)短的時(shí)間內(nèi)完成了從“單一材料密集使用（比如 F-150 皮卡）”到形成全新材料觀的轉(zhuǎn)變，即“將合適的材料，用合適的方法，應(yīng)用至合適的位置”。“現(xiàn)在，設(shè)計(jì)工程師可以使用 ANSYS 等各種各樣的工具，并選擇各種各樣的材料；組件工程師可以將這個(gè)部件制作成最復(fù)雜的形狀，而制造工程師則會(huì)處理這些材料豐富、形狀復(fù)雜的零部件，”Taub 博士指出，“那么我們不需要連接工程師了嗎？恰恰相反，他們可是所有公司都?jí)裘乱郧蟮娜恕?rdquo;
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
金屬成型和材料科學(xué)領(lǐng)域的創(chuàng)新帶來了強(qiáng)度更高的鋼材，但在制造過程中必須采用加壓淬火等更為復(fù)雜、更高成本的工藝。
材料專家 Alan Taub 博士曾就職于通用電氣、福特汽車和通用汽車等多家世界 500 強(qiáng)企業(yè)，自 2012 年起開始在密歇根大學(xué)投身教學(xué)工作。
