Moody’s Investors Service upgraded Ford Motor Co.’sdebt to investment grade today, culminating the automaker’s six-year struggle to win back the Blue Oval and other assets mortgaged in 2006.

Moody’s is the second agency to upgrade Ford’s debt from junk status, paving the way for the company to recover assets it mortgaged in 2006 to stave off bankruptcy. The company raised $23.5 billion in a move that allowed it to avoid government bailouts or bankruptcy when the industry collapsed in 2008.

“The Ford Blue Oval is back where it belongs with the Ford family of 166,000 employees around the world,” Bill Ford Jr., executive chairman of the automaker, said in a statement. “This is a great day for us and is the result of several years of hard work and progress by everyone associated with Ford.”

Moody’s said the upgrade reflects the automaker’s strength in North America and its expectation that “improvements Ford has made are likely to be lasting.”

Ford is expected to continue to manage its balance sheet well, Moody’s said.

The rating agency also said it believes Ford can maintain its investment grade rating even in the face of a European downturn.

“Moody’s believes that Ford has demonstrated its commitment to maintaining sound operating and financial disciplines by preserving a low break-even level; matching production levels to retail demand; limiting the use of incentives and price discounting; capitalizing on the use of global vehicle platforms; and, building healthy supplier relationships,” the agency said in a statement.

Moody’s cautioned that Ford would have to build its strength outside its North America stronghold before the agency will boost its rating further.

“In order to support a positive outlook or higher rating, Ford will have to demonstrate clear progress in building profitability outside of North America. This would require a sustained turnaround of its European operations and a profitable expansion of its position in China. The company would also have to maintain its solid position in North America and a healthy liquidity profile.”

Moody’s had ranked the second-largest U.S. automaker’s debt as junk since August 2005.

Ford — helped by deep cost cuts, staff reductions, plant closings, new models and a rebound in industry sales — has produced 12 consecutive profitable quarters through the first three months of 2012.

The automaker earned $29.5 billion in the last three years after racking up $30.1 billion in losses from 2006 through 2008.

Ford has also pared debt and reinstated a cash dividend.

In April, Ford’s rating was upgraded to investment grade by Fitch Ratings. Standard & Poor’s still rates the automaker below investment grade.

Bill Ford Jr., clearly elated at the move by Moody’s, announced the news of the upgrade to Ford employees via the automaker’s in-house public address system, “which is only ever used for fire drills,” he said.

He described the company’s decision to mortgage its assets back in 2006 as a “once-in-a-lifetime decision” and said he hoped the company would never have to go through anything like it again.

“By definition, today is also a once-in-a-lifetime event, which I couldn’t be happier about,” Bill Ford said.

He attributed the company’s comeback to an “heroic effort by every Ford employee.”

“When we pledged the Blue Oval, it was enormously emotionally for me and my family,” Bill Ford said. “We weren’t just pledging an asset, we were pledging our heritage. It feels wonderful.”

Asked whether he ever doubted Ford would win back its assets, Ford said: “No, I said back in the darkest days I was absolutely confident we had the right leadership and the right plan. The only question is whether we had enough time.”

Ford CEO Alan Mulally, 66, said the investment upgrade won’t change his plans. At the company’s recent annual meeting, Bill Ford and other directors endorsed Mulally’s performance and provided no timeline on his retirement.

Said Mulally: “Clearly it is a very significant milestone, but it changes none of my plans to continue to serve this great corporation.”

Mulally described the upgrade as “a very exciting moment for employees, dealers, suppliers. It’s way up there on the highlight film.”

Bob Shanks — who succeeded Lewis Booth as Ford CFO on April 1 — said the upgrade won’t have a major impact on the company’s operations but it will help lower borrowing costs.

“Operationally, it doesn’t have too much effect,” Shanks said. “We had pledged the assets but it hadn’t affected our operational flexibility. We will see a reduction in our borrowing rates.”