Federal Reserve Chair Janet Yellen on Friday said she expected the central bank to raise rates this year as the U.S. economy was on course to bounce back from a sluggish first quarter and as headwinds at home and abroad begin to wane. Yellen spoke amid growing concern at the Fed about possible market volatility once it begins to raise rates, and a desire to begin coaxing skeptical investors towards accepting the inevitable: ending about a roughly six-year stretch of near-zero interest rates.